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<p>The emergence of charge-neutral collective modes is a hallmark of correlated quantum phases but is often challenging to probe. In two-dimensional flatband systems, charge responses have been intensively investigated, but neutral excitations have not been directly probed. In particular, the intervalley-coherent state has been proposed as a unifying theme across graphene- and semiconductor-based systems and features a neutral Goldstone mode due to spontaneously broken valley <i>U</i>(1) symmetry. However, this mode has not been observed. Here we demonstrate the transport of neutral modes in twisted WSe<sub>2</sub> moiré superlattices using a space-and-time-resolved ultrafast imaging technique. We show two propagating collective modes with different velocities that emerge near the Van Hove singularity. The fast-propagating mode is consistent with a Goldstone mode for an intervalley-coherent state, whereas the slow-moving mode is probably a gapped amplitude mode. They can be understood as the spin–valley analogues of the collective modes of a superfluid. Our study provides a powerful approach for probing neutral modes in quantum materials and offers key insights into the interplay between charge and spin–valley physics in moiré superlattices.</p>

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Observation of propagating collective spin–valley modes in twisted WSe2

  • 期刊论文

  • 发表日期：

    2026年5月7日

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  • Richen Xiong,
  • Yi Guo,
  • Chenxin Qin,
  • Taige Wang,
  • Fanzhao Yin,
  • Samuel L. Brantly,
  • Youngjoon Choi,
  • Junhang Qi,
  • Jinfei Zhou,
  • Zihan Zhang,
  • Melike Erdi,
  • Kenji Watanabe,
  • Takashi Taniguchi,
  • Shu Zhang,
  • Seth Ariel Tongay,
  • Andrea F. Young,
  • Liang Fu,
  • Chenhao Jin

摘要

The emergence of charge-neutral collective modes is a hallmark of correlated quantum phases but is often challenging to probe. In two-dimensional flatband systems, charge responses have been intensively investigated, but neutral excitations have not been directly probed. In particular, the intervalley-coherent state has been proposed as a unifying theme across graphene- and semiconductor-based systems and features a neutral Goldstone mode due to spontaneously broken valley U(1) symmetry. However, this mode has not been observed. Here we demonstrate the transport of neutral modes in twisted WSe2 moiré superlattices using a space-and-time-resolved ultrafast imaging technique. We show two propagating collective modes with different velocities that emerge near the Van Hove singularity. The fast-propagating mode is consistent with a Goldstone mode for an intervalley-coherent state, whereas the slow-moving mode is probably a gapped amplitude mode. They can be understood as the spin–valley analogues of the collective modes of a superfluid. Our study provides a powerful approach for probing neutral modes in quantum materials and offers key insights into the interplay between charge and spin–valley physics in moiré superlattices.