Propagating Neutral Modes in an Intervalley Coherent State

TL;DR

This study uses ultrafast imaging to observe neutral collective modes in twisted WSe2 moiré superlattices, revealing a Goldstone mode and a gapped amplitude mode, advancing understanding of neutral excitations in quantum materials.

Contribution

It provides the first direct imaging of propagating neutral modes, including a Goldstone mode, in twisted WSe2, demonstrating their velocities and nature in moiré systems.

Findings

Detected two propagating neutral modes with different velocities.

Identified the fast mode as a Goldstone mode of an IVC state.

Observed modes near the van Hove singularity in twisted WSe2.

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.