Microscopic signatures of Chern number sign reversal in twisted bilayer WSe2

TL;DR

This paper experimentally demonstrates that the Chern numbers in twisted bilayer WSe2 undergo a sign reversal at a specific twist angle, linking microscopic layer-pseudospin textures to topological properties.

Contribution

First direct experimental verification of twist-angle-dependent Chern number sign reversal in twisted WSe2 bilayers using microscopy and spectroscopy.

Findings

Chern number sign reverses at 1.42° twist angle

Layer-pseudospin textures correlate with Chern number changes

Microscopic layer-pseudospin polarization explains sign reversal

Abstract

The discovery of quantized Chern numbers in twisted transition metal dichalcogenide (TMD) homobilayers,including 3.7{deg} twisted MoTe2 and 1.23{deg} twisted WSe2,has emerged as a defining breakthrough in physics. A striking and unresolved puzzle from these studies is the unexpected opposite sign of the observed Chern numbers between the two systems. Recent theory has proposed a twist-angle-dependent Chern number sign reversal in both twisted MoTe2 and WSe2, offering a potential explanation for the disparate experimental observations6. However, a direct experimental verification of the twist-angle-dependent Chern number sign reversal in a specific twisted TMD homobilayer is still elusive. Here, we report the first experimental demonstration that the Chern numbers of the moire frontier bands undergo sign reversal at a critical twist angle 1.42{deg} in twisted WSe2 bilayers (tWSe2). Using scanning tunnelling microscopy and spectroscopy, we direct measure layer-pseudospin skyrmion textures of tWSe2 and our results reveal that tWSe2 in the vicinity of the 1.42{deg} exhibits a twist-angle-dependent layer-pseudospin polarization, an effect that serves as the fundamental origin of the observed Chern number sigh reversal6-12.