Suchergebnisse

Realizations of some topological phases in two-dimensional systems rely on the challenge of jointly incorporating spin-orbit and magnetic exchange interactions. Here, we predict the formation and control of a fully valley-polarized quantum anomalous Hall effect in bilayer graphene, by separately imprinting spin-orbit and magnetic proximity effects in different layers. This results in varying spin splittings for the conduction and valence bands, which gives rise to a topological gap at a single Dirac cone. The topological phase can be controlled by a gate voltage and switched between valleys by reversing the sign of the exchange interaction. By performing quantum transport calculations in disordered systems, the chirality and resilience of the valley-polarized edge state are demonstrated. Our findings provide a promising route to engineer a topological phase that could enable low-power electronic devices and valleytronic applications as well as putting forward layer-dependent proximity effects in bilayer graphene as a way to create versatile topological states of matter. ; The authors were supported by the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (Graphene Flagship) and No. 824140 (TOCHA, H2020-FETPROACT-01-2018). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706).

We study the polarization dependence of optical absorption for a modified Haldane model, which exhibits antichiral edge modes in the presence of sample boundaries and has been argued to be realizable in transition metal dichalcogenides or Weyl semimetals. A rich optical phase diagram is unveiled, in which the correlations between perfect circular dichroism, pseudospin andvalley polarization can be tuned independently upon varying the Fermi energy. In particular, perfect circular dichroism and valley polarization are achieved simultaneously. This combination of optical properties suggests some interesting photonic device functionality (e.g., light polarizer) which could be combined with valleytronics applications (e.g., generation of valley currents). ; M.V. acknowledges the Graphene Flagship grant and the Department of Physics of Tohoku University for its hospitality. The research leading to these results has received funding from "La Caixa" Foundation by supporting M.V. S.R. was supported by the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 696656 (Graphene Flagship). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). R.S. acknowledges JSPS Kakenhi (Grant No. JP18H01810). N.T.H. acknowledges JSPS Kakenhi (Grant No. JP18J10151). ; Peer reviewed

Improved fabrication techniques have enabled the possibility of ballistic transport and unprecedented spin manipulation in ultraclean graphene devices. Spin transport in graphene is typically probed in a nonlocal spin valve and is analyzed using spin diffusion theory, but this theory is not necessarily applicable when charge transport becomes ballistic or when the spin diffusion length is exceptionally long. Here, we study these regimes by performing quantum simulations of graphene nonlocal spin valves. We find that conventional spin diffusion theory fails to capture the crossover to the ballistic regime as well as the limit of long spin diffusion length. We show that the latter can be described by an extension of the current theoretical framework. Finally, by covering the whole range of spin dynamics, our study opens a new perspective to predict and scrutinize spin transport in graphene and other two-dimensional material-based ultraclean devices. ; M. V. acknowledges support from "La Caixa" Foundation. S. R. P. acknowledges funding from the Irish Research Council under the Laureate awards programme. X. W. acknowledges the ANR GRANSPORT funding. All authors were supported by the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 785219 (Graphene Flagship). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). ; Peer reviewed

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. ; Chalmers researchers acknowledge financial support from the European Union (EU) Horizon 2020 Research and Innovation Programme GrapheneCore2 contract number 785219 (Graphene Flagship), EU FLAG-ERA project (from Swedish Research Council VR no. 2015- 06813), Swedish Research Council VR project grants (no. 2016-03658), Graphene Center, and the AoA Nano program at Chalmers University of Technology. Catalan Institute of Nanoscience and Nanotechnology (ICN2) was supported by the Severo Ochoa program from Spanish Ministry of Economy and Competitiveness (MINECO; grant no. SEV-2013-0295) and funded by the Centres de Recerca de Catalunya Programme/Generalitat de Cataluña. S.R. acknowledges the Spanish MINECO and the European Regional Development Fund (project no. FIS2015- 67767-P MINECO/FEDER), and the Secretaría de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalitat de Cataluña (2014 SGR 58). ; Peer reviewed

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer MoTe2 and WTe2 showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit λsθxy≈1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy≈80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities. ; X.W. acknowledges the Agence National pour la Recherche Flagera GRANSPORT funding. ICN2 authors were supported by the European Union Horizon 2020 research and innovation programme under Grant Agreements No. 881603 (Graphene Flagship) and No. 824140 (TOCHA, H2020-FETPROACT-01-2018). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish Ministerio de Ciencia e Innovación (Grants No. SEV-2017-0706 and No. PID2019-111773RB-I00/AEI/10.13039/501100011033). V.M.P. acknowledges the support of the National Research Foundation Singapore under its Medium-Sized Centre Programme.

Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices. ; Authors from Chalmers, ICN2, and University of Regensburg acknowledge funding from the European Union's Horizon 2020 research and innovation programme under Grant agreement no. 785219 (Graphene Flagship Core 2). The authors at Chalmers acknowledge financial support from EU FlagEra project (from Swedish Research Council VR No. 2015-06813), Swedish Research Council VR project Grants (No. 2016-03658), Graphene center and the AoA Nano program at Chalmers University of Technology. We acknowledge Dr Ron Jansen for useful discussions about proximity-induced exchange and stray Hall effects in the heterostructures. We acknowledge help from Bing Zhao in our group and staff at Nanofabrication laboratory and Quantum device laboratory for useful discussions and help in fabrication and measurements of devices. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). M. Vila acknowledges funding from 'La Caixa' Foundation. ; Peer reviewed

Graphene is an excellent material for long-distance spin transport but allows little spin manipulation. Transition-metal dichalcogenides imprint their strong spin-orbit coupling into graphene via the proximity effect, and it has been predicted that efficient spin-to-charge conversion due to spin Hall and Rashba-Edelstein effects could be achieved. Here, by combining Hall probes with ferromagnetic electrodes, we unambiguously demonstrate experimentally the spin Hall effect in graphene induced by MoS2 proximity and for varying temperatures up to room temperature. The fact that spin transport and the spin Hall effect occur in different parts of the same material gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Additionally, for a single graphene/MoS2 heterostructure-based device, we evidence a superimposed spin-to-charge current conversion that can be indistinguishably associated with either the proximity-induced Rashba-Edelstein effect in graphene or the spin Hall effect in MoS2. By a comparison of our results to theoretical calculations, the latter scenario is found to be the most plausible one. Our findings pave the way toward the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications. ; This work is supported by the Spanish MINECO under the Maria de Maeztu Units of Excellence Programme (MDM-2016-0618) and under projects MAT2015-65159-R and MAT2017-82071-ERC and by the European Union H2020 under the Marie Curie Actions (QUESTECH). The work in ICN2 is supported by Spanish MINECO under Severo Ochoa program (Grant No. Sev-2017-0706). S.R. and J.H.G. acknowledge support from the European Union Seventh Framework Programme under grant agreement no. 785219 Graphene Flagship and the computational resources from PRACE and the Barcelona Supercomputing Center (project no. 2015133194). M. V. acknowledges funding received from la Caixa foundation.