Observation of the double quantum spin Hall phase in moiré WSe2

Kavli Affiliate: Jie Shan

| First 5 Authors: Kaifei Kang, Yichen Qiu, Kenji Watanabe, Takashi Taniguchi, Jie Shan

| Summary:

Quantum spin Hall (QSH) insulators are a topologically protected phase of
matter in two dimensions that can support non-dissipative spin transport. A
hallmark of the phase is a pair of helical edge states surrounding an
insulating bulk. A higher (even) number of helical edge state pairs is usually
not possible in real materials because spin mixing would gap out the edge
states. Multiple pairs of helical edge states have been proposed in materials
with spin conservation symmetry and high spin Chern bands, but remained
experimentally elusive. Here, we demonstrate a QSH phase with one and two pairs
of helical edge states in twisted bilayer WSe2 at moir’e hole filling factor
{nu}= 2 and 4, respectively. We observe nearly quantized conductance or
resistance plateaus of h/({nu}e^2 ) at {nu} = 2 and 4 while the bulk is
insulating. The conductance is nearly independent of out-of-plane magnetic
field and decreases under an in-plane magnetic field. We also observe nonlocal
transport, which is sensitive only to the in-plane magnetic field. The results
agree with quantum transport of helical edge states protected by Ising spin
conservation symmetry and open a promising platform for low-power spintronics.

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