Staff directory Aron William Cummings

Publications

2018

  • 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.


  • Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2 /graphene/ h -BN heterostructures

    Zihlmann S., Cummings A.W., Garcia J.H., Kedves M., Watanabe K., Taniguchi T., Schönenberger C., Makk P. Physical Review B; 97 (7, 075434) 2018. 10.1103/PhysRevB.97.075434. IF: 3.813

    Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition-metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single-layer graphene encapsulated between the transition-metal dichalcogenide WSe2 and h-BN is found to exhibit exceptional quality with mobilities as high as 1×105 cm2 V-1 s-1. At the same time clear weak antilocalization indicates strong spin-orbit coupling, and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping-dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedom in graphene with the realization of new concepts in spin manipulation. © 2018 American Physical Society.


  • Proximity-induced spin-orbit coupling in graphene/ Bi1.5Sb0.5Te1.7Se1.3 heterostructures

    Jafarpisheh S., Cummings A.W., Watanabe K., Taniguchi T., Beschoten B., Stampfer C. Physical Review B; 98 (24) 2018. 10.1103/PhysRevB.98.241402. IF: 3.813

    The weak intrinsic spin-orbit coupling in graphene can be greatly enhanced by proximity coupling. Here, we report on the proximity-induced spin-orbit coupling in graphene transferred by hexagonal boron nitride (hBN) onto the topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) which was grown on a hBN substrate by vapor solid synthesis. Phase coherent transport measurements, revealing weak localization, allow us to extract the carrier density-dependent phase coherence length lφ. While lφ increases with increasing carrier density in the hBN/graphene/hBN reference sample, it decreases in graphene/BSTS due to the proximity coupling of BSTS to graphene. The latter behavior results from D'yakonov-Perel'-type spin scattering in graphene with a large proximity-induced spin-orbit coupling strength of at least 2.5 meV. © 2018 American Physical Society.


  • 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.


  • 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.


  • 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

  • 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.


  • 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.


  • 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.


  • 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.


2016

  • 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.


  • 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.


  • 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.


2015

  • 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.


  • 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.


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


  • 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


  • 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


  • 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