Giant Spin-valley Polarization and Multiple Hall Effect in Functionalized Bi Monolayers

TL;DR

This study predicts that functionalized Bi monolayers exhibit simultaneous quantum spin Hall effects and valley Hall effects, with large band gaps and controllable Hall phenomena, opening new avenues for electronics, spintronics, and valleytronics applications.

Contribution

It demonstrates the coexistence of large-gap QSHE and VHE in Bi monolayers with functionalization and magnetic doping, a novel combination not previously explored.

Findings

Large-band-gap QSHE and VHE coexist in Bi2XY monolayers.

Gate control enables observation of multiple Hall effects.

Significant valley splitting up to 513 meV achieved.

Abstract

Valleytronic materials, characterized by local extrema (valley) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects (VHEs) appear simultaneously in the Bi monolayers decorated with halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong SOC of Bi atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized Bi monolayers in electronics, spintronics, and valleytronics.