We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS2-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS2-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be ∼ 0.2ps −1 and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a'valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS2 system provides a possible platform for these novel edge states.

No abstract

Proximity effects induced in the 2D Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators. Here, we report a low temperature study of the electronic properties of high quality van der Waals heterostructures composed of a single graphene layer proximitized with -RuCl 3 . The latter is known to become antiferromagnetically ordered below 10 K. Shubnikov de Haas oscillations in the longitudinal resistance together with Hall resistance measurements provide clear evidence for a band realignment that is accompanied by a transfer of electrons originally occupying the graphene's spin degenerate Dirac cones into -RuCl 3 band states with in-plane spin polarization. Left behind are holes in two separate Fermi pockets, only the dispersion of one of which is distorted near the Fermi energy due to spin selective hybridization with these spin polarized -RuCl 3 band states. This interpretation is supported by our DFT calculations. An unexpected damping of the quantum oscillations as well as a zero field resistance upturn close to the Néel temperature of -RuCl 3 suggests the onset of additional spin scattering due to spin fluctuations in the -RuCl 3 .

We report a discovery, through first-principles calculations, that crystalline Ge-Sb-Te ͑GST͒ phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surfacelike interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb 2 Te 3 layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs.Topological insulator ͑TI͒ has a bulk-phase energy gap but contains conducting surface states that have linear energy-momentum dispersions near time-reversal invariant momenta ͑TRIM͒. 1-4 These surface states are chiral and robust to external perturbations because they are protected by time-reversal symmetry. Finding new TI materials and exploring implications to device applications have been the primary focus of current research on TI. 5-8 Also the change in topological insulating property and detailed emergent behaviors of the surface states when the composition and structure of TI are tailored are still yet to be investigated.Phase-change materials such as Ge-Sb-Te ͑GST͒ compounds are considered the best candidates for nextgeneration nonvolatile memories because of their rapid, and reversible cycles between the crystalline and amorphous structures. 9-12 Detailed atomic and electronic structures associated with the structural transition of GST compounds have been extensively studied 13-15 but the mechanism and factors responsible for the very fast atomic rearrangement are still unknown. Nor is the electronic structure of GST understood sufficiently to explain the conducting properties. Movement of Ge atoms from octahedral to tetrahedral sites has been proposed as the mechanism of structural transitions from metastable rocksalt or stable hexagonal structures to nonconducting amorphous phase. 11,13 Several candidate models have been suggested for ͑meta-͒ stable crystalline phases. Petrov proposed the layer sequence of Te-Sb-Te-Ge-Te-Te-Ge-Te-Sb ͑Ref. 16͒ ͑the Petrov sequence͒. Kooi and De Hosson ͑KH͒ proposed a different layer sequence of Te-Ge-Te-Sb-Te-Te-Sb-Te-Ge ͑Ref. 17͒ ͑the KH sequence͒. In firstprinciples calculations, the Petrov sequence is slightly less stable than the KH sequence. 18 GeTe and Sb 2 Te 3 are the main components of GSTs ͑Refs. 11, 12, and 19͒ and have finite band gaps in the bulk phase. Sb 2 Te 3 is topological insulator that has gapless edge states protected by time-reversal symmetry while maintaining bulk energy gap. 20,21 GeTe does not have such properties. For gapless edge states to exist, strong spin-orbit coupling ͑SOC͒ is needed to produce a parity inversion at TRIM. 4,20 Since the crystallin...

In symmetry-broken crystalline solids, pole structures of Berry curvature (BC) can emerge, and they have been utilized as a versatile tool for controlling transport properties. For example, the monopole component of the BC is induced by the time-reversal symmetry breaking, and the BC dipole arises from a lack of inversion symmetry, leading to the anomalous Hall and nonlinear Hall effects, respectively. Based on first-principles calculations, we show that the ferroelectricity in a tin telluride monolayer produces a unique BC distribution, which offers charge- and spin-controllable photocurrents. Even with the sizable band gap, the ferroelectrically driven BC dipole is comparable to those of small-gap topological materials. By manipulating the photon handedness and the ferroelectric polarization, charge and spin circular photogalvanic currents are generated in a controllable manner. The ferroelectricity in group-IV monochalcogenide monolayers can be a useful tool to control the BC dipole and the nonlinear optoelectronic responses.

To magnetize surfaces of topological insulators without damaging their topological feature is a crucial step for the realization of the quantum anomalous Hall effect (QAHE) and remains as a challenging task. Through density functional calculations, we found that adsorption of a semiconducting two-dimensional van der Waals (2D-vdW) ferromagnetic CrI3 monolayer can create a sizable spin splitting at the Dirac point of the topological surface states of Bi2Se3 films. Furthermore, general rules that connect different quantum and topological parameters are established through model analyses. This work provides a useful guideline for the realization of QAHE at high temperatures in heterostructures of 2D-vdW magnetic monolayers and topological insulators.

Magnetic anisotropy often plays a central role in various static and dynamic properties of magnetic materials. In particular, for two-dimensional (2D) van der Waals materials, as inferred from the Mermin–Wagner theorem, it is an essential prerequisite for stabilizing ferromagnetic order. In this work, we carry out first-principles calculations for a CrI3 monolayer and investigate how its magnetic anisotropy is interrelated to adjustable parameters governing the underlying electronic structure. We explore various routes for controlled manipulation of magnetic anisotropy: chemical adsorption, substitutional doping, optical excitation, and charge transfer through a heterostructure. In particular, the vertical stacking of CrI3 and graphene is noteworthy in regard to controlling magnetic anisotropy: the spin anisotropy axis is switchable between the out-of-plane and in-plane directions, which is accompanied by a variation in the anisotropy energy of up to 500%. Our results show the possibility that dynamic control of the anisotropy of the 2D magnet CrI3 may enable the development of an advanced spintronic device with enhanced energy efficiency and high operation speed.

Material with a nontrivial topology in its electronic structure enforces the existence of helical Dirac fermionic surface states. We discover emergent topological phases in the stacked structures of topological insulator and band insulator layers where the surface Dirac fermions interact with each other with a particular helicity ordering. Using first-principles calculations and a model Lagrangian, we explicitly demonstrate that such helicity ordering occurs in real materials of ternary chalcogen compounds and determines their topological-insulating phase. Our results reveal the rich collective nature of interacting surface Dirac fermions and pave the way for utilizing topological phases for technological devices such as nonvolatile memories.