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4 páginas, 4 figuras.-- PACS number(s): 81.05.ue, 62.25.−g, 72.80.Vp.-- et al. ; We report a theoretical study suggesting a novel type of electronic switching effect, driven by the geometrical reconstruction of nanoscale graphene-based junctions. We considered junction structures that have alternative metastable configurations transformed by rotations of local carbon dimers. The use of external mechanical strain allows a control of the energy-barrier heights of the potential profiles and also changes the reaction character from endothermic to exothermic or vice versa. The reshaping of the atomic details of the junction encode binary electronic "on" or "off" states, with on/off transmission ratio that can reach up to 104–105. Our results suggest the possibility to design modern logical switching devices or mechanophore sensors, monitored by mechanical strain and structural rearrangements. ; This work was partially funded by the Alexander vonHumboldt Foundation, the European Union, the Free State of Saxony (SAB Project No. A2-13996/2379), the NANOSIM-GRAPHENE (Project No. ANR-09-NANO-016- 01), and the South Korean Ministry of Education, Science, and Technology Program (WCU ITCE Project No. R31-2008- 000-10100-0). ; Peer reviewed

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. ; This work has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. S. R. acknowledges the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911), the Secretaria de Universidades e Investigacion del Departamento de Economia y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO SEV-2013-0295). ; Peer Reviewed

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. ; This work is supported by the European Union Seventh Framework Programme under grant agreement 604391 Graphene Flagship, the Severo Ochoa Program (MINECO, Grant SEV-2013-0295), the Spanish Ministry of Economy and Competitiveness (MAT2012-33911), and Secretaría de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña. ; Peer Reviewed

We report on the possibility of detecting hinge spin polarization in magnetic topological insulators by resistance measurements. By implementing a three-dimensional model of magnetic topological insulators into a multiterminal device with ferromagnetic contacts near the top surface, local spin features of the chiral edge modes are unveiled. We find local spin polarization at the hinges that inverts the sign between the top and bottom surfaces. At the opposite edge, the topological state with inverted spin polarization propagates in the reverse direction. A large resistance switch between forward and backward propagating states is obtained, driven by the matching between the spin polarized hinges and the ferromagnetic contacts. This feature is general to the ferromagnetic, antiferromagnetic, and canted antiferromagnetic phases, and enables the design of spin-sensitive devices, with the possibility of reversing the hinge spin polarization of the currents. ; We acknowledge the European Union Horizon 2020 research and innovation programme under Grant Agreement 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).

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We use numerical simulations to predict peculiar magnetotransport fingerprints in polycrystalline graphene, driven by the presence of grain boundaries of varying size and orientation. The formation of Landau levels is shown to be restricted by the polycrystalline morphology, requiring the magnetic length to be smaller than the average grain radius. The nature of localization is also found to be unusual, with strongly localized states at the center of Landau levels (including the usually highly robust zero-energy state) and extended electronic states lying between Landau levels. These extended states percolate along the network of grain boundaries, resulting in a finite value for the bulk dissipative conductivity and suppression of the quantized Hall conductance. Such breakdown of the quantum Hall regime provoked by extended structural defects is also illustrated through two-terminal Landauer-Büttiker conductance calculations, indicating how a single grain boundary induces cross linking between edge states lying at opposite sides of a ribbon geometry. ; The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. This work was also funded by Spanish Ministry of Economy and Competitiveness under Contract No. MAT2012-33911. ; Peer Reviewed

4 páginas, 4 figuras.-- PACS number(s): 61.46.Km, 73.63.−b, 62.25.−g.-- et al. ; The electrical response of graphene-based materials can be tailored under mechanical stress. We report different switching behaviors that take place in mechanically deformed graphene nanoribbons prior to the breakage of the junction. By performing tight-binding molecular dynamics, the study of structural changes of graphene nanoribbons with different widths is achieved, revealing that carbon chains are the ultimate bridges before the junction breaks. The electronic and transport calculations show that binary on/off states can be switched prior to and during breakage depending on the atomic details of the nanoribbon. This work supports the interpretation of recent experiments on nonvolatile memory element engineering based on graphene break junctions. ; This work is supported by the European Union and the Freistaat Sachsen (Project No. 13857/2379), the NANOSIM-GRAPHENE Project (ANR-09-NANO-016- 01) of ANR/P3N2009, and the Alexander von Humboldt Foundation. ; Peer reviewed

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 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. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 696656, Graphene Flagship. S.R. acknowledges Funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund [Project No. FIS2015- 67767-P (MINECO/FEDER)] and the Severo Ochoa Program (MINECO SEV-2013-0295). S.A. acknowledges support from the National Research Foundation of Singapore under its Fellowship programme (NRF-NRFF2012-01) and computational resources from the Centre for Advanced 2D Materials (an NRF midsized center). ; Peer Reviewed

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. ; This research is partly funded by the European Union Seventh Framework Programme under Grant No. 604391 Graphene Flagship. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant No. SEV-2013-0295). F.O. would like to thank the Deutsche Forschungsgemeinschaft for financial support (Grant No. OR 349/1-1). ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We report on the possibility to generate highly anisotropic quantum conductivity in disordered graphene-based superlattices. Our quantum simulations, based on an efficient real-space implementation of the Kubo-Greenwood formula, show that in disordered graphene superlattices the strength of multiple scattering phenomena can strongly depend on the transport measurement geometry. This eventually yields the coexistence of a ballistic waveguide and a highly resistive channel (Anderson insulator) in the same two-dimensional platform, evidenced by a σyy/σxx ratio varying over several orders of magnitude, and suggesting the possibility of building graphene electronic circuits based on the unique properties of chiral massless Dirac fermions in graphene. © 2014 American Physical Society. ; The work by J.G.P. is financially supported by the Danish Council for Independent Research, FTP Grants No. 11-105204 and No. 11-120941. S.R. and A.W.C. acknowledge the Spanish Ministry of Economy and Competitiveness for national project funding (MAT2012-33911) and SAMSUNG for support within the Global Innovation Program. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant No. 604391 Graphene Flagship. ; Peer Reviewed

We demonstrate the emergence and dynamics of intraparticle entanglement in massless Dirac fermions. This entanglement, generated by spin-orbit coupling, arises between the spin and sublattice pseudospin of electrons in graphene. The entanglement is a complex dynamic quantity but is generally large, independent of the initial state. Its time dependence implies a dynamical violation of a Bell inequality, while its magnitude indicates that large intraparticle entanglement is a general feature of graphene on a substrate. These features are also expected to impact entanglement between pairs of particles, and may be detectable in experiments that combine Cooper pair splitting with nonlocal measurements of spin-spin correlation in mesoscopic devices based on Dirac materials. ; 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). The authors acknowledge the European Union's Horizon 2020 research and innovation programme under Grants Agreement No. 785219 and No. 881603 (Graphene Flagship). B.G. deM. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 754558 (PREBIST). ; Peer reviewed

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

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. In 2005, several new phenomena such as the spin Hall effect and the quantum spin Hall effect were predicted in the presence of strong spinorbit coupling and vanishing 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 symmetrybreaking 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-Büttiker scattering and nonequilibrium Green function approaches). In contrast to usual two-dimensional electron gases, the presence of defects in graphene generates more complex electronic features such as electron-hole asymmetry, defect resonances or percolation effect between localized impurity states, which, together with extra degrees of freedom (sublattice pseudospin, valley isospin), bring a higher degree of complexity and enlarge the transport phase diagram. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 696656. S. R. acknowledges funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (Project No. FIS2015-67767-P (MINECO/FEDER)), the Secretaria de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO, Grant SEV2013-0295). We acknowledge computational resources from PRACE and the Barcelona Supercomputing Center (Mare Nostrum), under Project 2015133194. B. K. N. was supported by NSF Grant No. ECCS 1509094. The supercomputing time was provided by in part by XSEDE, which is supported by NSF Grant No. ACI-1053575. ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We report peculiar transport fingerprints at the secondary Dirac points created by the interaction between graphene and boron nitride layers. By performing ab initio calculations, the electronic characteristics of the moiré patterns produced by the interaction between layers are first shown to be in good agreement with experimental data, and further used to calibrate the tight-binding model implemented for the transport study. By means of a real-space order-N quantum transport (Kubo) methodology, low-energy (Dirac point) transport properties are contrasted with those of high-energy (secondary) Dirac points, including both Anderson disorder and Gaussian impurities to respectively mimic short-range and long-range scattering potentials. Mean free paths at the secondary Dirac points are found to range from 10 nm to a few hundreds of nm depending on the static disorder, while the observation of satellite resistivity peaks depends on the strength of quantum interferences and localization effects. ; This work was funded by Spanish FEDER-MINECO (Grants No. FIS2009-12721-C04-01 and No. CSD2007-00041) and AGAUR (Grant No. FI-DGR 2011FI B 00993). S.R. acknowledges support from the Spanish Ministry of Economy and Competitiveness (under Contract No. MAT2012-33911). The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 "Graphene Flagship". ; Peer Reviewed

Under the terms of the Creative Commons Attribution (CC BY) license to their work. ; While graphene-based technology shows great promise for a variety of electronic applications, including radio-frequency devices, the resistance of the metal-graphene contact is a technological bottleneck for the realization of viable graphene electronics. One of the most important factors in determining the resistance of a metal-graphene junction is the contact resistivity. Despite the large number of experimental works that exist in the literature measuring the contact resistivity, a simple model of it is still lacking. In this paper, we present a comprehensive physical model for the contact resistivity of these junctions, based on the Bardeen Transfer Hamiltonian method. This model unveils the role played by different electrical and physical parameters in determining the specific contact resistivity, such as the chemical potential of interaction, the work metal-graphene function difference, and the insulator thickness between the metal and graphene. In addition, our model reveals that the contact resistivity is strongly dependent on the bias voltage across the metal-graphene junction. This model is applicable to a wide variety of graphene-based electronic devices and thus is useful for understanding how to optimize the contact resistance in these systems. ; We acknowledge support from SAMSUNG within the Global Innovation Program. The research leading to these results has received funding from Ministerio of Economía y Competitividad of Spain under the Project Nos. TEC2012-31330 and MAT2012-33911, and from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. ; Peer Reviewed