Theoretical and Computational Nanoscience Group

Group Leader: Stephan Roche

Publications

2017

  • 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.


  • Electron trajectories and magnetotransport in nanopatterned graphene under commensurability conditions

    Power S.R., Thomsen M.R., Jauho A.-P., Pedersen T.G. Physical Review B; 96 (7, 075425) 2017. 10.1103/PhysRevB.96.075425. IF: 3.836

    Commensurability oscillations in the magnetotransport of periodically patterned systems, emerging from the interplay of cyclotron orbit and pattern periodicity, are a benchmark of mesoscopic physics in electron gas systems. Exploiting similar effects in two-dimensional materials would allow exceptional control of electron behavior, but it is hindered by the requirement to maintain ballistic transport over large length scales. Recent experiments have overcome this obstacle and observed distinct magnetoresistance commensurability peaks for perforated graphene sheets (antidot lattices). Interpreting the exact mechanisms behind these peaks is of key importance, particularly in graphene, where a range of regimes are accessible by varying the electron density. In this work, a fully atomistic, device-based simulation of magnetoresistance experiments allows us to analyze both the resistance peaks and the current flow at commensurability conditions. Magnetoresistance spectra are found in excellent agreement with experiment, but we show that a semiclassical analysis, in terms of simple skipping or pinned orbits, is insufficient to fully describe the corresponding electron trajectories. Instead, a generalized mechanism in terms of states bound to individual antidots, or to groups of antidots, is required. Commensurability features are shown to arise when scattering between such states is enhanced. The emergence and suppression of commensurability peaks is explored for different antidot sizes, magnetic field strengths, and electron densities. The insights gained from our study will guide the design and optimization of future experiments with nanostructured graphene. © 2017 American Physical Society.


  • 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.

    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.


  • 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.


  • 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.


  • 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.


  • Nanostructured graphene for spintronics

    Gregersen Sø.S., Power S.R., Jauho A.-P. Physical Review B; 95 (12, 121406) 2017. 10.1103/PhysRevB.95.121406. IF: 3.836

    Zigzag edges of the honeycomb structure of graphene exhibit magnetic polarization, making them attractive as building blocks for spintronic devices. Here, we show that devices with zigzag-edged triangular antidots perform essential spintronic functionalities, such as spatial spin splitting or spin filtering of unpolarized incoming currents. Near-perfect performance can be obtained with optimized structures. The device performance is robust against substantial disorder. The gate-voltage dependence of transverse resistance is qualitatively different for spin-polarized and spin-unpolarized devices, and can be used as a diagnostic tool. Importantly, the suggested devices are feasible within current technologies. © 2017 American Physical Society.


  • 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.

    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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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

  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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

  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • Velocity renormalization and Dirac cone multiplication in graphene superlattices with various barrier-edge geometries

    De Jamblinne De Meux A., Leconte N., Charlier J.-C., Lherbier A. Physical Review B - Condensed Matter and Materials Physics; 91 (23, 235139) 2015. 10.1103/PhysRevB.91.235139. IF: 3.736

    The electronic properties of one-dimensional graphene superlattices strongly depend on the atomic size and orientation of the 1D external periodic potential. Using a tight-binding approach, we show that the armchair and zigzag directions in these superlattices have a different impact on the renormalization of the anisotropic velocity of the charge carriers. For symmetric potential barriers, the velocity perpendicular to the barrier is modified for the armchair direction while remaining unchanged in the zigzag case. For asymmetric barriers, the initial symmetry between the forward and backward momentum with respect to the Dirac cone symmetry is broken for the velocity perpendicular (armchair case) or parallel (zigzag case) to the barriers. At last, Dirac cone multiplication at the charge neutrality point occurs only for the zigzag geometry. In contrast, band gaps appear in the electronic structure of the graphene superlattice with barrier in the armchair direction. © 2015 American Physical Society.


2014

  • 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


  • 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


  • Anomalous exchange interaction between intrinsic spins in conducting graphene systems

    Santos, H.; Soriano, D.; Palacios, J.J. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.89.195416. IF: 3.664


  • 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


  • 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


  • 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


  • 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


  • 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


  • Physical model of the contact resistivity of metal-graphene junctions

    Chaves, F.A.; Jimenez, D.; Cummings, A.W.; Roche, S. Journal of Applied Physics; 2014. . IF: 2.185


  • 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


  • 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


  • 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


  • 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


  • 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

  • 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


  • 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


  • 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


  • 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


  • 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


  • 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


  • 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


  • 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


  • 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

  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • Interplay between sublattice and spin symmetry breaking in graphene

    Soriano, D.; Fernández-Rossier, J. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.195433.


  • Large bandwidths in synthetic one-dimensional stacks of biological molecules

    Oetzel, B.; Ortmann, F.; Matthes, L.; Tandetzky, F.; Bechstedt, F.; Hannewald, K. Physical Review B - Condensed Matter and Materials Physics; 86 2012. 10.1103/PhysRevB.86.195407.


  • 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.


  • 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.


  • 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.


  • Spin-filtered edge states in graphene

    Gosálbez-Martínez, D.; Soriano, D.; Palacios, J.J.; Fernández-Rossier, J. Solid State Communications; 152: 1469 - 1476. 2012. 10.1016/j.ssc.2012.04.046.


  • 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.


  • 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

  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • Nanoelectronics: Graphene gets a better gap

    Roche, S. Nature Nanotechnology; 2011. 10.1038/nnano.2010.262 .


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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

  • 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. .


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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.


  • 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

  • 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.