Bilayer graphene encapsulated within monolayers of WS$_2$ or Cr$_2$Ge$_2$Te$_6$: Tunable proximity spin-orbit or exchange coupling

TL;DR

This study explores how encapsulating bilayer graphene with WS$_2$ or Cr$_2$Ge$_2$Te$_6$ allows for tunable spin-orbit and exchange interactions via twist angles, magnetization, and gate fields, with potential applications in spintronics.

Contribution

It provides first-principles models of proximity-induced spin interactions in bilayer graphene heterostructures, demonstrating tunability through structural and electric control.

Findings

Twisting WS$_2$ layers can turn off spin splittings away from $K$ points.

Gate fields can flip out-of-plane spin polarization near $K$ points.

Antiparallel magnetizations in CGT layers open a tunable gap in BLG.

Abstract

Van der Waals (vdW) heterostructures consisting of bilayer graphene (BLG) encapsulated within monolayers of strong spin-orbit semiconductor WS$_2$ or ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (CGT), are investigated. By performing realistic first-principles calculations we capture the essential BLG band structure features, including layer- and sublattice-resolved proximity spin-orbit or exchange couplings. For different relative twist angles (0 or 60$^{\circ}$) of the WS$_2$ layers, and the magnetizations (parallel or antiparallel) of the CGT layers, with respect to BLG, the low energy bands are found and characterized by a series of fit parameters of model Hamiltonians. These effective models are then employed to investigate the tunability of the relevant energy dispersions by a gate field. For WS$_2$/BLG/WS$_2$ encapsulation we find that twisting allows to turn off the spin splittings away from the $K$ points, due to opposite proximity-induced valley-Zeeman couplings in the two sheets of BLG. Close to the $K$ points the electron spins are polarized out of the plane. This polarization can be flipped by applying a gate field. As for magnetic CGT/BLG/CGT structures, we realize the recently proposed spin-valve effect, whereby a gap opens for antiparallel magnetizations of the CGT layers. Furthermore, we find that for the antiferromagnetic orientation the electron states away from $K$ have vanishingly weak proximity exchange, while the states close to $K$ remain spin polarized in the presence of an electric field. The induced magnetization can be flipped by changing the gate field. These findings should be useful for spin transport, spin filtering, and spin relaxation anisotropy studies of BLG-based vdW heterostructures.