Propagating Neutral Modes in an Intervalley Coherent State
Abstract
The emergence of neutral collective modes is a hallmark of correlated quantum phases but is often challenging to probe experimentally. In two-dimensional flatband systems, charge responses have been intensively investigated, yet neutral excitations remain largely unexplored. In particular, intervalley coherent state (IVC) features a neutral Goldstone mode due to spontaneously broken valley U(1) symmetry. While IVC state has been proposed as a unifying theme across graphene- and semiconductor-based systems, its defining feature - the neutral Goldstone mode - remains elusive in experiment. Here we investigate space-and-time-resolved transport of neutral modes in twisted WSe2 moir\'e superlattices through a novel ultrafast imaging technique. We uncover two new propagating collective modes with very different velocities, which emerge near the van Hove singularity (VHS) in both intermediate- (3.5~4 degree) and large-angle (~5 degree) twisted WSe2. The fast-propagating mode has a surprisingly large speed of ~3 km/s and is consistent with a Goldstone mode for an IVC state, while the slow-moving mode is likely a gapped amplitude mode. They can be understood as the spin-valley analogues of collective modes of a superfluid, whose propagation are imaged for the first time in a condensed matter system. Our study sets a new paradigm for probing charge-neutral modes in quantum materials and offers key insights into the interplay between charge and spin-valley physics in moir\'e superlattices.
Cite
@article{arxiv.2507.18770,
title = {Propagating Neutral Modes in an Intervalley Coherent State},
author = {Richen Xiong and Yi Guo and Chenxin Qin and Fanzhao Yin and Taige Wang and Samuel L. Brantly and Junhang Qi and Jinfei Zhou and Zihan Zhang and Melike Erdi and Kenji Watanabe and Takashi Taniguchi and Shu Zhang and Seth Ariel Tongay and Andrea F. Young and Liang Fu and Chenhao Jin},
journal= {arXiv preprint arXiv:2507.18770},
year = {2025}
}