Staff directory
Stephan Roche
ICREA Research Professor and Group Leader
stephan.roche(ELIMINAR)@icn2.cat
Theoretical and Computational Nanoscience
- ORCID: 0000-0003-0323-4665
Publications
2024
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Exploring Dielectric Properties in Atomistic Models of Amorphous Boron Nitride
Thomas Galvani; Ali Hamze; Laura Caputo; Onurcan Kaya; Simon Dubois; Luigi Colombo; Việt Hùng Nguyễn; Yongwoo Shin; Hyu‐Soung Shin; Jean‐Christophe Charlier; Stephan Roche Jphys Materials; 2024. 10.1088/2515-7639/ad4c06.
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Giant spin transport anisotropy in magnetic topological insulators
Vila, Marc; Cummings, Aron W; Roche, Stephan Physical Review b; 109 (19): 195435. 2024. 10.1103/PhysRevB.109.195435.
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Impact of hydrogenation on the stability and mechanical properties of amorphous boron nitride
Kaya, Onurcan; Colombo, Luigi; Antidormi, Aleandro; Villena, Marco A; Lanza, Mario; Cole, Ivan; Roche, Stephan Journal Of Physics-Materials; 7 (2): 025010. 2024. 10.1088/2515-7639/ad367b.
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Resilient intraparticle entanglement and its manifestation in spin dynamics of disordered Dirac materials
Romeral, Jorge Martinez; Cummings, Aron W; Roche, Stephan Physical Review b; 109 (17): 174313. 2024. 10.1103/PhysRevB.109.174313.
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Tailoring giant quantum transport anisotropy in nanoporous graphenes under electrostatic disorder
Alcon, Isaac; Cummings, Aron W; Roche, Stephan Nanoscale Horizons; 9 (3) 2024. 10.1039/d3nh00416c.
2023
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Revealing the improved stability of amorphous boron-nitride upon carbon doping
Kaya, O; Colombo, L; Antidormi, A; Lanza, MR; Roche, S Nanoscale Horizons; 8 (3): 361 - 367. 2023. 10.1039/d2nh00520d. IF: 9.700
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When matter and information merge into "Quantum"
Aguado, R; Cervera-Lierta, A; Correia, A; de Franceschi, S; Diez Muiño, R; Garcia Ripoll, JJ; Levi-Yeyati, A; Platero, G; Roche, S; Sanchez-Portal, D Communications Physics; 6 (1): 266. 2023. 10.1038/s42005-023-01391-x. IF: 5.500
2022
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Electrical control of spin-polarized topological currents in monolayer WTe2
Garcia, JH; You, JX; Garcia-Mota, M; Koval, P; Ordejon, P; Cuadrado, R; Verstraete, MJ; Zanolli, Z; Roche, S Physical Review b; 106 (16) 2022. 10.1103/PhysRevB.106.L161410. IF: 3.908
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Emerging properties of non-crystalline phases of graphene and boron nitride based materials
Antidormi A., Colombo L., Roche S. Nano Materials Science; 4 (1): 10 - 17. 2022. 10.1016/j.nanoms.2021.03.003. IF: 0.000
We review recent developments on the synthesis and properties of two-dimensional materials which, although being mainly of an sp2 bonding character, exhibit highly disordered, non-uniform and structurally random morphologies. The emergence of such class of amorphous materials, including amorphous graphene and boron nitride, have shown superior properties compared to their crystalline counterparts when used as interfacial films. In this paper we discuss their structural, vibrational and electronic properties and present a perspective of their use for electronic applications. © 2021 Chongqing University
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Giant valley-polarized spin splittings in magnetized Janus Pt dichalcogenides
Sattar S., Larsson J.A., Canali C.M., Roche S., Garcia J.H. Physical Review B; 105 (4, A100) 2022. 10.1103/PhysRevB.105.L041402. IF: 4.036
We reveal giant proximity-induced magnetism and valley-polarization effects in Janus Pt dichalcogenides (such as SPtSe), when bound to the europium oxide (EuO) substrate. Using first-principles simulations, it is surprisingly found that the charge redistribution, resulting from proximity with EuO, leads to the formation of two K and K′ valleys in the conduction bands. Each of these valleys displays its own spin polarization and a specific spin texture dictated by broken inversion and time-reversal symmetries, and valley-exchange and Rashba splittings as large as hundreds of meV. This provides a platform for exploring spin-valley physics in low-dimensional semiconductors, with potential spin transport mechanisms such as spin-orbit torques much more resilient to disorder and temperature effects. © 2022 American Physical Society.
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Have mysterious topological valley currents been observed in graphene superlattices?
Roche S., Power S.R., Nikolić B.K., García J.H., Jauho A.-P. JPhys Materials; 5 (2, 021001) 2022. 10.1088/2515-7639/ac452a. IF: 0.000
We provide a critical discussion concerning the claim of topological valley currents, driven by a global Berry curvature and valley Hall effect proposed in recent literature. After pointing out a major inconsistency of the theoretical scenario proposed to interpret giant nonlocal resistance, we discuss various possible alternative explanations and open directions of research to solve the mystery of nonlocal transport in graphene superlattices. © 2022 The Author(s).
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Magnetism, symmetry and spin transport in van der Waals layered systems
Kurebayashi H., Garcia J.H., Khan S., Sinova J., Roche S. Nature Reviews Physics; 4 (3): 150 - 166. 2022. 10.1038/s42254-021-00403-5. IF: 31.068
The discovery of an ever-increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin–orbit coupling and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here, we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals magnetism and spin–orbit coupling effects, we discuss how their coexistence, manipulation and competition could ultimately establish new ways to engineer robust spin textures and drive the generation and dynamics of spin current and magnetization switching in 2D-materials-based van der Waals heterostructures. Grounding our analysis on existing experimental results and theoretical considerations, we draw a prospective analysis about how intertwined magnetism and spin–orbit torque phenomena combine at interfaces with well-defined symmetries and how this dictates the nature and figures of merit of spin–orbit torque and angular momentum transfer. This will serve as a guiding role in designing future non-volatile memory devices that utilize the unique properties of 2D materials with the spin degree of freedom. © 2022, Springer Nature Limited.
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Manipulation of spin transport in graphene/transition metal dichalcogenide heterobilayers upon twisting
Pezo A., Zanolli Z., Wittemeier N., Ordejón P., Fazzio A., Roche S., Garcia J.H. 2D Materials; 9 (1, 015008) 2022. 10.1088/2053-1583/ac3378. IF: 7.103
Proximity effects between layered materials trigger a plethora of novel and exotic quantum transport phenomena. Besides, the capability to modulate the nature and strength of proximity effects by changing crystalline and interfacial symmetries offers a vast playground to optimize physical properties of relevance for innovative applications. In this work, we use large-scale first principles calculations to demonstrate that strain and twist-angle strongly vary the spin–orbit coupling (SOC) in graphene/transition metal dichalcogenide heterobilayers. Such a change results in a modulation of the spin relaxation times by up to two orders of magnitude. Additionally, the relative strengths of valley-Zeeman and Rashba SOC can be tailored upon twisting, which can turn the system into an ideal Dirac–Rashba regime or generate transitions between topological states of matter. These results shed new light on the debated variability of SOC and clarify how lattice deformations can be used as a knob to control spin transport. Our outcomes also suggest complex spin transport in polycrystalline materials, due to the random variation of grain orientation, which could reflect in large spatial fluctuations of SOC fields. © 2021 IOP Publishing Ltd
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Toward Optimized Charge Transport in Multilayer Reduced Graphene Oxides
Çlnar M.N., Antidormi A., Nguyen V.-H., Kovtun A., Lara-Avila S., Liscio A., Charlier J.-C., Roche S., Sevinçli H. Nano Letters; 22 (6): 2202 - 2208. 2022. 10.1021/acs.nanolett.1c03883. IF: 11.189
In the context of graphene-based composite applications, a complete understanding of charge conduction in multilayer reduced graphene oxides (rGO) is highly desirable. However, these rGO compounds are characterized by multiple and different sources of disorder depending on the chemical method used for their synthesis. Most importantly, the precise role of interlayer interaction in promoting or jeopardizing electronic flow remains unclear. Here, thanks to the development of a multiscale computational approach combining first-principles calculations with large-scale transport simulations, the transport scaling laws in multilayer rGO are unraveled, explaining why diffusion worsens with increasing film thickness. In contrast, contacted films are found to exhibit an opposite trend when the mean free path becomes shorter than the channel length, since conduction becomes predominantly driven by interlayer hopping. These predictions are favorably compared with experimental data and open a road toward the optimization of graphene-based composites with improved electrical conduction. © 2022 American Chemical Society. All rights reserved.
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Two-dimensional materials prospects for non-volatile spintronic memories
Yang H., Valenzuela S.O., Chshiev M., Couet S., Dieny B., Dlubak B., Fert A., Garello K., Jamet M., Jeong D.-E., Lee K., Lee T., Martin M.-B., Kar G.S., Sénéor P., Shin H.-J., Roche S. Nature; 606 (7915): 663 - 673. 2022. 10.1038/s41586-022-04768-0.
Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin–orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes. © 2022, Springer Nature Limited.
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Unveiling the Multiradical Character of the Biphenylene Network and Its Anisotropic Charge Transport
Alcón I., Calogero G., Papior N., Antidormi A., Song K., Cummings A.W., Brandbyge M., Roche S. Journal of the American Chemical Society; 2022. 10.1021/jacs.2c02178.
Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics. © 2022 American Chemical Society. All rights reserved.
2021
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Advanced Data Encryption using 2D Materials
Wen C., Li X., Zanotti T., Puglisi F.M., Shi Y., Saiz F., Antidormi A., Roche S., Zheng W., Liang X., Hu J., Duhm S., Roldan J.B., Wu T., Chen V., Pop E., Garrido B., Zhu K., Hui F., Lanza M. Advanced Materials; 33 (27, 2100185) 2021. 10.1002/adma.202100185. IF: 30.849
Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO2 and Al2O3, are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 224 − 1 bits), and high throughput of 1 Mbit s−1 by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation. © 2021 Wiley-VCH GmbH.
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Control of spin-charge conversion in van der Waals heterostructures
Galceran R., Tian B., Li J., Bonell F., Jamet M., Vergnaud C., Marty A., García J.H., Sierra J.F., Costache M.V., Roche S., Valenzuela S.O., Manchon A., Zhang X., Schwingenschlögl U. APL Materials; 9 (10, 100901) 2021. 10.1063/5.0054865. IF: 5.096
The interconversion between spin and charge degrees of freedom offers incredible potential for spintronic devices, opening routes for spin injection, detection, and manipulation alternative to the use of ferromagnets. The understanding and control of such interconversion mechanisms, which rely on spin-orbit coupling, is therefore an exciting prospect. The emergence of van der Waals materials possessing large spin-orbit coupling (such as transition metal dichalcogenides or topological insulators) and/or recently discovered van der Waals layered ferromagnets further extends the possibility of spin-to-charge interconversion to ultrathin spintronic devices. Additionally, they offer abundant room for progress in discovering and analyzing novel spin-charge interconversion phenomena. Modifying the properties of van der Waals materials through proximity effects is an added degree of tunability also under exploration. This Perspective discusses the recent advances toward spin-to-charge interconversion in van der Waals materials. It highlights scientific developments which include techniques for large-scale growth, device physics, and theoretical aspects. © 2021 Author(s).
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Graphene on two-dimensional hexagonal BN, AlN, and GaN: Electronic, spin-orbit, and spin relaxation properties
Zollner K., Cummings A.W., Roche S., Fabian J. Physical Review B; 103 (7, 075129) 2021. 10.1103/PhysRevB.103.075129. IF: 4.036
We investigate the electronic band structure of graphene on a series of two-dimensional hexagonal nitride insulators hXN, X=B, Al, and Ga, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and spin-orbit coupling (SOC) from the low-energy Dirac bands of the proximitized graphene. While commensurate hBN induces a staggered potential of about 10 meV into the Dirac band structure, less lattice-matched hAlN and hGaN disrupt the Dirac point much less, giving a staggered gap below 100 μeV. Proximitized intrinsic SOC surprisingly does not increase much above the pristine graphene value of 12 μeV; it stays in the window of 1-16 μeV, depending strongly on stacking. However, Rashba SOC increases sharply when increasing the atomic number of the boron group, with calculated maximal values of 8, 15, and 65 μeV for B-, Al-, and Ga-based nitrides, respectively. The individual Rashba couplings also depend strongly on stacking, vanishing in symmetrically sandwiched structures, and can be tuned by a transverse electric field. The extracted spin-orbit parameters were used as input for spin transport simulations based on Chebyshev expansion of the time-evolution of the spin expectation values, yielding interesting predictions for the electron spin relaxation. Spin lifetime magnitudes and anisotropies depend strongly on the specific (hXN)/graphene/hXN system, and they can be efficiently tuned by an applied external electric field as well as the carrier density in the graphene layer. A particularly interesting case for experiments is graphene/hGaN, in which the giant Rashba coupling is predicted to induce spin lifetimes of 1-10 ns, short enough to dominate over other mechanisms, and lead to the same spin relaxation anisotropy as that observed in conventional semiconductor heterostructures: 50%, meaning that out-of-plane spins relax twice as fast as in-plane spins. © 2021 American Physical Society.
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Hinge Spin Polarization in Magnetic Topological Insulators Revealed by Resistance Switch
Perez-Piskunow P.M., Roche S. Physical Review Letters; 126 (16, 167701) 2021. 10.1103/PhysRevLett.126.167701. IF: 9.161
We report on the possibility of detecting hinge spin polarization in magnetic topological insulators by resistance measurements. By implementing a three-dimensional model of magnetic topological insulators into a multiterminal device with ferromagnetic contacts near the top surface, local spin features of the chiral edge modes are unveiled. We find local spin polarization at the hinges that inverts the sign between the top and bottom surfaces. At the opposite edge, the topological state with inverted spin polarization propagates in the reverse direction. A large resistance switch between forward and backward propagating states is obtained, driven by the matching between the spin polarized hinges and the ferromagnetic contacts. This feature is general to the ferromagnetic, antiferromagnetic, and canted antiferromagnetic phases, and enables the design of spin-sensitive devices, with the possibility of reversing the hinge spin polarization of the currents. © 2021 American Physical Society.
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Janus monolayers of magnetic transition metal dichalcogenides as an all-in-one platform for spin-orbit torque
Smaili I., Laref S., Garcia J.H., Schwingenschlögl U., Roche S., Manchon A. Physical Review B; 104 (10, 104415) 2021. 10.1103/PhysRevB.104.104415. IF: 4.036
We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit torque memories. Using first-principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of nonvolatile magnetic random access memories. ©2021 American Physical Society.
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Linear scaling quantum transport methodologies
Fan Z., Garcia J.H., Cummings A.W., Barrios-Vargas J.E., Panhans M., Harju A., Ortmann F., Roche S. Physics Reports; 903: 1 - 69. 2021. 10.1016/j.physrep.2020.12.001. IF: 25.600
In recent years, predictive computational modeling has become a cornerstone for the study of fundamental electronic, optical, and thermal properties in complex forms of condensed matter, including Dirac and topological materials. The simulation of quantum transport in realistic models calls for the development of linear scaling, or order-N, numerical methods, which then become enabling tools for guiding experimental research and for supporting the interpretation of measurements. In this review, we describe and compare different order-N computational methods that have been developed during the past twenty years, and which have been used extensively to explore quantum transport phenomena in disordered media. We place particular focus on the zero-frequency electrical conductivities derived within the Kubo–Greenwood and Kubo–Streda formalisms, and illustrate the capabilities of these methods to tackle the quasi-ballistic, diffusive, and localization regimes of quantum transport in the noninteracting limit. The fundamental issue of computational cost versus accuracy of various proposed numerical schemes is addressed in depth. We then illustrate the usefulness of these methods with various examples of transport in disordered materials, such as polycrystalline and defected graphene models, 3D metals and Dirac semimetals, carbon nanotubes, and organic semiconductors. Finally, we extend the review to the study of spin dynamics and topological transport, for which efficient approaches for calculating charge, spin, and valley Hall conductivities are described. © 2020 The Author(s)
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Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers
Vila M., Hsu C.-H., Garcia J.H., Benítez L.A., Waintal X., Valenzuela S.O., Pereira V.M., Roche S. Physical Review Research; 3 (4, 043230) 2021. 10.1103/PhysRevResearch.3.043230. IF: 0.000
The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit λsθxy≈1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy≈80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. © 2021 authors. Published by the American Physical Society.
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Observation of giant and tunable thermal diffusivity of a Dirac fluid at room temperature
Block A., Principi A., Hesp N.C.H., Cummings A.W., Liebel M., Watanabe K., Taniguchi T., Roche S., Koppens F.H.L., van Hulst N.F., Tielrooij K.-J. Nature Nanotechnology; 16 (11): 1195 - 1200. 2021. 10.1038/s41565-021-00957-6. IF: 39.213
Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management. © 2021, The Author(s).
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Reply to: On the measured dielectric constant of amorphous boron nitride
Hong S., Lee M.-H., Kim S.W., Lee C.-S., Ma K.Y., Kim G., Yoon S.I., Antidormi A., Roche S., Shin H.-J., Chhowalla M., Shin H.S. Nature; 590 (7844): E8 - E10. 2021. 10.1038/s41586-020-03163-x. IF: 49.962
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Room-temperature tunnel magnetoresistance across biomolecular tunnel junctions based on ferritin
Karuppannan S.K., Pasula R.R., Herng T.S., Ding J., Chi X., Barco E.D., Roche S., Yu X., Yakovlev N., Lim S., Nijhuis C.A. JPhys Materials; 4 (3, 035003) 2021. 10.1088/2515-7639/abfa79. IF: 0.000
We report exceptionally large tunnel magnetoresistance (TMR) for biomolecular tunnel junctions based on ferritins immobilized between Ni and EGaIn electrodes. Ferritin stores iron in the form of ferrihydrite nanoparticles (NPs) and fulfills the following roles: (a) it dictates the tunnel barrier, (b) it magnetically decouples the NPs from the ferromagnetic (FM) electrode, (c) it stabilizes the NPs, and (d) it acts as a spin filter reducing the complexity of the tunnel junctions since only one FM electrode is required. The mechanism of charge transport is long-range tunneling which results in TMR of 60 ± 10% at 200 K and 25 ± 5% at room temperature. We propose a magnon-assisted transmission to explain the substantially larger TMR switching fields (up to 1 Tesla) than the characteristic coercive fields (a few Gauss) of ferritin ferrihydrite particles at T < 20 K. These results highlight the genuine potential of biomolecular tunnel junctions in designing functional nanoscale spintronic devices. © 2021 The Author(s). Published by IOP Publishing Ltd.
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Thermal transport in amorphous graphene with varying structural quality
Antidormi A., Colombo L., Roche S. 2D Materials; 8 (1, 015028) 2021. 10.1088/2053-1583/abc7f8. IF: 7.103
The synthesis of wafer-scale two-dimensional amorphous carbon monolayers has been recently demonstrated. This material presents useful properties when integrated as coating of metals, semiconductors or magnetic materials, such as enabling efficient atomic layer deposition and hence fostering the development of ultracompact technologies. Here we propose a characterization of how the structural degree of amorphousness of such carbon membranes could be controlled by the crystal growth temperature. We also identify how energy is dissipated in this material by a systematic analysis of emerging vibrational modes whose localization increases with the loss of spatial symmetries, resulting in a tunable thermal conductivity varying by more than two orders of magnitude. Our simulations provide some recipe to design most suitable 'amorphous graphene' based on the target applications such as ultrathin heat spreaders, energy harvesters or insulating thermal barriers. © 2020 IOP Publishing Ltd.
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Valley Hall effect and nonlocal resistance in locally gapped graphene
Aktor T., Garcia J.H., Roche S., Jauho A.-P., Power S.R. Physical Review B; 103 (11, 115406) 2021. 10.1103/PhysRevB.103.115406. IF: 4.036
We report on the emergence of bulk, valley-polarized currents in graphene-based devices, driven by spatially varying regions of broken sublattice symmetry, and revealed by nonlocal resistance (RNL) fingerprints. By using a combination of quantum transport formalisms, giving access to bulk properties as well as multiterminal device responses, the presence of a nonuniform local band gap is shown to give rise to valley-dependent scattering and a finite Fermi-surface contribution to the valley Hall conductivity, related to characteristics of RNL. These features are robust against disorder and provide a plausible interpretation of controversial experiments in graphene/hexagonal boron nitride superlattices. Our findings suggest both an alternative mechanism for the generation of valley Hall effect in graphene and a route towards valley-dependent electron optics, by materials and device engineering. © 2021 American Physical Society.
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Valley-polarized quantum anomalous Hall phase in bilayer graphene with layer-dependent proximity effects
Vila M., Garcia J.H., Roche S. Physical Review B; 104 (16, A84) 2021. 10.1103/PhysRevB.104.L161113. IF: 4.036
Realizations of some topological phases in two-dimensional systems rely on the challenge of jointly incorporating spin-orbit and magnetic exchange interactions. Here, we predict the formation and control of a fully valley-polarized quantum anomalous Hall effect in bilayer graphene, by separately imprinting spin-orbit and magnetic proximity effects in different layers. This results in varying spin splittings for the conduction and valence bands, which gives rise to a topological gap at a single Dirac cone. The topological phase can be controlled by a gate voltage and switched between valleys by reversing the sign of the exchange interaction. By performing quantum transport calculations in disordered systems, the chirality and resilience of the valley-polarized edge state are demonstrated. Our findings provide a promising route to engineer a topological phase that could enable low-power electronic devices and valleytronic applications as well as putting forward layer-dependent proximity effects in bilayer graphene as a way to create versatile topological states of matter. © 2021 American Physical Society.
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Van der Waals heterostructures for spintronics and opto-spintronics
Sierra J.F., Fabian J., Kawakami R.K., Roche S., Valenzuela S.O. Nature Nanotechnology; 16 (8): 856 - 868. 2021. 10.1038/s41565-021-00936-x. IF: 39.213
The large variety of 2D materials and their co-integration in van der Waals heterostructures enable innovative device engineering. In addition, their atomically thin nature promotes the design of artificial materials by proximity effects that originate from short-range interactions. Such a designer approach is particularly compelling for spintronics, which typically harnesses functionalities from thin layers of magnetic and non-magnetic materials and the interfaces between them. Here we provide an overview of recent progress in 2D spintronics and opto-spintronics using van der Waals heterostructures. After an introduction to the forefront of spin transport research, we highlight the unique spin-related phenomena arising from spin–orbit and magnetic proximity effects. We further describe the ability to create multifunctional hybrid heterostructures based on van der Waals materials, combining spin, valley and excitonic degrees of freedom. We end with an outlook on perspectives and challenges for the design and production of ultracompact all-2D spin devices and their potential applications in conventional and quantum technologies. © 2021, Springer Nature Limited.
2020
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Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN
Kim G., Ma K.Y., Park M., Kim M., Jeon J., Song J., Barrios-Vargas J.E., Sato Y., Lin Y.-C., Suenaga K., Roche S., Yoo S., Sohn B.-H., Jeon S., Shin H.S. Nature Communications; 11 (1, 5359) 2020. 10.1038/s41467-020-19181-2. IF: 12.121
Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics. © 2020, The Author(s).
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Canted Persistent Spin Texture and Quantum Spin Hall Effect in WTe2
Garcia J.H., Vila M., Hsu C.-H., Waintal X., Pereira V.M., Roche S. Physical Review Letters; 125 (25, 256603) 2020. 10.1103/PhysRevLett.125.256603. IF: 8.385
We report an unconventional quantum spin Hall phase in the monolayer WTe2, which exhibits hitherto unknown features in other topological materials. The low symmetry of the structure induces a canted spin texture in the yz plane, which dictates the spin polarization of topologically protected boundary states. Additionally, the spin Hall conductivity gets quantized (2e2/h) with a spin quantization axis parallel to the canting direction. These findings are based on large-scale quantum simulations of the spin Hall conductivity tensor and nonlocal resistances in multiprobe geometries using a realistic tight-binding model elaborated from first-principle methods. The observation of this canted quantum spin Hall effect, related to the formation of topological edge states with nontrivial spin polarization, demands for specific experimental design and suggests interesting alternatives for manipulating spin information in topological materials. © 2020 American Physical Society.
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Efficient machine-learning based interatomic potentialsfor exploring thermal conductivity in two-dimensional materials
Bohayra Mortazavi, V Evgeny Podryabinkin, Ivan S Novikov, Stephan Roche, Timon Rabczuk,Xiaoying Zhuang; V Alexander Shapeev Journal of Physics-Materials; 3 (2, 02LT02) 2020. 10.1088/2515-7639/ab7cbb. IF: 0.000
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Emergence of intraparticle entanglement and time-varying violation of Bell's inequality in Dirac matter
De Moraes B.G., Cummings A.W., Roche S. Physical Review B; 102 (4, 041403) 2020. 10.1103/PhysRevB.102.041403. IF: 3.575
We demonstrate the emergence and dynamics of intraparticle entanglement in massless Dirac fermions. This entanglement, generated by spin-orbit coupling, arises between the spin and sublattice pseudospin of electrons in graphene. The entanglement is a complex dynamic quantity but is generally large, independent of the initial state. Its time dependence implies a dynamical violation of a Bell inequality, while its magnitude indicates that large intraparticle entanglement is a general feature of graphene on a substrate. These features are also expected to impact entanglement between pairs of particles, and may be detectable in experiments that combine Cooper pair splitting with nonlocal measurements of spin-spin correlation in mesoscopic devices based on Dirac materials. © 2020 American Physical Society.
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Exploring event horizons and Hawking radiation through deformed graphene membranes
Morresi T., Binosi D., Simonucci S., Piergallini R., Roche S., Pugno N.M., Simone T. 2D Materials; 7 (4, 041006) 2020. 10.1088/2053-1583/aba448. IF: 7.140
Analogue gravitational systems are becoming an increasing popular way of studying the behaviour of quantum systems in curved spacetime. Setups based on ultracold quantum gases in particular, have been recently harnessed to explore the thermal nature of Hawking's and Unruh's radiation that was theoretically predicted almost 50 years ago. For solid state implementations, a promising system is graphene, in which a link between the Dirac-like low-energy electronic excitations and relativistic quantum field theories has been unveiled soon after its discovery. This link could be extended to the case of curved quantum field theory when the graphene sheet is shaped in a surface of constant negative curvature, known as Beltrami's pseudosphere. Here we provide numerical evidence that energetically stable negative curvature graphene surfaces can be realized. Owing to large-scale simulations, our geometrical realizations are characterized by a ratio between the carbon-carbon bond length and the pseudosphere radius small enough to allow the formation of an analog of a black hole event horizon. Additionally, from the energy dependence of the spatially resolved density of states, we infer some thermal properties of the corresponding gravitational system, which could be investigated using low temperature scanning tunnelling microscopy or optical near field spectroscopy. These findings pave the way to the realization of a solid-state system in which the curved spacetime dynamics of quantum many body systems can be investigated. © 2020 IOP Publishing Ltd.
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Exploring phononic properties of two-dimensional materials using machine learning interatomic potentials
Mortazavi B., Novikov I.S., Podryabinkin E.V., Roche S., Rabczuk T., Shapeev A.V., Zhuang X. Applied Materials Today; 20 (100685) 2020. 10.1016/j.apmt.2020.100685. IF: 8.352
Phononic properties are commonly studied by calculating force constants using the density functional theory (DFT) simulations. Although DFT simulations offer accurate estimations of phonon dispersion relations or thermal properties, but for low-symmetry and nanoporous structures the computational cost quickly becomes very demanding. Moreover, the computational setups may yield nonphysical imaginary frequencies in the phonon dispersion curves, impeding the assessment of phononic properties and the dynamical stability of the considered system. Here, we compute phonon dispersion relations and examine the dynamical stability of a large ensemble of novel materials and compositions. We propose a fast and convenient alternative to DFT simulations which derived from machine-learning interatomic potentials passively trained over computationally efficient ab-initio molecular dynamics trajectories. Our results for diverse two-dimensional (2D) nanomaterials confirm that the proposed computational strategy can reproduce fundamental thermal properties in close agreement with those obtained via the DFT approach. The presented method offers a stable, efficient, and convenient solution for the examination of dynamical stability and exploring the phononic properties of low-symmetry and porous 2D materials. © 2020 Elsevier Ltd
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Impact of oxidation morphology on reduced graphene oxides upon thermal annealing
Aleandro Antidormi, Stephan Roche, Luciano Colombo Journal Of Physics-Materials; 3 (1, 15011) 2020. 10.1088/2515-7639/ab5ef2. IF: 0.000
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Machine-learning interatomic potentials enable first-principles multiscale modeling of lattice thermal conductivity in graphene/borophene heterostructures
Mortazavi B., Podryabinkin E.V., Roche S., Rabczuk T., Zhuang X., Shapeev A.V. Materials Horizons; 7 (9): 2359 - 2367. 2020. 10.1039/d0mh00787k. IF: 12.319
One of the ultimate goals of computational modeling in condensed matter is to be able to accurately compute materials properties with minimal empirical information. First-principles approaches such as density functional theory (DFT) provide the best possible accuracy on electronic properties but they are limited to systems up to a few hundreds, or at most thousands of atoms. On the other hand, classical molecular dynamics (CMD) simulations and the finite element method (FEM) are extensively employed to study larger and more realistic systems, but conversely depend on empirical information. Here, we show that machine-learning interatomic potentials (MLIPs) trained over short ab initio molecular dynamics trajectories enable first-principles multiscale modeling, in which DFT simulations can be hierarchically bridged to efficiently simulate macroscopic structures. As a case study, we analyze the lattice thermal conductivity of coplanar graphene/borophene heterostructures, recently synthesized experimentally (Sci. Adv., 2019, 5, eaax6444), for which no viable classical modeling alternative is presently available. Our MLIP-based approach can efficiently predict the lattice thermal conductivity of graphene and borophene pristine phases, the thermal conductance of complex graphene/borophene interfaces and subsequently enable the study of effective thermal transport along the heterostructures at continuum level. This work highlights that MLIPs can be effectively and conveniently employed to enable first-principles multiscale modeling via hierarchical employment of DFT/CMD/FEM simulations, thus expanding the capability for computational design of novel nanostructures. © 2020 The Royal Society of Chemistry.
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Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene
Karpiak B., Cummings A.W., Zollner K., Vila M., Khokhriakov D., Hoque A.M., Dankert A., Svedlindh P., Fabian J., Roche S., Dash S.P. 2D Materials; 7 (1, 015026) 2020. 10.1088/2053-1583/ab5915. IF: 7.140
Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices. © 2019 IOP Publishing Ltd.
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Magnetism, spin dynamics, and quantum transport in two-dimensional systems
Savero Torres W., Sierra J.F., Benítez L.A., Bonell F., García J.H., Roche S., Valenzuela S.O. MRS Bulletin; 45 (5): 357 - 365. 2020. 10.1557/mrs.2020.121. IF: 5.061
Two-dimensional (2D) quantum materials offer a unique platform to explore mesoscopic phenomena driven by interfacial and topological effects. Their tunable electric properties and bidimensional nature enable their integration into sophisticated heterostructures with engineered properties, resulting in the emergence of new exotic phenomena not accessible in other platforms. This has fostered many studies on 2D ferromagnetism, proximity-induced effects, and quantum transport, demonstrating their relevance for fundamental research and future device applications. Here, we review ongoing progress in this lively research field with special emphasis on spin-related phenomena. © Materials Research Society 2020.
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Nonlocal Spin Dynamics in the Crossover from Diffusive to Ballistic Transport
Vila M., Garcia J.H., Cummings A.W., Power S.R., Groth C.W., Waintal X., Roche S. Physical Review Letters; 124 (19, 196602) 2020. 10.1103/PhysRevLett.124.196602. IF: 8.385
Improved fabrication techniques have enabled the possibility of ballistic transport and unprecedented spin manipulation in ultraclean graphene devices. Spin transport in graphene is typically probed in a nonlocal spin valve and is analyzed using spin diffusion theory, but this theory is not necessarily applicable when charge transport becomes ballistic or when the spin diffusion length is exceptionally long. Here, we study these regimes by performing quantum simulations of graphene nonlocal spin valves. We find that conventional spin diffusion theory fails to capture the crossover to the ballistic regime as well as the limit of long spin diffusion length. We show that the latter can be described by an extension of the current theoretical framework. Finally, by covering the whole range of spin dynamics, our study opens a new perspective to predict and scrutinize spin transport in graphene and other two-dimensional material-based ultraclean devices. © 2020 American Physical Society.
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Optimization of the sensitivity of a double-dot magnetic detector
Macucci M., Marconcini P., Roche S. Electronics (Switzerland); 9 (7, 1134): 1 - 13. 2020. 10.3390/electronics9071134. IF: 2.412
We investigate, by means of numerical simulations, the lowest magnetic field level that can be detected with a given relative accuracy with a sensor based on a double-dot device fabricated in a high-mobility two-dimensional electron gas. The double dot consists of a cavity delimited by an input and an output constriction, with a potential barrier exactly in the middle. In conditions of perfect symmetry, a strong conductance enhancement effect appears as a consequence of the constructive interference between symmetric trajectories. When the symmetry is broken, for example by the presence of an applied magnetic field, this enhancement effect is suppressed. We explore the design parameter space and assess the minimum magnetic field value that can be measured with a given accuracy in the presence of flicker noise. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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The 2021 quantum materials roadmap
Giustino F., Lee J.H., Trier F., Bibes M., Winter S.M., Valentí R., Son Y.-W., Taillefer L., Heil C., Figueroa A.I., Plaçais B., Wu Q., Yazyev O.V., Bakkers E.P.A.M., Nygård J., Forn-Díaz P., de Franceschi S., McIver J.W., Foa Torres L.E.F., Low T., Kumar A., Galceran R., Valenzuela S.O., Costache M.V., Manchon A., Kim E.-A., Schleder G.R., Fazzio A., Roche S. JPhys Materials; 3 (4, 042006) 2020. 10.1088/2515-7639/abb74e. IF: 0.000
In recent years, the notion of ‘Quantum Materials’ has emerged as a powerful unifying concept across diverse fields of science and engineering, from condensed-matter and coldatom physics to materials science and quantum computing. Beyond traditional quantum materials such as unconventional superconductors, heavy fermions, and multiferroics, the field has significantly expanded to encompass topological quantum matter, two-dimensional materials and their van der Waals heterostructures, Moiré materials, Floquet time crystals, as well as materials and devices for quantum computation with Majorana fermions. In this Roadmap collection we aim to capture a snapshot of the most recent developments in the field, and to identify outstanding challenges and emerging opportunities. The format of the Roadmap, whereby experts in each discipline share their viewpoint and articulate their vision for quantum materials, reflects the dynamic and multifaceted nature of this research area, and is meant to encourage exchanges and discussions across traditional disciplinary boundaries. It is our hope that this collective vision will contribute to sparking new fascinating questions and activities at the intersection of materials science, condensed matter physics, device engineering, and quantum information, and to shaping a clearer landscape of quantum materials science as a new frontier of interdisciplinary scientific inquiry. We stress that this article is not meant to be a fully comprehensive review but rather an up-to-date snapshot of different areas of research on quantum materials with a minimal number of references focusing on the latest developments. © 2020 The Author(s). Published by IOP Publishing Ltd
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Tunable room-temperature spin galvanic and spin Hall effects in van der Waals heterostructures
Benítez L.A., Savero Torres W., Sierra J.F., Timmermans M., Garcia J.H., Roche S., Costache M.V., Valenzuela S.O. Nature Materials; 19 (2): 170 - 175. 2020. 10.1038/s41563-019-0575-1. IF: 38.663
Spin–orbit coupling stands as a powerful tool to interconvert charge and spin currents and to manipulate the magnetization of magnetic materials through spin-torque phenomena. However, despite the diversity of existing bulk materials and the recent advent of interfacial and low-dimensional effects, control of this interconversion at room temperature remains elusive. Here, we demonstrate strongly enhanced room-temperature spin-to-charge interconversion in graphene driven by the proximity of WS2. By performing spin precession experiments in appropriately designed Hall bars, we separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign, peaking near the charge neutrality point with an equivalent magnitude that is comparable to the largest efficiencies reported to date. Such electric-field tunability provides a building block for spin generation free from magnetic materials and for ultra-compact magnetic memory technologies. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
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Ultralow-dielectric-constant amorphous boron nitride
Hong S., Lee C.-S., Lee M.-H., Lee Y., Ma K.Y., Kim G., Yoon S.I., Ihm K., Kim K.-J., Shin T.J., Kim S.W., Jeon E.-C., Jeon H., Kim J.-Y., Lee H.-I., Lee Z., Antidormi A., Roche S., Chhowalla M., Shin H.-J., Shin H.S. Nature; 582 (7813): 511 - 514. 2020. 10.1038/s41586-020-2375-9. IF: 42.779
Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics1–3. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal–oxide–semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends4 the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties5. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
2019
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Green function, quasi-classical Langevin and Kubo-Greenwood methods in quantum thermal transport
Sevinçli H., Roche S., Cuniberti G., Brandbyge M., Gutierrez R., Sandonas L.M. Journal of Physics Condensed Matter; 31 (27, 273003) 2019. 10.1088/1361-648X/ab119a. IF: 2.711
With the advances in fabrication of materials with feature sizes at the order of nanometers, it has been possible to alter their thermal transport properties dramatically. Miniaturization of device size increases the power density in general, hence faster electronics require better thermal transport, whereas better thermoelectric applications require the opposite. Such diverse needs bring new challenges for material design. Shrinkage of length scales has also changed the experimental and theoretical methods to study thermal transport. Unsurprisingly, novel approaches have emerged to control phonon flow. Besides, ever increasing computational power is another driving force for developing new computational methods. In this review, we discuss three methods developed for computing vibrational thermal transport properties of nano-structured systems, namely Green function, quasi-classical Langevin, and Kubo-Green methods. The Green function methods are explained using both nonequilibrium expressions and the Landauer-type formula. The partitioning scheme, decimation techniques and surface Green functions are reviewed, and a simple model for reservoir Green functions is shown. The expressions for the Kubo-Greenwood method are derived, and Lanczos tridiagonalization, continued fraction and Chebyshev polynomial expansion methods are discussed. Additionally, the quasi-classical Langevin approach, which is useful for incorporating phonon-phonon and other scatterings is summarized. © 2019 IOP Publishing Ltd.
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Nonvolatile Memories Based on Graphene and Related 2D Materials
Bertolazzi S., Bondavalli P., Roche S., San T., Choi S.-Y., Colombo L., Bonaccorso F., Samorì P. Advanced Materials; 31 (10, 1806663) 2019. 10.1002/adma.201806663. IF: 25.809
The pervasiveness of information technologies is generating an impressive amount of data, which need to be accessed very quickly. Nonvolatile memories (NVMs) are making inroads into high-capacity storage to replace hard disk drives, fuelling the expansion of the global storage memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory cell layers, innovative device concepts, and novel materials are being investigated. In this context, emerging 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous, offer a host of physical and chemical properties, which could both improve existing memory technologies and enable the next generation of low-cost, flexible, and wearable storage devices. Herein, an overview of graphene and related 2D materials (GRMs) in different types of NVM cells is provided, including resistive random-access, flash, magnetic and phase-change memories. The physical and chemical mechanisms underlying the switching of GRM-based memory devices studied in the last decade are discussed. Although at this stage most of the proof-of-concept devices investigated do not compete with state-of-the-art devices, a number of promising technological advancements have emerged. Here, the most relevant material properties and device structures are analyzed, emphasizing opportunities and challenges toward the realization of practical NVM devices. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Room-Temperature Spin Hall Effect in Graphene/MoS 2 van der Waals Heterostructures
Safeer C.K., Ingla-Aynés J., Herling F., Garcia J.H., Vila M., Ontoso N., Calvo M.R., Roche S., Hueso L.E., Casanova F. Nano Letters; 2019. 10.1021/acs.nanolett.8b04368. IF: 12.279
Graphene is an excellent material for long-distance spin transport but allows little spin manipulation. Transition-metal dichalcogenides imprint their strong spin-orbit coupling into graphene via the proximity effect, and it has been predicted that efficient spin-to-charge conversion due to spin Hall and Rashba-Edelstein effects could be achieved. Here, by combining Hall probes with ferromagnetic electrodes, we unambiguously demonstrate experimentally the spin Hall effect in graphene induced by MoS 2 proximity and for varying temperatures up to room temperature. The fact that spin transport and the spin Hall effect occur in different parts of the same material gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Additionally, for a single graphene/MoS 2 heterostructure-based device, we evidence a superimposed spin-to-charge current conversion that can be indistinguishably associated with either the proximity-induced Rashba-Edelstein effect in graphene or the spin Hall effect in MoS 2 . By a comparison of our results to theoretical calculations, the latter scenario is found to be the most plausible one. Our findings pave the way toward the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications. © 2019 American Chemical Society.
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The phase diagram of 2D antiferromagnets
Valenzuela S.O., Roche S. Nature Nanotechnology; 14 (12): 1088 - 1089. 2019. 10.1038/s41565-019-0592-x. IF: 33.407
[No abstract available]
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Tunable circular dichroism and valley polarization in the modified Haldane model
Vila M., Hung N.T., Roche S., Saito R. Physical Review B; 99 (16, 161404) 2019. 10.1103/PhysRevB.99.161404. IF: 3.736
We study the polarization dependence of optical absorption for a modified Haldane model, which exhibits antichiral edge modes in the presence of sample boundaries and has been argued to be realizable in transition metal dichalcogenides or Weyl semimetals. A rich optical phase diagram is unveiled, in which the correlations between perfect circular dichroism, pseudospin andvalley polarization can be tuned independently upon varying the Fermi energy. In particular, perfect circular dichroism and valley polarization are achieved simultaneously. This combination of optical properties suggests some interesting photonic device functionality (e.g., light polarizer) which could be combined with valleytronics applications (e.g., generation of valley currents). © 2019 American Physical Society.
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Universal Spin Diffusion Length in Polycrystalline Graphene
Cummings A.W., Dubois S.M.-M., Charlier J.-C., Roche S. Nano Letters; 19 (10): 7418 - 7426. 2019. 10.1021/acs.nanolett.9b03112. IF: 12.279
Graphene grown by chemical vapor deposition (CVD) is the most promising material for industrial-scale applications based on graphene monolayers. It also holds promise for spintronics; despite being polycrystalline, spin transport in CVD graphene has been measured over lengths up to 30 μm, which is on par with the best measurements made in single-crystal graphene. These results suggest that grain boundaries (GBs) in CVD graphene, while impeding charge transport, may have little effect on spin transport. However, to date very little is known about the true impact of disordered networks of GBs on spin relaxation. Here, by using first-principles simulations, we derive an effective tight-binding model of graphene GBs in the presence of spin-orbit coupling (SOC), which we then use to evaluate spin transport in realistic morphologies of polycrystalline graphene. The spin diffusion length is found to be independent of the grain size, and it is determined only by the strength of the substrate-induced SOC. This result is consistent with the D'yakonov-Perel' mechanism of spin relaxation in the diffusive regime, but we find that it also holds in the presence of quantum interference. These results clarify the role played by GBs and demonstrate that the average grain size does not dictate the upper limit for spin transport in CVD-grown graphene, a result of fundamental importance for optimizing large-scale graphene-based spintronic devices. Copyright © 2019 American Chemical Society.
2018
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1D ferromagnetic edge contacts to 2D graphene/h-BN heterostructures
Karpiak B., Dankert A., Cummings A.W., Power S.R., Roche S., Dash S.P. 2D Materials; 5 (1, 014001) 2018. 10.1088/2053-1583/aa8d2b. IF: 7.042
We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a change in sign of the signal for the electron- and hole-doped regimes, which is in contrast to the expected behavior of the spin signal. Calculations show that the contact-induced fringe fields are typically on the order of hundreds of mT, but can be reduced below 100 mT with careful optimization of the contact geometry. There may be an additional contribution from magnetoresistance effects due to tunneling anisotropy in the contacts, which needs further investigation. These studies are useful for optimization of spin injection and detection in 2D material heterostructures through 1D edge contacts. © 2017 IOP Publishing Ltd.
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A barrier to spin filters
Valenzuela S.O., Roche S. Nature Electronics; 1 (6): 328 - 329. 2018. 10.1038/s41928-018-0089-x.
Electron tunnelling through a two-dimensional magnetic insulator is assisted by magnon inelastic processes that provide spin-filtering. © 2018 The Author(s).
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Ballistic tracks in graphene nanoribbons
Aprojanz J., Power S.R., Bampoulis P., Roche S., Jauho A.-P., Zandvliet H.J.W., Zakharov A.A., Tegenkamp C. Nature Communications; 9 (1, 4426) 2018. 10.1038/s41467-018-06940-5. IF: 12.353
High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts. © 2018, The Author(s).
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Charge and spin transport anisotropy in nanopatterned graphene
Gregersen, Soren Schou; Garcia, Jose H.; Jauho, Antti-Pekka; Roche, Stephan; Power, Stephen R.; Journal Of Physics-Materials; 1 (1) 2018. 10.1088/2515-7639/aadca3.
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Deciphering the origin of nonlocal resistance in multiterminalgraphene on hexagonal-boron-nitride withab initioquantumtransport: Fermi surface edge currents rather than Fermi seatopological valley currents
J M Marmolejo-Tejada, J H García, M D Petrovic, P-H Chang, X-L Sheng, A Cresti, P Plechác, S Roche and B K Nikolic Journal of Physics: Materials; 1 (1) 2018. 10.1088/2515-7639/aad585.
The recent observation (Gorbachev et al 2014 Science 346 448) of nonlocal resistance R NL near the Dirac point (DP) of multiterminal graphene on aligned hexagonal-boron nitride (G/hBN) has been interpreted as the consequence of topological valley Hall currents carried by the Fermi sea states just beneath the bulk gap E g induced by inversion symmetry breaking. However, the corresponding valley Hall conductivity ${\sigma }_{{xy}}^{v}$, quantized inside E g , is not directly measurable. Conversely, the Landauer–Büttiker formula, as a numerically exact approach to observable nonlocal transport quantities, yields R NL ≡ 0 for the same simplistic Hamiltonian of gapped graphene that generates ${\sigma }_{{xy}}^{v}\ne 0$ via the Kubo formula. We combine ab initio with quantum transport calculations to demonstrate that G/hBN wires with zigzag edges host dispersive edge states near the DP that are absent in theories based on the simplistic Hamiltonian. Although such edge states exist also in isolated zigzag graphene wires, aligned hBN is required to modify their energy–momentum dispersion and generate ${R}_{\mathrm{NL}}\ne 0$ near the DP. The Fermi surface-determined edge currents carrying the nonlocal signal persist also in the presence of edge disorder and over long distances. Concurrently, they resolve the long-standing puzzle of why the highly insulating state of G/hBN is rarely observed. Thus, we conclude that the observed R NL is unrelated to Fermi sea topological valley currents conjectured for gapped Dirac spectra.
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Effect of the channel length on the transport characteristics of transistors based on boron-doped graphene ribbons
Marconcini P., Cresti A., Roche S. Materials; 11 (5, 667) 2018. 10.3390/ma11050667. IF: 2.467
Substitutional boron doping of devices based on graphene ribbons gives rise to a unipolar behavior, a mobility gap, and an increase of the ION/IOFF ratio of the transistor. Here we study how this effect depends on the length of the doped channel. By means of self-consistent simulations based on a tight-binding description and a non-equilibrium Green's function approach, we demonstrate a promising increase of the ION/IOFF ratio with the length of the channel, as a consequence of the different transport regimes in the ON and OFF states. Therefore, the adoption of doped ribbons with longer aspect ratios could represent a significant step toward graphene-based transistors with an improved switching behavior. © 2018 by the authors.
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Sensing ion channel in neuron networks with graphene field effect transistors
Veliev F., Cresti A., Kalita D., Bourrier A., Belloir T., Briançon-Marjollet A., Albrieux M., Roche S., Bouchiat V., Delacour C. 2D Materials; 5 (4, 045020) 2018. 10.1088/2053-1583/aad78f. IF: 7.042
Graphene, the atomically-thin honeycomb carbon lattice, is a highly conducting 2D material whose exposed electronic structure offers an ideal platform for chemical and biological sensing. Its biocompatible, flexible and chemically inert nature associated with the lack of dangling bonds, offers novel perspectives for direct interfacing with biological molecules. Combined with its exceptional electronic and optical properties, this promotes graphene as a unique platform for bioelectronics. Among the successful bio-integrations of graphene, the detection of action potentials in numerous electrogenic cells including neurons has paved the road for the high spatio-temporal and wide-field mapping of neuronal activity. Ultimate resolution of sensing ion channel activity can be achieved with neural interfaces, and it was shown that macroscale electrodes can record extracellular current of individual ion channels in model systems, by charging the quantum capacitance of large graphene monolayer (mm2). Here, we show the field effect detection of ion channel activity within neuron networks, cultured during several weeks above graphene transistor arrays. Dependences upon drugs, reference potential gating and device geometry confirm the field effect detection of individual ion channel and suggest a significant contribution of grain boundaries, which provide highly sensitive nanoscale-sized sensing sites. Our theoretical analysis and simulations demonstrate that the ion gating of a single grain boundary in liquid affects the electronic transmission of the whole transistor channel, resulting in significant conductance variations. Monitoring the ion channels activity is of great interest as most of neurodegenerative diseases relied on channelopathies, which rely on ion channel abnormal activity. Thus, such highly sensitive and biocompatible neuro-electronics which open the way to FET detection at the sub-cell precision should be useful for a wide range of fundamental and applied research areas, including brain-on-chip, pharmacology, and in vivo monitoring or diagnosis. © 2018 IOP Publishing Ltd.
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Spin Proximity Effects in Graphene/Topological Insulator Heterostructures
Song K., Soriano D., Cummings A.W., Robles R., Ordejón P., Roche S. Nano Letters; 18 (3): 2033 - 2039. 2018. 10.1021/acs.nanolett.7b05482. IF: 12.080
Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. Here, we use ab initio simulations and tight-binding models to determine the precise spin texture of electronic states in graphene interfaced with a Bi2Se3 topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices. © 2018 American Chemical Society.
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Spin transport in graphene/transition metal dichalcogenide heterostructures
Garcia J.H., Vila M., Cummings A.W., Roche S. Chemical Society Reviews; 47 (9): 3359 - 3379. 2018. 10.1039/c7cs00864c. IF: 40.182
Since its discovery, graphene has been a promising material for spintronics: its low spin-orbit coupling, negligible hyperfine interaction, and high electron mobility are obvious advantages for transporting spin information over long distances. However, such outstanding transport properties also limit the capability to engineer active spintronics, where strong spin-orbit coupling is crucial for creating and manipulating spin currents. To this end, transition metal dichalcogenides, which have larger spin-orbit coupling and good interface matching, appear to be highly complementary materials for enhancing the spin-dependent features of graphene while maintaining its superior charge transport properties. In this review, we present the theoretical framework and the experiments performed to detect and characterize the spin-orbit coupling and spin currents in graphene/transition metal dichalcogenide heterostructures. Specifically, we will concentrate on recent measurements of Hanle precession, weak antilocalization and the spin Hall effect, and provide a comprehensive theoretical description of the interconnection between these phenomena. © 2018 The Royal Society of Chemistry.
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Tailoring emergent spin phenomena in Dirac material heterostructures
Khokhriakov D., Cummings A.W., Song K., Vila M., Karpiak B., Dankert A., Roche S., Dash S.P. Science Advances; 4 (9, aat9349) 2018. 10.1126/sciadv.aat9349. IF: 11.511
Dirac materials such as graphene and topological insulators (TIs) are known to have unique electronic and spintronic properties. We combine graphene with TIs in van der Waals heterostructures to demonstrate the emergence of a strong proximity-induced spin-orbit coupling in graphene. By performing spin transport and precession measurements supported by ab initio simulations, we discover a strong tunability and suppression of the spin signal and spin lifetime due to the hybridization of graphene and TI electronic bands. The enhanced spin-orbit coupling strength is estimated to be nearly an order of magnitude higher than in pristine graphene. These findings in graphene-TI heterostructures could open interesting opportunities for exploring exotic physical phenomena and new device functionalities governed by topological proximity effects. Copyright © 2018 The Authors.
2017
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Electrical and Thermal Transport in Coplanar Polycrystalline Graphene-hBN Heterostructures
Barrios-Vargas J.E., Mortazavi B., Cummings A.W., Martinez-Gordillo R., Pruneda M., Colombo L., Rabczuk T., Roche S. Nano Letters; 17 (3): 1660 - 1664. 2017. 10.1021/acs.nanolett.6b04936. IF: 12.712
We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm-1 K-1. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials. © 2017 American Chemical Society.
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Giant Spin Lifetime Anisotropy in Graphene Induced by Proximity Effects
Cummings A.W., Garcia J.H., Fabian J., Roche S. Physical Review Letters; 119 (20, 206601) 2017. 10.1103/PhysRevLett.119.206601. IF: 8.462
We report on fundamental aspects of spin dynamics in heterostructures of graphene and transition metal dichalcogenides (TMDCs). By using realistic models derived from first principles we compute the spin lifetime anisotropy, defined as the ratio of lifetimes for spins pointing out of the graphene plane to those pointing in the plane. We find that the anisotropy can reach values of tens to hundreds, which is unprecedented for typical 2D systems with spin-orbit coupling and indicates a qualitatively new regime of spin relaxation. This behavior is mediated by spin-valley locking, which is strongly imprinted onto graphene by TMDCs. Our results indicate that this giant spin lifetime anisotropy can serve as an experimental signature of materials with strong spin-valley locking, including graphene-TMDC heterostructures and TMDCs themselves. Additionally, materials with giant spin lifetime anisotropy can provide an exciting platform for manipulating the valley and spin degrees of freedom, and for designing novel spintronic devices. © 2017 American Physical Society.
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Grain boundary-induced variability of charge transport in hydrogenated polycrystalline graphene
Vargas J.E.B., Falkenberg J.T., Soriano D., Cummings A.W., Brandbyge M., Roche S. 2D Materials; 4 (2, 025009) 2017. 10.1088/2053-1583/aa59de. IF: 6.937
Chemical functionalization has proven to be a promising means of tailoring the unique properties of graphene. For example, hydrogenation can yield a variety of interesting effects, including a metal-insulator transition or the formation of localized magnetic moments. Meanwhile, graphene grown by chemical vapor deposition is the most suitable for large-scale production, but the resulting material tends to be polycrystalline. Up to now there has been relatively little focus on how chemical functionalization, and hydrogenation in particular, impacts the properties of polycrystalline graphene. In this work, we use numerical simulations to study the electrical properties of hydrogenated polycrystalline graphene. We find a strong correlation between the spatial distribution of the hydrogen adsorbates and the charge transport properties. Charge transport is weakly sensitive to hydrogenation when adsorbates are confined to the grain boundaries, while a uniform distribution of hydrogen degrades the electronic mobility. This difference stems from the formation of the hydrogen-induced resonant impurity states, which are inhibited when the honeycomb symmetry is locally broken by the grain boundaries. These findings suggest a tunability of electrical transport of polycrystalline graphene through selective hydrogen functionalization, and also have implications for hydrogen-induced magnetization and spin lifetime of this material. © 2017 IOP Publishing Ltd.
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Graphene spintronics
Cummings A.W., Valenzuela S.O., Ortmann F., Roche S. 2D Materials: Properties and Devices; : 197 - 218. 2017. 10.1017/9781316681619.012.
Charge and spin are two fundamental properties of the electron which are currently exploited in advanced technologies, but to date they have been used separately in information processing and data storage, respectively. Charge currents drive the operation of elementary electronic devices and logic circuits that encode and process binary or analogue information. Meanwhile, the spin degree of freedom is used in its collective form of magnetic domains for switching magneto resistance signals and realizing long-term data storage, from ferrite core memories to modern hard disk drives [1]. The field of spintronics aims to combine the charge and spin of electrons to create novel functionalities [2]. In the simplest spintronic device, called a spin valve, an electronic current flows between two ferromagnetic electrodes through a non-magnetic channel. A spin signal is carried along with the charge current and is normally detected through its magneto resistance [3], which is influenced by the magnetic ordering of the electrodes. The requirement for non-magnetic channels is to transport spin currents with minimum spin information loss due to spin-scattering events, which in most cases are caused by spin-orbit coupling. The field of spintronics emerged from scientific discoveries in the 1980s, which concerned spin-dependent electron transport phenomena in solid-state devices. Following the observation in 1985 by Johnson and Silsbee [4] of spin-polarized electron injection from a ferromagnetic metal to a normal metal, the foundational step of the field of spintronics was the discovery, by Albert Fert et al. [5] and Peter Grünberg et al. [6], of giant magneto resistance in thin film structures composed of alternating ferromagnetic and non-magnetic conductive layers. Control of magneto resistance has required the use of various magnetic and non-magnetic metallic and semiconducting materials, and has resulted in a massive technological impact on magnetic field sensors, which today are used in hard disk drives, biosensors, microelectromechanical systems (MEMS), and magneto resistive random-access memory (MRAM) [7]. Following the discovery of the spin transfer torque effect, which permits the control of the magnetization with an electrical current, a second revolution in spintronics is currently underway [8]. © Materials Research Society 2017.
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Growth of Twin-Free and Low-Doped Topological Insulators on BaF2(111)
Bonell F., Cuxart M.G., Song K., Robles R., Ordejón P., Roche S., Mugarza A., Valenzuela S.O. Crystal Growth and Design; 17 (9): 4655 - 4660. 2017. 10.1021/acs.cgd.7b00525. IF: 4.055
We demonstrate the growth of twin-free Bi2Te3 and Sb2Te3 topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi2Se3 on lattice-matched BaF2(111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi2Se3 films are achieved by increasing the substrate roughness, the quality of our Bi2Te3 is superior on the flattest BaF2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab initio calculations. © 2017 American Chemical Society.
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Large edge magnetism in oxidized few-layer black phosphorus nanomeshes
Nakanishi Y., Ishi A., Ohata C., Soriano D., Iwaki R., Nomura K., Hasegawa M., Nakamura T., Katsumoto S., Roche S., Haruyama J. Nano Research; 10 (2): 718 - 728. 2017. 10.1007/s12274-016-1355-8. IF: 7.354
The formation and control of a room-temperature magnetic order in two-dimensional (2D) materials is a challenging quest for the advent of innovative magnetic- and spintronic-based technologies. To date, edge magnetism in 2D materials has been experimentally observed in hydrogen (H)-terminated graphene nanoribbons (GNRs) and graphene nanomeshes (GNMs), but the measured magnetization remains far too small to allow envisioning practical applications. Herein, we report experimental evidences of large room-temperature edge ferromagnetism (FM) obtained from oxygen (O)-terminated zigzag pore edges of few-layer black phosphorus (P) nanomeshes (BPNMs). The magnetization values per unit area are ~100 times larger than those reported for H-terminated GNMs, while the magnetism is absent for H-terminated BPNMs. The magnetization measurements and the first-principles simulations suggest that the origin of such a magnetic order could stem from ferromagnetic spin coupling between edge P with O atoms, resulting in a strong spin localization at the edge valence band, and from uniform oxidation of full pore edges over a large area and interlayer spin interaction. Our findings pave the way for realizing high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements. [Figure not available: see fulltext.] © 2017, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
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Record Low Thermal Conductivity of Polycrystalline MoS2 Films: Tuning the Thermal Conductivity by Grain Orientation
Sledzinska M., Quey R., Mortazavi B., Graczykowski B., Placidi M., Saleta Reig D., Navarro-Urrios D., Alzina F., Colombo L., Roche S., Sotomayor Torres C.M. ACS Applied Materials and Interfaces; 9 (43): 37905 - 37911. 2017. 10.1021/acsami.7b08811. IF: 7.504
We report a record low thermal conductivity in polycrystalline MoS2 obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS2 films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m-1 K-1, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity. © 2017 American Chemical Society.
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Scale-invariant large nonlocality in polycrystalline graphene
Ribeiro M., Power S.R., Roche S., Hueso L.E., Casanova F. Nature Communications; 8 (1, 2198) 2017. 10.1038/s41467-017-02346-x. IF: 12.124
The observation of large nonlocal resistances near the Dirac point in graphene has been related to a variety of intrinsic Hall effects, where the spin or valley degrees of freedom are controlled by symmetry breaking mechanisms. Engineering strong spin or valley Hall signals on scalable graphene devices could stimulate further practical developments of spin- and valleytronics. Here we report on scale-invariant nonlocal transport in large-scale chemical vapor deposition graphene under an applied external magnetic field. Contrary to previously reported Zeeman spin Hall effect, our results are explained by field-induced spin-filtered edge states whose sensitivity to grain boundaries manifests in the nonlocal resistance. This phenomenon, related to the emergence of the quantum Hall regime, persists up to the millimeter scale, showing that polycrystalline morphology can be imprinted in nonlocal transport. This suggests that topological Hall effects in large-scale graphene materials are highly sensitive to the underlying structural morphology, limiting practical realizations. © 2017 The Author(s).
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Scaling properties of polycrystalline graphene: A review
Isacsson A., Cummings A.W., Colombo L., Colombo L., Kinaret J.M., Roche S. 2D Materials; 4 (1, 012002) 2017. 10.1088/2053-1583/aa5147. IF: 6.937
We present an overview of the electrical, mechanical, and thermal properties of polycrystalline graphene. Most global properties of this material, such as the charge mobility, thermal conductivity, or Young's modulus, are sensitive to its microstructure, for instance the grain size and the presence of line or point defects. Both the local and global features of polycrystalline graphene have been investigated by a variety of simulations and experimental measurements. In this review, we summarize the properties of polycrystalline graphene, and by establishing a perspective on how the microstructure impacts its large-scale physical properties, we aim to provide guidance for further optimization and improvement of applications based on this material, such as flexible and wearable electronics, and high-frequency or spintronic devices. © 2016 IOP Publishing Ltd.
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Spin hall effect and weak antilocalization in graphene/transition metal dichalcogenide heterostructures
Garcia J.H., Cummings A.W., Roche S. Nano Letters; 17 (8): 5078 - 5083. 2017. 10.1021/acs.nanolett.7b02364. IF: 12.712
We report on a theoretical study of the spin Hall Effect (SHE) and weak antilocalization (WAL) in graphene/transition metal dichalcogenide (TMDC) heterostructures, computed through efficient real-space quantum transport methods, and using realistic tight-binding models parametrized from ab initio calculations. The graphene/WS2 system is found to maximize spin proximity effects compared to graphene on MoS2, WSe2, or MoSe2 with a crucial role played by disorder, given the disappearance of SHE signals in the presence of strong intervalley scattering. Notably, we found that stronger WAL effects are concomitant with weaker charge-to-spin conversion efficiency. For further experimental studies of graphene/TMDC heterostructures, our findings provide guidelines for reaching the upper limit of spin current formation and for fully harvesting the potential of two-dimensional materials for spintronic applications. © 2017 American Chemical Society.
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Spin precession in anisotropic media
Raes B., Cummings A.W., Bonell F., Costache M.V., Sierra J.F., Roche S., Valenzuela S.O. Physical Review B; 95 (8, 085403) 2017. 10.1103/PhysRevB.95.085403. IF: 3.836
We generalize the diffusive model for spin injection and detection in nonlocal spin structures to account for spin precession under an applied magnetic field in an anisotropic medium, for which the spin lifetime is not unique and depends on the spin orientation. We demonstrate that the spin precession (Hanle) line shape is strongly dependent on the degree of anisotropy and on the orientation of the magnetic field. In particular, we show that the anisotropy of the spin lifetime can be extracted from the measured spin signal, after dephasing in an oblique magnetic field, by using an analytical formula with a single fitting parameter. Alternatively, after identifying the fingerprints associated with the anisotropy, we propose a simple scaling of the Hanle line shapes at specific magnetic field orientations that results in a universal curve only in the isotropic case. The deviation from the universal curve can be used as a complementary means of quantifying the anisotropy by direct comparison with the solution of our generalized model. Finally, we applied our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution. © 2017 American Physical Society.
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Tailoring magnetic insulator proximity effects in graphene: First-principles calculations
Hallal A., Ibrahim F., Yang H., Roche S., Chshiev M. 2D Materials; 4 (2, 025074) 2017. 10.1088/2053-1583/aa6663. IF: 6.937
We report a systematic first-principles investigation of the influence of different magnetic insulators on the magnetic proximity effect induced in graphene. Four different magnetic insulators are considered: two ferromagnetic europium chalcogenides namely EuO and EuS and two ferrimagnetic insulators yttrium iron garnet (YIG) and cobalt ferrite (CFO). The obtained exchange-splitting in graphene varies from tens to hundreds of meV depending on substrates. We find an electron doping to graphene induced by YIG and europium chalcogenides substrates, that shift the Fermi level above the Dirac cone up to 0.78 eV and 1.3 eV respectively, whereas hole doping shifts the Fermi level down below the Dirac cone about 0.5 eV in graphene/CFO. Furthermore, we study the variation of the extracted exchange and tight-binding parameters as a function of the EuO and EuS thicknesses. We show that those parameters are robust to thickness variation such that a single monolayer of magnetic insulator can induce a strong magnetic proximity effect on graphene. Those findings pave the way towards possible engineering of graphene spin-gating by proximity effect especially in view of recent experimental advancements. © 2017 IOP Publishing Ltd.
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Valley-polarized quantum transport generated by gauge fields in graphene
Settnes M., Garcia J.H., Roche S. 2D Materials; 4 (3, 031006) 2017. 10.1088/2053-1583/aa7cbd. IF: 6.937
We report on the possibility to simultaneously generate in graphene a bulk valley-polarized dissipative transport and a quantum valley Hall effect by combining strain-induced gauge fields and real magnetic fields. Such unique phenomenon results from a ‘resonance/anti-resonance’ effect driven by the superposition/cancellation of superimposed gauge fields which differently affect time reversal symmetry. The onset of a valley-polarized Hall current concomitant to a dissipative valley-polarized current flow in the opposite valley is revealed by a e2 /h Hall conductivity plateau. We employ efficient linear scaling Kubo transport methods combined with a valley projection scheme to access valley-dependent conductivities and show that the results are robust against disorder.
2016
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Anomalous ballistic transport in disordered bilayer graphene: A Dirac semimetal induced by dimer vacancies
Van Tuan D., Roche S. Physical Review B; 93 (4, 041403) 2016. 10.1103/PhysRevB.93.041403.
We report anomalous quantum transport features in bilayer graphene in the presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus nondimer vacancies is investigated in very large scale disordered models. While nondimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer band structure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a realization of Dirac semimetals in high dimension. © 2016 American Physical Society.
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Charge, spin and valley Hall effects in disordered grapheme
Cresti A., Nikolíc B.K., Garćia J.H., Roche S. Rivista del Nuovo Cimento; 39 (12): 587 - 667. 2016. 10.1393/ncr/i2016-10130-6. IF: 1.250
The discovery of the integer quantum Hall effect in the early eighties of the last century, with highly precise quantization values for the Hall conductance in multiples of e2/h, has been the first fascinating manifestation of the topological state of matter driven by magnetic field and disorder, and related to the formation of non-dissipative current flow. Throughout the 2000's, several new phenomena such as the spin Hall effect and the quantum spin Hall effect were confirmed experimentally for systems with strong spin-orbit coupling effects and in the absence of external magnetic field. More recently, the Zeeman spin Hall effect and the formation of valley Hall topological currents have been introduced for graphene-based systems, under time-reversal or inversion symmetry-breaking conditions, respectively. This review presents a comprehensive coverage of all these Hall effects in disordered graphene from the perspective of numerical simulations of quantum transport in two-dimensional bulk systems (by means of the Kubo formalism) and multiterminal nanostructures (by means of the Landauer-Buttiker scattering and non-equilibrium Green's function approaches). In contrast to usual two-dimensional electron gases in semiconductor heterostructures, the presence of defects in graphene generates more complex electronic features such as electron-hole asymmetry, defect-induced resonances in the electron density of states or percolation effect between localized impurity states, which, together with extra degrees of freedom (sublattice pseudospin and valley isospin), bring a higher degree of complexity and enlarge the transport phase diagram.
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Effects of Dephasing on Spin Lifetime in Ballistic Spin-Orbit Materials
Cummings A.W., Roche S. Physical Review Letters; 116 (8, 086602) 2016. 10.1103/PhysRevLett.116.086602. IF: 7.645
We theoretically investigate spin dynamics in spin-orbit-coupled materials. In the ballistic limit, the spin lifetime is dictated by dephasing that arises from energy broadening plus a nonuniform spin precession. For the case of clean graphene, we find a strong anisotropy with spin lifetimes that can be short even for modest energy scales, on the order of a few ns. These results offer deeper insight into the nature of spin dynamics in graphene, and are also applicable to the investigation of other systems where spin-orbit coupling plays an important role. © 2016 American Physical Society.
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Gate-tunable atomically thin lateral MoS2 Schottky junction patterned by electron beam
Katagiri Y., Nakamura T., Ishii A., Ohata C., Hasegawa M., Katsumoto S., Cusati T., Fortunelli A., Iannaccone G., Fiori G., Roche S., Haruyama J. Nano Letters; 16 (6): 3788 - 3794. 2016. 10.1021/acs.nanolett.6b01186. IF: 13.779
Among atomically thin two-dimensional (2D) materials, molybdenum disulfide (MoS2) is attracting considerable attention because of its direct bandgap in the 2H-semiconducting phase. On the other hand, a 1T-metallic phase has been revealed, bringing complementary application. Recently, thanks to top-down fabrication using electron beam (EB) irradiation techniques, in-plane 1T-metal/2H-semiconductor lateral (Schottky) MoS2 junctions were demonstrated, opening a path toward the co-integration of active and passive two-dimensional devices. Here, we report the first transport measurements evidencing the formation of a MoS2 Schottky barrier (SB) junction with barrier height of 0.13-0.18 eV created at the interface between EB-irradiated (1T)/nonirradiated (2H) regions. Our experimental findings, supported by state-of-the-art simulation, reveal unique device fingerprint of SB-based field-effect transistors made from atom-thin 1T layers. © 2016 American Chemical Society.
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How disorder affects topological surface states in the limit of ultrathin Bi2Se3 films
Song K., Soriano D., Robles R., Ordejon P., Roche S. 2D Materials; 3 (4, 045007) 2016. 10.1088/2053-1583/3/4/045007. IF: 9.611
We present a first-principles study of electronic properties of ultrathin films of topological insulators (TIs) and scrutinize the role of disorder on the robustness of topological surface states, which can be analysed through their spin textures. The presence of twin grain boundaries is found to increase the band gap of the film, while preserving the spin texture of states in first conduction and valence bands. Differently, partial hydrogenation of one surface not only results in some self-doping effect, but also provokes some alteration of the spin texture symmetry of the electronic states. The formation of a new Dirac cone at M-point of the Brillouin zone of the hydrogenated surface, together with a modified spin texture characteristics are consistent with a dominant Dresselhaus spin-orbit interaction type, more usually observed in 3D materials. Our findings indicate that defects can either be detrimental or beneficial for exploring spin transport of surface states in the limit of ultrathin films of TIs, which maximizes surface over bulk phenomena. © 2016 IOP Publishing Ltd.
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Introduction to Quantum Transport
Triozon F., Roche S., Niquet Y.-M. Simulation of Transport in Nanodevices; : 163 - 222. 2016. 10.1002/9781118761793.ch5.
This chapter presents a detailed study of wavepacket propagation in various physical situations. This approach provides an intuitive understanding of quantum transport and its semiclassical limit. The chapter addresses the transmission formalism, which treats transport through a small conductor connected to electrodes. It introduces the Landauer-Buttiker formulation of transport, based on the quantum transmission of wavepackets through a conductor connected to electrodes. This formalism is very well suited to the study of electron transport through a small system connected to macroscopic electrodes. The chapter presents a step-by-step introduction to the Green's function method, which allows calculating the quantum transmission efficiently. It highlights the link between the Green's function and wavepacket propagation. Stationary states built from the Green's functions can be viewed as wavepackets with a spatial extension tending to infinity. Simulating transient regimes and noise is of great importance for technological applications. © ISTE Ltd 2016. All rights reserved.
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Localized electronic states at grain boundaries on the surface of graphene and graphite
Luican-Mayer A., Barrios-Vargas J.E., Falkenberg J.T., Autès G., Cummings A.W., Soriano D., Li G., Brandbyge M., Yazyev O.V., Roche S., Yandrei E. 2D Materials; 3 (3, 031005) 2016. 10.1088/2053-1583/3/3/031005. IF: 9.611
Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors.Wehere report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy"Q and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure.Weshow that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance. © 2016 IOP Publishing Ltd.
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Near-field photocurrent nanoscopy on bare and encapsulated graphene
Woessner A., Alonso-González P., Lundeberg M.B., Gao Y., Barrios-Vargas J.E., Navickaite G., Ma Q., Janner D., Watanabe K., Cummings A.W., Taniguchi T., Pruneri V., Roche S., Jarillo-Herrero P., Hone J., Hillenbrand R., Koppens F.H.L. Nature Communications; 7 ( 10783) 2016. 10.1038/ncomms10783. IF: 11.329
Optoelectronic devices utilizing graphene have demonstrated unique capabilities and performances beyond state-of-the-art technologies. However, requirements in terms of device quality and uniformity are demanding. A major roadblock towards high-performance devices are nanoscale variations of the graphene device properties, impacting their macroscopic behaviour. Here we present and apply non-invasive optoelectronic nanoscopy to measure the optical and electronic properties of graphene devices locally. This is achieved by combining scanning near-field infrared nanoscopy with electrical read-out, allowing infrared photocurrent mapping at length scales of tens of nanometres. Using this technique, we study the impact of edges and grain boundaries on the spatial carrier density profiles and local thermoelectric properties. Moreover, we show that the technique can readily be applied to encapsulated graphene devices. We observe charge build-up near the edges and demonstrate a solution to this issue.
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On the possibility of observing tunable laser-induced bandgaps in graphene
Calvo H.L., Pastawski H.M., Roche S., Foa Torres L.E.F. Graphene, Carbon Nanotubes, and Nanostructures: Techniques and Applications; : 41 - 59. 2016. .
[No abstract available]
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Quantum transport in graphene in presence of strain-induced pseudo-Landau levels
Settnes M., Leconte N., Barrios-Vargas J.E., Jauho A.-P., Roche S. 2D Materials; 3 (3, 034005) 2016. 10.1088/2053-1583/3/3/034005. IF: 9.611
Wereport on mesoscopic transport fingerprints in disordered graphene caused by strain-field induced pseudomagnetic Landau levels (pLLs). Efficient numerical real space calculations of the Kubo formula are performed for an ordered network of nanobubbles in graphene, creating pseudomagnetic fields up to several hundreds of Tesla, values inaccessible by real magnetic fields. Strain-induced pLLs yield enhanced scattering effects across the energy spectrum resulting in lower mean free path and enhanced localization effects. In the vicinity of the zeroth order pLL, we demonstrate an anomalous transport regime, where the mean free paths increases with disorder.Weattribute this puzzling behavior to the low-energy sub-lattice polarization induced by the zeroth order pLL, which is unique to pseudomagnetic fields preserving time-reversal symmetry. These results, combined with the experimental feasibility of reversible deformation fields, open the way to tailor a metal-insulator transition driven by pseudomagnetic fields. © 2016 IOP Publishing Ltd.
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Spin dynamics and relaxation in graphene dictated by electron-hole puddles
Van Tuan D., Ortmann F., Cummings A.W., Soriano D., Roche S. Scientific Reports; 6 ( 21046) 2016. 10.1038/srep21046. IF: 5.228
The understanding of spin dynamics and relaxation mechanisms in clean graphene, and the upper time and length scales on which spin devices can operate, are prerequisites to realizing graphene-based spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with Rashba spin-orbit fields as low as a few tens of μeV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO2 substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in the presence of electron-hole puddles. © 2016, Nature Publishing Group. All rights reserved.
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Spin dynamics in bilayer graphene: Role of electron-hole puddles and Dyakonov-Perel mechanism
Van Tuan D., Adam S., Roche S. Physical Review B; 94 (4, 041405) 2016. 10.1103/PhysRevB.94.041405.
We report on spin transport features which are unique to high quality bilayer graphene, in the absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of a propagating wave packet is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy dependence of the spin lifetime is similar to monolayer graphene with an M-shaped profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hole puddles (characteristics of supported graphene either on SiO2 or hBN) surprisingly results in a large enhancement of the low-energy spin lifetime and a lowering of its high-energy values. Such a feature, unique to the bilayer, is explained in terms of a reinforced Dyakonov-Perel mechanism at the Dirac point, whereas spin relaxation at higher energies is driven by pure dephasing effects. This suggests further electrostatic control of the spin transport length scales in graphene devices. © 2016 American Physical Society.
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Spin Hall Effect and Origins of Nonlocal Resistance in Adatom-Decorated Graphene
Van Tuan D., Marmolejo-Tejada J.M., Waintal X., Nikolić B.K., Valenzuela S.O., Roche S. Physical Review Letters; 117 (17, 176602) 2016. 10.1103/PhysRevLett.117.176602. IF: 7.645
Recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms have raised a fierce controversy. We apply numerically exact Kubo and Landauer-Büttiker formulas to realistic models of gold-decorated disordered graphene (including adatom clustering) to obtain the spin Hall conductivity and spin Hall angle, as well as the nonlocal resistance as a quantity accessible to experiments. Large spin Hall angles of ∼0.1 are obtained at zero temperature, but their dependence on adatom clustering differs from the predictions of semiclassical transport theories. Furthermore, we find multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, as well as a strong suppression of the SHE at room temperature. This motivates us to design a multiterminal graphene geometry which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials. © 2016 American Physical Society.
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Spin Manipulation in Graphene by Chemically Induced Pseudospin Polarization
Van Tuan D., Roche S. Physical Review Letters; 116 (10, 106601) 2016. 10.1103/PhysRevLett.116.106601. IF: 7.645
Spin manipulation is one of the most critical challenges to realize spin-based logic devices and spintronic circuits. Graphene has been heralded as an ideal material to achieve spin manipulation, but so far new paradigms and demonstrators are limited. Here we show that certain impurities such as fluorine adatoms, which locally break sublattice symmetry without the formation of strong magnetic moment, could result in a remarkable variability of spin transport characteristics. The impurity resonance level is found to be associated with a long-range sublattice pseudospin polarization, which by locally decoupling spin and pseudospin dynamics provokes a huge spin lifetime electron-hole asymmetry. In the dilute impurity limit, spin lifetimes could be tuned electrostatically from 100 ps to several nanoseconds, providing a protocol to chemically engineer an unprecedented spin device functionality. © 2016 American Physical Society.
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Thermal conductivity of MoS2 polycrystalline nanomembranes
Sledzinska M., Graczykowski B., Placidi M., Reig D.S., El Sachat A., Reparaz J.S., Alzina F., Mortazavi B., Quey R., Colombo L., Roche S., Torres C.M.S. 2D Materials; 3 (3, 035016) 2016. 10.1088/2053-1583/3/3/035016. IF: 9.611
Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS2 nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS2 by means of 2-laser Raman thermometry. The free-standing, drum-like MoS2 nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS2 and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m-1 K-1. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed. © 2016 IOP Publishing Ltd.
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Unconventional features in the quantum Hall regime of disordered graphene: Percolating impurity states and Hall conductance quantization
Leconte N., Ortmann F., Cresti A., Roche S. Physical Review B; 93 (11, 115404) 2016. 10.1103/PhysRevB.93.115404.
We report on the formation of critical states in disordered graphene, at the origin of variable and unconventional transport properties in the quantum Hall regime, such as a zero-energy Hall conductance plateau in the absence of an energy band gap and Landau-level degeneracy breaking. By using efficient real-space transport methodologies, we compute both the dissipative and Hall conductivities of large-size graphene sheets with random distribution of model single and double vacancies. By analyzing the scaling of transport coefficients with defect density, system size, and magnetic length, we elucidate the origin of anomalous quantum Hall features as magnetic-field-dependent impurity states, which percolate at some critical energies. These findings shed light on unidentified states and quantum-transport anomalies reported experimentally. © 2016 American Physical Society.
2015
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Efficient linear scaling approach for computing the Kubo Hall conductivity
Ortmann F., Leconte N., Roche S. Physical Review B - Condensed Matter and Materials Physics; 91 (16, 165117) 2015. 10.1103/PhysRevB.91.165117. IF: 3.736
We report an order-N approach to compute the Kubo Hall conductivity for disorderd two-dimensional systems reaching tens of millions of orbitals, and realistic values of the applied external magnetic fields (as low as a few Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity σxy using a wave-packet propagation method and a continued fraction expansion for the computation of diagonal and off-diagonal matrix elements of the Green functions. The validity of the method is demonstrated by comparison of results with brute-force diagonalization of the Kubo formula, using (disordered) graphene as the system of study. This approach to mesoscopic system sizes is opening an unprecedented perspective for so-called reverse engineering in which the available experimental transport data are used to get a deeper understanding of the microscopic structure of the samples. Besides, this will not only allow addressing subtle issues in terms of resistance standardization of large-scale materials (such as wafer scale polycrystalline graphene), but will also enable the discovery of new quantum transport phenomena in complex two-dimensional materials, out of reach with classical methods. © 2015 American Physical Society.
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Graphene spintronics: The European Flagship perspective
Roche S., Åkerman J., Beschoten B., Charlier J.-C., Chshiev M., Dash S.P., Dlubak B., Fabian J., Fert A., Guimarães M., Guinea F., Grigorieva I., Schönenberger C., Seneor P., Stampfer C., Valenzuela S.O., Waintal X., Van Wees B. 2D Materials; 2 (3, 030202) 2015. 10.1088/2053-1583/2/3/030202. IF: 0.000
Wereview current challenges and perspectives in graphene spintronics, which is one of themost promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes ofmaterials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. The general context of spintronics is first discussed togetherwith open issues and recent advances achieved by theGraphene SpintronicsWork Package consortiumwithin theGraphene Flagship project. Based on such progress, which establishes the state of the art, several novel opportunities for spinmanipulation such as the generation of pure spin current (through spinHall effect) and the control of magnetization through the spin torque phenomena appear on the horizon. Practical applications arewithin reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is a specially commissioned editorial from the Graphene Flagship Work Package on Spintronics. This editorial is part of the 2DMaterials focus collection on 'Progress on the science and applications of twodimensionalmaterials,' published in association with theGraphene Flagship. It provides an overviewof key recent advances of the spintronicswork package aswell as the mid-term objectives of the consortium. © 2015 IOP Publishing Ltd.
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Quantum Spin Hall Effect and Topological Insulators
Ortmann F., Roche S., Valenzuela S.O. Topological Insulators: Fundamentals and Perspectives; : 3 - 10. 2015. 10.1002/9783527681594.ch1.
Topological insulators (TIs), which exist in two and three dimensions, represent a new electronic phase stemming from the topological character of the bulk wave functions of certain materials and compounds. To overcome the limitations imposed by the low spin-orbit interaction in graphene, the quantum spin Hall (QSH) phase was early on proposed by Bernevig and Zhang in intricate strain architecture promoted by strong spin-orbit coupling. Three-dimensional TIs have several attributes in common with graphene, such as their low-energy electronic properties dominated by massless Dirac Fermion excitations, where the energy dispersion relations are described by a Dirac cone. Several possibilities for generating photo-induced bandgaps in graphene and the formation of states akin to those of TIs have been proposed theoretically, opening another field of research in which light illumination becomes an intriguing enabling tool to switch on and off the formation of the topological state. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
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Role of grain boundaries in tailoring electronic properties of polycrystalline graphene by chemical functionalization
Seifert M., Vargas J.E.B., Bobinger M., Sachsenhauser M., Cummings A.W., Roche S., Garrido J.A. 2D Materials; 2 (2, 024008) 2015. 10.1088/2053-1583/2/2/024008. IF: 0.000
Grain boundaries, inevitably present in chemical vapor deposited graphene, are expected to have considerable impact on the development of graphene-based hybrid materials with tailored material properties.Wedemonstrate here the critical role of polycrystallinity on the chemical functionalization of graphene comparing ozone-induced oxidation with remote plasma hydrogenation.Weshow that graphene oxidation and hydrogenation occur in two consecutive stages upon increasing defect density: an initial step in which surface-bound functional groups are generated, followed by the creation of vacancies. Remarkably, we find that hydrogenation yields homogeneously distributed defects while ozone-induced defects are preferentially accumulated at the grain boundaries eventually provoking local cracking of the structure. Supported by quantum simulations, our experimental findings reveal distinct electronic transport regimes depending on the density and distribution of induced defects on the polycrystalline graphene films. Our findings highlight the key role played by grain boundaries during graphene functionalization, and at the same time provide a novel perspective to tailor the properties of polycrystalline graphene. © 2015 IOP Publishing Ltd.
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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems
Ferrari A.C., Bonaccorso F., Fal'ko V., Novoselov K.S., Roche S., Bøggild P., Borini S., Koppens F.H.L., Palermo V., Pugno N., Garrido J.A., Sordan R., Bianco A., Ballerini L., Prato M., Lidorikis E., Kivioja J., Marinelli C., Ryhänen T., Morpurgo A., Coleman J.N., Nicolosi V., Colombo L., Fert A., Garcia-Hernandez M., Bachtold A., Schneider G.F., Guinea F., Dekker C., Barbone M., Sun Z., Galiotis C., Grigorenko A.N., Konstantatos G., Kis A., Katsnelson M., Vandersypen L., Loiseau A., Morandi V., Neumaier D., Treossi E., Pellegrini V., Polini M., Tredicucci A., Williams G.M., Hee Hong B., Ahn J.-H., Min Kim J., Zirath H., Van Wees B.J., Van Der Zant H., Occhipinti L., Di Matteo A., Kinloch I.A., Seyller T., Quesnel E., Feng X., Teo K., Rupesinghe N., Hakonen P., Neil S.R.T., Tannock Q., Löfwander T., Kinaret J. Nanoscale; 7 (11): 4598 - 4810. 2015. 10.1039/c4nr01600a. IF: 7.394
We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field. © The Royal Society of Chemistry 2015.
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Spin transport in hydrogenated graphene
Soriano D., Van Tuan D., Dubois S.M.-M., Gmitra M., Cummings A.W., Kochan D., Ortmann F., Charlier J.-C., Fabian J., Roche S. 2D Materials; 2 (2, 022002) 2015. 10.1088/2053-1583/2/2/022002. IF: 0.000
In this review we discuss the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification.We first discuss the spin-orbit coupling induced by local σ-π re-hybridization and sp3 C-Hdefect formation togetherwith the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances, recently introduced by Kochan et al (2014 Phys. Rev. Lett. 112 116602) is revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space orderNwavepacket propagation method. Typical spin lifetimes on the order of 1 ns are obtained for 1 ppm of hydrogen impurities (corresponding to a transport time of about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that local defect-induced magnetism can be at the origin of the substantial spin polarization loss in the clean graphene limit. © 2015 IOP Publishing Ltd.
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Topological Insulators: Fundamentals and Perspectives
Ortmann F., Roche S., Valenzuela S.O. Topological Insulators: Fundamentals and Perspectives; : 1 - 407. 2015. 10.1002/9783527681594.
There are only few discoveries and new technologies in physical sciences that have the potential to dramatically alter and revolutionize our electronic world. Topological insulators are one of them. The present book for the first time provides a full overview and in-depth knowledge about this hot topic in materials science and condensed matter physics. Techniques such as angle-resolved photoemission spectrometry (ARPES), advanced solid-state Nuclear Magnetic Resonance (NMR) or scanning-tunnel microscopy (STM) together with key principles of topological insulators such as spin-locked electronic states, the Dirac point, quantum Hall effects and Majorana fermions are illuminated in individual chapters and are described in a clear and logical form. Written by an international team of experts, many of them directly involved in the very first discovery of topological insulators, the book provides the readers with the knowledge they need to understand the electronic behavior of these unique materials. Being more than a reference work, this book is essential for newcomers and advanced researchers working in the field of topological insulators. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
2014
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Anisotropic behavior of quantum transport in graphene superlattices: Coexistence of ballistic conduction with Anderson insulating regime
Pedersen, J.G.; Cummings, A.W.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.89.165401. IF: 3.664
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Anomalous dissipation mechanism and Hall quantization limit in polycrystalline graphene grown by chemical vapor deposition
Lafont, F.; Ribeiro-Palau, R.; Han, Z.; Cresti, A.; Delvallee, A.; Cummings, A.W.; Roche, S.; Bouchiat, V.; Ducourtieux, S.; Schopfer, F.; Poirier, W. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.90.115422. IF: 3.664
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Charge transport in polycrystalline graphene: Challenges and opportunities
Cummings, A.W.; Duong, D.L.; Nguyen, V.L.; Van Tuan, D.; Kotakoski, J.; Barrios Vargas, J.E.; Lee, Y.H.; Roche, S. Advanced Materials; 26 (30): 5079 - 5094. 2014. 10.1002/adma.201401389. IF: 15.409
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Fingerprints of inelastic transport at the surface of the topological insulator Bi 2 Se 3: Role of electron-phonon coupling
Costache, M.V.; Neumann, I.; Sierra, J.F.; Marinova, V.; Gospodinov, M.M.; Roche, S.; Valenzuela, S.O. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.086601. IF: 7.728
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Graphene spintronics: Puzzling controversies and challenges for spin manipulation
Roche, S.; Valenzuela, S.O. Journal of Physics D - Applied Physics; 2014. 10.1088/0022-3727/47/9/094011. IF: 2.521
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Impact of graphene polycrystallinity on the performance of graphene field-effect transistors
Jiménez, D.; Cummings, A.W.; Chaves, F.; Van Tuan, D.; Kotakoski, J.; Roche, S. Applied Physics Letters; 2014. 10.1063/1.4863842. IF: 3.515
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Multiple quantum phases in graphene with enhanced spin-orbit coupling: From the quantum spin hall regime to the spin hall effect and a robust metallic state
Cresti, A.; Van Tuan, D.; Soriano, D.; Cummings, A.W.; Roche, S. Physical Review Letters; 113 (24): NC. 2014. 10.1103/PhysRevLett.113.246603. IF: 7.728
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Physical model of the contact resistivity of metal-graphene junctions
Chaves F.A., Jiménez D., Cummings A.W., Roche S. Journal of Applied Physics; 115 (16, 164513) 2014. 10.1063/1.4874181. IF: 2.101
While graphene-based technology shows great promise for a variety of electronic applications, including radio-frequency devices, the resistance of the metal-graphene contact is a technological bottleneck for the realization of viable graphene electronics. One of the most important factors in determining the resistance of a metal-graphene junction is the contact resistivity. Despite the large number of experimental works that exist in the literature measuring the contact resistivity, a simple model of it is still lacking. In this paper, we present a comprehensive physical model for the contact resistivity of these junctions, based on the Bardeen Transfer Hamiltonian method. This model unveils the role played by different electrical and physical parameters in determining the specific contact resistivity, such as the chemical potential of interaction, the work metal-graphene function difference, and the insulator thickness between the metal and graphene. In addition, our model reveals that the contact resistivity is strongly dependent on the bias voltage across the metal-graphene junction. This model is applicable to a wide variety of graphene-based electronic devices and thus is useful for understanding how to optimize the contact resistance in these systems. © 2014 AIP Publishing LLC.
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Pseudospin-driven spin relaxation mechanism in graphene
Tuan, D.V.; Ortmann, F.; Soriano, D.; Valenzuela, S.O.; Roche, S. Nature Physics; 10 (11): 857 - 863. 2014. 10.1038/nphys3083. IF: 20.603
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Quantum Hall Effect in Polycrystalline Graphene: The Role of Grain Boundaries
Cummings, A.W.; Cresti, A.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 90: 161401 (R). 2014. 10.1103/PhysRevB.90.161401. IF: 3.664
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Quantum transport in chemically functionalized graphene at high magnetic field: defect-induced critical states and breakdown of electron-hole symmetry
Leconte, N; Ortmann, F.; Cresti, A.; Charlier, J.C.; Roche, S. 2D Materials; 2014. 10.1088/2053-1583/1/2/021001. IF: 0.000
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Transport fingerprints at graphene superlattice Dirac points induced by a boron nitride substrate
Martinez-Gordillo, R.; Roche, S.; Ortmann, F.; Pruneda, M. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.89.161401. IF: 3.664
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Tunneling magnetoresistance phenomenon utilizing graphene magnet electrode
Hashimoto, T.; Kamikawa, S.; Soriano, D.; Pedersen, J. G.; Roche, S.; Haruyama, J. Applied Physics Letters; 105: 183111. 2014. 10.1063/1.4901279. IF: 3.515
2013
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Band gap engineering via edge-functionalization of graphene nanoribbons
Wagner, P.; Ewels, C.P.; Adjizian, J.-J.; Magaud, L.; Pochet, P.; Roche, S.; Lopez-Bezanilla, A.; Ivanovskaya, V.V.; Yaya, A.; Rayson, M.; Briddon, P.; Humbert, B. Journal of Physical Chemistry C; 117 (50): 26790 - 26796. 2013. 10.1021/jp408695c. IF: 4.814
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Broken symmetries, zero-energy modes, and quantum transport in disordered graphene: From supermetallic to insulating regimes
Cresti, A.; Ortmann, F.; Louvet, T.; Van Tuan, D.; Roche, S. Physical Review Letters; 110 2013. 10.1103/PhysRevLett.110.196601. IF: 7.943
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Highly defective graphene: A key prototype of two-dimensional Anderson insulators
Lherbier, A.; Roche, S.; Restrepo, O.A.; Niquet, Y.-M.; Delcorte, A.; Charlier, J.-C. Nano Research; 6: 326 - 334. 2013. 10.1007/s12274-013-0309-7. IF: 7.392
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Impact of vacancies on diffusive and pseudodiffusive electronic transport in graphene
Cresti, A.; Louvet, T.; Ortmann, F.; Van Tuan, D.; Lenarczyk, P.; Huhs, G.; Roche, S. Crystals; 3 (2): 289 - 305. 2013. 10.3390/cryst3020289. IF: 0.000
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Multiscale simulation of carbon nanotube transistors
Maneux, C.; Fregonese, S.; Zimmer, T.; Retailleau, S.; Nguyen, H.N.; Querlioz, D.; Bournel, A.; Dollfus, P.; Triozon, F.; Niquet, Y.M.; Roche, S. Solid-State Electronics; 89: 26 - 67. 2013. 10.1016/j.sse.2013.06.013. IF: 1.482
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Non-perturbative effects of laser illumination on the electrical properties of graphene nanoribbons
Calvo, H.L.; Perez-Piskunow, P.M.; Pastawski, H.M.; Roche, S.; Foa Torres, L.E.F. Journal of Physics Condensed Matter; 25 2013. 10.1088/0953-8984/25/14/144202. IF: 2.355
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Proximity effects induced in graphene by magnetic insulators: First-principles calculations on spin filtering and exchange-splitting gaps
Yang, H.X.; Hallal, A.; Terrade, D.; Waintal, X.; Roche, S.; Chshiev, M. Physical Review Letters; 110 2013. 10.1103/PhysRevLett.110.046603. IF: 7.943
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Scaling properties of charge transport in polycrystalline graphene
Van Tuan, D.; Kotakoski, J.; Louvet, T.; Ortmann, F.; Meyer, J.C.; Roche, S. Nano Letters; 13: 1730 - 1735. 2013. 10.1021/nl400321r. IF: 13.025
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Splitting of the zero-energy Landau level and universal dissipative conductivity at critical points in disordered graphene
Ortmann, F.; Roche, S. Physical Review Letters; 2013. 10.1103/PhysRevLett.110.086602. IF: 7.943
2012
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Atomistic boron-doped graphene field-effect transistors: A route toward unipolar characteristics
Marconcini, P.; Cresti, A.; Triozon, F.; Fiori, G.; Biel, B.; Niquet, Y.M.; Macucci, M.; Roche, S. ACS Nano; 6: 7942 - 7947. 2012. 10.1021/nn3024046.
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Chemically enriched graphene-based switching devices: A novel principle driven by impurity-induced quasibound states and quantum coherence
Roche, S.; Biel, B.; Cresti, A.; Triozon, F. Physica E: Low-Dimensional Systems and Nanostructures; 44: 960 - 962. 2012. 10.1016/j.physe.2011.06.008.
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Electron-hole transport asymmetry in boron-doped graphene field effect transistors
Marconcini, P. ; Cresti, A. ; Triozon, F.; Fiori, G. ; Biel, B. ; Niquet, Y.M.; Macucci, M.; Roche, S. Journal of Computational Electronics; 1: 1 - 4. 2012. 10.1109/IWCE.2012.6242844.
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Embedded boron nitride domains in graphene nanoribbons for transport gap engineering
Lopez-Bezanilla, A.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 86 2012. 10.1103/PhysRevB.86.165420.
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Insulating behavior of an amorphous graphene membrane
Van Tuan D., Kumar A., Roche S., Ortmann F., Thorpe M.F., Ordejon P. Physical Review B - Condensed Matter and Materials Physics; 86 (12, 121408) 2012. 10.1103/PhysRevB.86.121408.
We investigate the charge transport properties of planar amorphous graphene that is fully topologically disordered, in the form of sp2 threefold coordinated networks consisting of hexagonal rings but also including many pentagons and heptagons distributed in a random fashion. Using the Kubo transport methodology and the Lanczos method, the density of states, mean free paths, and semiclassical conductivities of such amorphous graphene membranes are computed. Despite a large increase in the density of states close to the charge neutrality point, all electronic properties are dramatically degraded, evidencing an Anderson insulating state caused by topological disorder alone. These results are supported by Landauer-Büttiker conductance calculations, which show a localization length as short as 5 nm. © 2012 American Physical Society.
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Laser-induced effects on the electronic features of graphene nanoribbons
Calvo, H.L.; Perez-Piskunow, P.M.; Roche, S.; Foa Torres, L.E.F. Applied Physics Letters; 101 2012. 10.1063/1.4772496.
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Quantum transport in disordered graphene: A theoretical perspective
Roche, S.; Leconte, N.; Ortmann, F.; Lherbier, A.; Soriano, D.; Charlier, J.-C. Solid State Communications; 152: 1404 - 1410. 2012. 10.1016/j.ssc.2012.04.030.
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Quenching of the quantum hall effect in graphene with scrolled edges
Cresti, A.; Fogler, M.M.; Guinea, F.; Castro Neto, A.H.; Roche, S. Physical Review Letters; 108 2012. 10.1103/PhysRevLett.108.166602.
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Three-dimensional models of topological insulators: Engineering of Dirac cones and robustness of the spin texture
Soriano, D.; Ortmann, F.; Roche, S. Physical Review Letters; 109 2012. 10.1103/PhysRevLett.109.266805.
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Transport properties of graphene containing structural defects
Lherbier, A.; Dubois, S.M.-M.; Declerck, X.; Niquet, Y.-M.; Roche, S.; Charlier, J.-C. Physical Review B - Condensed Matter and Materials Physics; 86 2012. 10.1103/PhysRevB.86.075402.
2011
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Effects of domains in phonon conduction through hybrid boron nitride and graphene sheets
Sevinçli, H.; Li, W.; Mingo, N.; Cuniberti, G.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 84 2011. 10.1103/PhysRevB.84.205444.
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Efficient linear scaling method for computing the thermal conductivity of disordered materials
Li, W.; Sevinçli, H.; Roche, S.; Cuniberti, G. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.155416.
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Engineering carbon chains from mechanically stretched graphene-based materials
Erdogan, E.; Popov, I.; Rocha, C.G.; Cuniberti, G.; Roche, S.; Seifert, G. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.041401.
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Graphene: Piecing it together
Rümmeli, M.H.; Rocha, C.G.; Ortmann, F.; Ibrahim, I.; Sevincli, H.; Börrnert, F.; Kunstmann, J.; Bachmatiuk, A.; Pötschke, M.; Shiraishi, M.; Meyyappan, M.; Büchner, B.; Roche, S.; Cuniberti, G. Advanced Materials; 23: 4471 - 4490. 2011. 10.1002/adma.201101855.
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Inducing and optimizing magnetism in graphene nanomeshes
Yang, H.-X.; Chshiev, M.; Boukhvalov, D.W.; Waintal, X.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 84 2011. 10.1103/PhysRevB.84.214404.
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Integer quantum Hall effect in trilayer graphene
Kumar, A.; Escoffier, W.; Poumirol, J.M.; Faugeras, C.; Arovas, D.P.; Fogler, M.M.; Guinea, F.; Roche, S.; Goiran, M.; Raquet, B. Physical Review Letters; 107 2011. 10.1103/PhysRevLett.107.126806.
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Magnetism-dependent transport phenomena in hydrogenated graphene: From spin-splitting to localization effects
Leconte N., Soriano D., Roche S., Ordejon P., Charlier J.-C., Palacios J.J. ACS Nano; 5 (5): 3987 - 3992. 2011. 10.1021/nn200558d.
Spin-dependent transport in hydrogenated two-dimensional graphene is explored theoretically. Adsorbed atomic hydrogen impurities can either induce a local antiferromagnetic, ferromagnetic, or nonmagnetic state depending on their density and relative distribution. To describe the various magnetic possibilities of hydrogenated graphene, a self-consistent Hubbard Hamiltonian, optimized by ab initio calculations, is first solved in the mean field approximation for small graphene cells. Then, an efficient order N Kubo transport methodology is implemented, enabling large scale simulations of functionalized graphene. Depending on the underlying intrinsic magnetic ordering of hydrogen-induced spins, remarkably different transport features are predicted for the same impurity concentration. Indeed, while the disordered nonmagnetic graphene system exhibits a transition from diffusive to localization regimes, the intrinsic ferromagnetic state exhibits unprecedented robustness toward quantum interference, maintaining, for certain resonant energies, a quasiballistic regime up to the micrometer scale. Consequently, low temperature transport measurements could unveil the presence of a magnetic state in weakly hydrogenated graphene. © 2011 American Chemical Society.
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Magnetoresistance and magnetic ordering fingerprints in hydrogenated graphene
Soriano D., Leconte N., Ordejón P., Charlier J.-C., Palacios J.-J., Roche S. Physical Review Letters; 107 (1, 016602) 2011. 10.1103/PhysRevLett.107.016602.
Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to ∼7% are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene. © 2011 American Physical Society.
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Magnetoresistance in disordered graphene: The role of pseudospin and dimensionality effects unraveled
Ortmann, F.; Cresti, A.; Montambaux, G.; Roche, S. Europhysics Letters; 94 2011. 10.1209/0295-5075/94/47006.
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Mechanically-induced transport switching effect in graphene-based nanojunctions
Kawai, T.; Poetschke, M.; Miyamoto, Y.; Rocha, C.G.; Roche, S.; Cuniberti, G. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.241405.
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Nanoelectronics: Graphene gets a better gap
Roche, S. Nature Nanotechnology; 2011. 10.1038/nnano.2010.262 .
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Oxygen surface functionalization of graphene nanoribbons for transport gap engineering
Cresti A., Lopez-Bezanilla A., Ordejón P., Roche S. ACS Nano; 5 (11): 9271 - 9277. 2011. 10.1021/nn203573y.
We numerically investigate the impact of epoxide adsorbates on the transport properties of graphene nanoribbons with width varying from a few nanometers to 15 nm. For the wider ribbons, a scaling analysis of conductance properties is performed for adsorbate density ranging from 0.1% to 0.5%. Oxygen atoms introduce a large electron-hole transport asymmetry with mean free paths changing by up to 1 order of magnitude, depending on the hole or electron nature of charge carriers. The opening of a transport gap on the electron side for GNRs as wide as 15 nm could be further exploited to control current flow and achieve larger ON/OFF ratios, despite the initially small intrinsic energy gap. The effect of the adsorbates in narrow ribbons is also investigated by full ab initio calculations to explore the limit of ultimate downsized systems. In this case, the inhomogeneous distribution of adsorbates and their interplay with the ribbon edge are found to play an important role. © 2011 American Chemical Society.
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Polaron transport in organic crystals: Temperature tuning of disorder effects
Ortmann, F.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 84 2011. 10.1103/PhysRevB.84.180302.
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Quantum transport in chemically modified two-dimensional graphene: From minimal conductivity to Anderson localization
Leconte N., Lherbier A., Varchon F., Ordejon P., Roche S., Charlier J.-C. Physical Review B - Condensed Matter and Materials Physics; 84 (23, 235420) 2011. 10.1103/PhysRevB.84.235420.
An efficient computational methodology is used to explore charge transport properties in chemically modified (and randomly disordered) graphene-based materials. The Hamiltonians of various complex forms of graphene are constructed using tight-binding models enriched by first-principles calculations. These atomistic models are further implemented into a real-space order-N Kubo-Greenwood approach, giving access to the main transport length scales (mean free paths, localization lengths) as a function of defect density and charge carrier energy. An extensive investigation is performed for epoxide impurities with specific discussions on both the existence of a minimum semiclassical conductivity and a crossover between weak to strong localization regime. The 2D generalization of the Thouless relationship linking transport length scales is here illustrated based on a realistic disorder model. © 2011 American Physical Society.
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Tuning laser-induced band gaps in graphene
Calvo, H.L.; Pastawski, H.M.; Roche, S.; Torres, L.E.F.F. Applied Physics Letters; 98 2011. 10.1063/1.3597412.
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Two-dimensional graphene with structural defects: Elastic mean free path, minimum conductivity, and anderson transition
Lherbier, A.; Dubois, S.M.M.; Declerck, X.; Roche, S.; Niquet, Y.M.; Charlier, J.C. Physical Review Letters; 106 2011. 10.1103/PhysRevLett.106.046803.
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Unveiling the magnetic structure of graphene nanoribbons
Ribeiro, R.; Poumirol, J.-M.; Cresti, A.; Escoffier, W.; Goiran, M.; Broto, J.-M.; Roche, S.; Raquet, B. Physical Review Letters; 107 2011. 10.1103/PhysRevLett.107.086601.
2010
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Conductance of functionalized nanotubes, graphene and nanowires: from ab initio to mesoscopic physics
Blase, X. ; Adessi, C.; Biel, B.; Lopez-Bezanilla, A.; Fernández-Serra, M.V.; Margine, E. R.; Triozon, F.; Roche, S. Physica Status Solidi (B): Basic Research; 2010. .
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Damaging graphene with ozone treatment: A chemically tunable metal - Insulator transition
Leconte N., Moser J., Ordejón P., Tao H., Lherbier A., Bachtold A., Alsina F., Sotomayor Torres C.M., Charlier J.-C., Roche S. ACS Nano; 4 (7): 4033 - 4038. 2010. 10.1021/nn100537z.
We present a multiscale ab initio study of electronic and transport properties of two-dimensional graphene after epoxide functionalization via ozone treatment. The orbital rehybridization induced by the epoxide groups triggers a strong intervalley scattering and changes dramatically the conduction properties of graphene. By varying the coverage density of epoxide defects from 0.1 to 4%, charge conduction can be tuned from a diffusive to a strongly localized regime, with localization lengths down to a few nanometers long. Experimental results supporting the interpretation as a metal - insulator transition are also provided. © 2010 American Chemical Society.
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Edge magnetotransport fingerprints in disordered graphene nanoribbons
Poumirol, J.-M.; Cresti, A.; Roche, S.; Escoffier, W.; Goiran, M.; Wang, X.; Li, X.; Dai, H.; Raquet, B. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.041413.
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Inelastic transport in vibrating disordered carbon nanotubes: Scattering times and temperature-dependent decoherence effects
Ishii, H.; Roche, S.; Kobayashi, N.; Hirose, K. Physical Review Letters; 104 2010. 10.1103/PhysRevLett.104.116801.
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Magnetotransport in disordered graphene exposed to ozone: From weak to strong localization
Moser, J.; Tao, H.; Roche, S.; Alzina, F.; Sotomayor Torres, C.M.; Bachtold, A. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.205445.
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Mobility gaps in disordered graphene-based materials: An ab initio -based tight-binding approach to mesoscopic transport
Biel, B.; Cresti, A.; Avriller, R.; Dubois, S.; López-Bezanilla, A.; Triozon, F.; Blase, X.; Charlier, J.-C.; Roche, S. Physica Status Solidi (C) Current Topics in Solid State Physics; 7: 2628 - 2631. 2010. 10.1002/pssc.200983826.
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Modeling graphene-based nanoelectromechanical devices
Poetschke, M.; Rocha, C.G.; Foa Torres, L.E.F.; Roche, S.; Cuniberti, G. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.193404.
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Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology
Li, W.; Sevinçli, H.; Cuniberti, G.; Roche, S. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.041410.
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Preface: Phys. stat. sol. (c) 7/11-12
Correia, A.; Sáenz, J.J.; Ordejon, P.; Roche, S. Physica Status Solidi (C) Current Topics in Solid State Physics; 7: 2593 - 2595. 2010. 10.1002/pssc.201060100.
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Quantum transport in graphene nanoribbons: Effects of edge reconstruction and chemical reactivity
Dubois, S.M.-M.; Lopez-Bezanilla, A.; Cresti, A.; Triozon, F.; Biel, B.; Charlier, J.-C.; Roche, S. ACS Nano; 4: 1971 - 1976. 2010. 10.1021/nn100028q.
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Simulation, modelling and characterisation of quasi-ballistic transport in nanometer sized field effect transistors: from TCAD to atomistic simulation
Roche, S.; Poiroux, T.; Lecarval, G.; Barraud, S.; Triozon, F.; Persson, M.; Niquet, Y.M. International Journal of Nanotechnology; 7 (04-ag.): 348 - 366. 2010. 10.1504/IJNT.2010.031724.
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Tuning the band gap of semiconducting carbon nanotube by an axial magnetic field
Fedorov, G.; Barbara, P.; Smirnov, D.; Jiménez, D.; Roche, S. Applied Physics Letters; 2010. .
2009
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Propagative Landau states and Fermi level pinning in carbon nanotubes
Nanot, S.; Avriller, R.; Escoffier, W.; Broto, J.-M.; Roche, S.; Raquet, B. Physical Review Letters; 103 2009. 10.1103/PhysRevLett.103.256801.