Double Quantum Spin Hall Phase in Moiré WSe₂

Quantum spin Hall (QSH) insulators are topologically protected phases of matter in two dimensions that can support 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. Here, we report experimental evidence for a QSH phase with one and two pairs of helical edge states in twisted bilayer WSe₂ at Moiré hole filling factor ν = 2 and 4, respectively. We observe nearly quantized (within 10%) resistance plateaus of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mfrac><mml:mrow><mml:mi>h</mml:mi></mml:mrow><mml:mrow><mml:mi>ν</mml:mi><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:mfrac></mml:math> and large nonlocal transport at ν = 2 and 4 while the bulk is insulating. The resistance is independent of the out-of-plane magnetic field and increases under an in-plane magnetic field. The results agree with quantum transport of helical edge states in a material with high spin Chern bands protected by Ising spin conservation symmetry.