Speaker: Prof Branislav K. Nikolić
Affiliation: University of Delaware USA
Abstract Details: The recent experimental observation of nonlocal voltage, several microns away from the nominal current path, near the Dirac point (DP) in multiterminal graphene devices with adatom-induced spin-orbit coupling or in multiterminal graphene on hexagonal boron nitride (G/hBN) heterostructures has been interpreted as the result of the direct and inverse spin Hall effect (SHE) or the direct and inverse valley Hall effect (VHE), respectively . However, subsequent experiments reproducing the nonlocal signal in graphene with adatoms have also demonstrated insensitivity to the applied in-plane magnetic field, thereby suggesting its disconnect with SHE physics or any other spin-related mechanism. The theoretical interpretation of nonlocal signal in G/hBN heterostructures in terms of topological valley currents carried by the Fermi sea states just beneath the gap opened in graphene due to inversion symmetry breaking does not explain the long-standing puzzle of why the highly insulating state of G/hBN is rarely observed. Furthermore, using Landauer-Büttiker (LB) theory, as a rigorous quantum transport approach employed over the past three decades to obtain observable nonlocal voltage and the corresponding nonlocal resistance, we obtained  zero nonlocal signal in the same geometry used in experiments (where the channel connecting the two crossbars is much longer that its width) and for the same simplistic Hamiltonian which gives (not directly observable) quantized VH conductivity characterizing topological valley currents. In this talk, I will show how to resolve these puzzles by using first-principles Hamiltonians of graphene with adatoms or G/hBN heterostructures combined with numerically exact calculations of the nonlocal resistance based on the multiterminal LB formula [2,3]. In the case of multiterminal graphene with adatoms, we find several background mechanisms which generate nonlocal resistance even when spin-orbit coupling is switched off . We also proposed a specific device geometry where nonlocal resistance due to the SHE can be isolated by removing such background contributions . This will be compared with the direct and inverse intrinsic SHE as the sole origin of nonlocal resistance in graphene/transition-metal-dichalcogenide heterostructures where graphene acquires homogeneous proximity spin-orbit coupling. In the case of multiterminal G/hBN heterostructure, we demonstrate  the key role played by the Fermi surface edge states and the corresponding edge currents (which were missed in previous theoretical analyses based on simplistic Hamiltonian) that can explain both the nonlocal resistance and metallic-like resistivity observed in experiments while being in full accord with the very recent Josephson interferometry-based imaging of the spatial profile of edge supercurrents in G/hBN wires.
A. Cresti, B. K. Nikolić, J. H. García, and S. Roche, Riv. Nuovo Cimento 39, 587 (2016).
D. V. Tuan, J. M. Marmolejo-Tejada, X. Waintal, B. K. Nikolić, S. O. Valenzuela, and S. Roche, Phys. Rev. Lett., 117, 176602 (2016).
J. M. Marmolejo-Tejada, J. H. Garcìa, P.-H. Chang, X.-L. Sheng, A. Cresti, S. Roche, and B. K. Nikolić, arXiv:1706.09361
About the Speaker: Branislav K. Nikolić is a Professor of Physics at the University of Delaware and a Senior Visiting Scientist at RIKEN Center for Emergent Matter Science in Japan. He received his Ph.D. in theoretical condensed matter physics from Stony Brook University, and B.Sc. degree from the University of Belgrade, Serbia. He was visiting Professor at the University of Regensburg, National Taiwan University and Centre de Physique Théorique de Grenoble-Alpes. His research is focused on nonequilibrium many-body quantum systems, first-principles quantum transport and high-performance computing applied to nanostructures of interest to spintronics, nanoelectronics, thermoelectrics and nano-bio interface. His most notable contributions include studies of the spin Hall effect, spin pumping and spin torque in the presence of spin-orbit coupling, decoherence of transported spins, spin-dependent shot noise, nonequilibrium electron-magnon and electron-phonon systems, topological insulator based devices for spintronic and thermoelectric applications and graphene based devices for ultrafast DNA sequencing.