QF Fellow and Associate seminar: Richen Xiong

Collective spin-valley modes in twisted bilayer WSe2: moire superlattices, intervalley coherence and spin superfluid 

Abstract: Quantum transport of charge has revealed a wealth of quantum phases, yet many other exotic quantum states in 2D flatland host charge-neutral modes, remaining challenging to probe transport experimentally. One example Is intervalley coherent states a unifying theme in graphene and semiconductor moire systems, predicted to host a neutral Goldstone mode arising from spontaneously broken valley U(1) symmetry. Here we directly image the transport of collective spin-valley modes in twisted WSe2 using an ultrafast imaging technique. Near the van Hove singularity, we uncover two new collective modes. One mode propagates at a large speed of ~3 km/s and is consistent with the Goldstone mode, while the slower mode is likely a gapped excitation. These can be interpreted as the phase and amplitude modes of a spin-valley superfluid. Our study opens new opportunities to study the transport of charge neutral modes in condensed matter systems.