UCSB Researchers Directly Observe Goldstone Modes in Twisted Tungsten Diselenide Superlattices

A team at the University of California, Santa Barbara has recorded, for the first time, Goldstone modes—charge-neutral collective excitations—in a twisted bilayer of tungsten diselenide (WSe2). Using an ultrafast pump-probe optical technique, the researchers excited the sample with a brief laser pulse and captured the ensuing space-time evolution with a delayed probe, effectively filming the quasiparticles’ dynamics. The superlattices were assembled with twist angles between 3.5° and 5°.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

Transition-metal dichalcogenides such as WSe2 exhibit strong two-dimensional electronic behavior. When two monolayers are stacked with a small relative twist, a moiré pattern forms, enhancing electron correlations and theoretically enabling exotic phases, including superconductivity and intervalley coherent states. Goldstone modes tied to intervalley order have been predicted for decades, but their charge-neutral nature prevents detection by conventional electrical or scanning-tunnelling methods, creating a persistent experimental gap.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

The work was led by principal investigator Chenhao Jin, with co-author Xiong and an interdisciplinary group of theorists and materials scientists from multiple institutions. The team synthesized high-quality twisted WSe2 heterostructures, precisely controlled the twist angle, and integrated the optical setup with theoretical modeling to identify the observed excitations.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

• Twist angles: 3.5°, 4°, and 5° between WSe2 layers.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

• Technique: Sub-picosecond pump-probe optics.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

• Observation: Direct imaging of an intervalley coherent (IVC) state and its Goldstone mode, a phase-only oscillation that does not generate charge current.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

• Result: Valley polarization propagates without resistance while charge transport remains insulating.

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Visualizing dissipationless valley currents offers a pathway to valleytronic devices that encode information in the valley degree of freedom rather than charge. This could enable ultralow-power electronics, quantum computing, and new control over quantum phases in two-dimensional materials. The approach also establishes ultrafast optics as a versatile probe for other charge-neutral quasiparticles in moiré systems.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

Jin noted that the emergence of collective excitations beyond simple metallic behavior highlights the richness of interacting electron systems. Xiong said the shift from static to dynamical measurements allows optical probing of phenomena invisible to charge-based techniques. Both described the work as a paradigm shift linking theory with empirical validation.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted

• “Among the most intriguing consequences of interacting electrons are the emergence of new collective excitations that are qualitatively different free electrons, which lead to exotic material properties beyond simple metal,” — Chenhao Jin, Lead Investigator

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• “What we’re doing is using optics to probe the space-and-time-resolved properties of the material,” — Xiong, Co-author

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• “Our method is different from the traditional method in that we want to approach these things from a dynamical view,” — Xiong, Co-author

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The team will apply ultrafast real-space imaging to other two-dimensional and moiré materials, seeking correlated phenomena such as unconventional superconductivity and topological quantum matter. Advancing optical probes may accelerate discoveries across magnetism, quantum fluid dynamics, and related fields.

First Direct Observation of Exotic Charge-Neutral Quasiparticles in Twisted