Showing papers by "Martin Gmitra published in 2020"

Quantum Anomalous Hall Effects in Graphene from Proximity-Induced Uniform and Staggered Spin-Orbit and Exchange Coupling

Abstract: We investigate an effective model of proximity modified graphene (or symmetrylike materials) with broken time-reversal symmetry. We predict the appearance of quantum anomalous Hall phases by computing bulk band gap and Chern numbers for benchmark combinations of system parameters. Allowing for staggered exchange field enables quantum anomalous Hall effect in flat graphene with Chern number C=1. We explicitly show edge states in zigzag and armchair nanoribbons and explore their localization behavior. Remarkably, the combination of staggered intrinsic spin-orbit and uniform exchange coupling gives topologically protected (unlike in time-reversal systems) pseudohelical states, whose spin is opposite in opposite zigzag edges. Rotating the magnetization from out of plane to in plane makes the system trivial, allowing us to control topological phase transitions. We also propose, using density functional theory, a material platform—graphene on Ising antiferromagnet MnPSe3—to realize staggered exchange (pseudospin Zeeman) coupling.

Swapping Exchange and Spin-Orbit Coupling in 2D van der Waals Heterostructures

Abstract: The concept of swapping the two most important spin interactions---exchange and spin-orbit coupling---is proposed based on two-dimensional multilayer van der Waals heterostructures. Specifically, we show by performing realistic ab initio simulations, that a single device consisting of a bilayer graphene sandwiched by a 2D ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) and a monolayer ${\mathrm{WS}}_{2}$, is able not only to generate, but also to swap the two interactions. The highly efficient swapping is enabled by the interplay of gate-dependent layer polarization in bilayer graphene and short-range spin-orbit and exchange proximity effects affecting only the layers in contact with the sandwiching materials. We call these structures ex-so-tic, for supplying either exchange (ex) or spin-orbit (so) coupling in a single device, by gating. Such bifunctional devices demonstrate the potential of van der Waals spintronics engineering using 2D crystal multilayers.

Chiral Majorana fermions in graphene from proximity-induced superconductivity

Abstract: We present a detailed theoretical study of chiral topological superconductor phases in proximity-superconducting graphene systems based on an effective model inspired by density functional theory simulations. Inducing $s$-wave superconductivity in quantum anomalous Hall effect systems leads to chiral topological superconductors. For out-of-plane magnetization we find topological superconducting phases with even numbers of chiral Majorana fermions per edge, which is correlated with the opening of a nontrivial gap in the bulk system in the $K$ points and their connection under particle-hole symmetry. We show that in a quantum anomalous Hall insulator with in-plane magnetization and a nontrivial gap opening at $M$, the corresponding topological superconductor can be tuned to host only single chiral Majorana states at its edge, which is promising for proposals exploiting such states for braiding operations.

Fully spin-polarized bulk states in ferroelectric GeTe

Abstract: By measuring the spin polarization of GeTe films as a function of light polarization we observed that the bulk states are fully spin polarized in the initial state, in strong contrast with observations for other systems with a strong spin-orbit interaction and the surface derived states in the same system. In agreement with state-of-the-art theory, our experimental results show that fully spin-polarized bulk states are an intrinsic property of the ferroelectric Rashba semiconductor alpha-GeTe(111). The fact that the measured spin-polarization vector does not change with light polarization can be explained by the absence of a mixing of states with a different total angular momentum J.

Swapping exchange and spin-orbit coupling in ex-so-tic 2D heterostructures

Abstract: The concept of swapping the two most important spin interactions -- exchange and spin-orbit coupling -- is proposed based on two-dimensional multilayer van der Waals heterostructures. Specifically, we show by performing realistic ab initio simulations, that a single device consisting of a bilayer graphene sandwiched by a 2D ferromagnet Cr2Ge2Te6 (CGT) and a monolayer WS2, is able not only to generate, but also to swap the two interactions. The highly efficient swapping is enabled by the interplay of gate-dependent layer polarization in bilayer graphene and short-range spin-orbit and exchange proximity effects affecting only the layers in contact with the sandwiching materials. We call these structures ex-so-tic, for supplying either exchange (ex) or spin-orbit (so) coupling in a single device, by gating. Such bifunctional devices demonstrate the potential of van der Waals spintronics engineering using 2D crystal multilayers.