In a groundbreaking experiment, researchers at the University of California, Santa Barbara, have achieved a milestone in condensed matter physics by capturing for the first time the elusive Goldstone modes within a twisted tungsten diselenide superlattice. These charge-neutral quasiparticles, theorized for decades, are manifestations of collective quantum phenomena that underpin extraordinary behaviors such as superconductivity and exotic quantum phases. Using an innovative ultrafast optical imaging technique, the research team has shed unprecedented light on the dynamic evolution of these quantum excitations, overcoming previous experimental limitations associated with their charge neutrality.
The study focuses on tungsten diselenide (WSe2), a transition metal dichalcogenide (TMD) known for its distinct electronic characteristics in two-dimensional form. By stacking atomically thin sheets of WSe2 at carefully calibrated twist angles between 3.5° and 5°, the researchers engineered moiré superlattices where the electrons’ quantum mechanical wavefunctions from different layers interact in novel ways. These twisted bilayer systems serve as fertile grounds for emergent quantum phenomena, with electron correlations becoming significantly enhanced due to reduced kinetic energy bandwidths and modulated interlayer coupling.
Detecting Goldstone modes, especially in charge-neutral systems, presents profound experimental challenges because their oscillations do not produce an electric current, which is the signal source for conventional detection methods like scanning tunneling microscopy or electrical transport measurements. To circumvent this, the team employed a cutting-edge combination of pump-probe ultrafast optics—a technique that uses synchronized bursts of laser light to first excite the system and then probe its transient, time-resolved response. This approach effectively records a “movie” of the quasiparticles’ space-time evolution, allowing direct observation of collective excitations that reflect changes in quantum phase coherence rather than charge flow.
Central to the findings is the observation of an intervalley coherent (IVC) state, a quantum superposition in which electrons residing in distinct valleys—energy minima in the momentum space of the material—synchronize their phases, giving rise to macroscopic coherence across the system. The Goldstone mode detected corresponds to slow, dissipationless variations in the relative phase of this valley coherence, effectively functioning as a superfluid-like transport channel for valley polarization rather than electrical charge. Remarkably, this implies that valley polarization currents can flow without resistance even when the system remains insulating to charge transport, highlighting fundamental novel conduits for quantum information and energy transfer.
The successful measurement of these modes marks a paradigm shift in understanding the complex quantum phases emerging from electron interactions in twisted TMD superlattices. The experimental data reveal the intricate dance of electronic degrees of freedom in moiré-engineered materials and forge a vital link between theoretical predictions and empirical validation. By directly tracing the dynamical response of these collective excitations, the study unlocks the potential to manipulate quantum states in two-dimensional moiré materials with a precision and versatility unattainable through static measurements alone.
Moreover, the implications of this research extend beyond fundamental physics. The ability to harness dissipationless valley currents suggests new architectures for ultralow-power electronic and spintronic devices, where information can be encoded and transmitted without the typical energy losses associated with charge movement. This capability could revolutionize technologies in quantum computing, communication, and energy-efficient electronics by enabling control over quantum coherence and collective modes in scalable solid-state platforms.
The experimentation was supported by an interdisciplinary team including theorists and materials scientists working collaboratively across institutions. The team synthesized the high-quality twisted WSe2 heterostructures and carefully controlled the twist angle to achieve the desired moiré periodicity. Sophisticated ultrafast optical setups captured the transient responses, and thorough theoretical modeling confirmed the identification of Goldstone modes tied to intervalley order. This synergy between synthesis, measurement, and theory exemplifies the holistic approach essential for unraveling complex quantum phenomena in novel materials.
This research not only highlights the unique properties of transition metal dichalcogenide superlattices but also underscores the transformative power of new experimental methodologies in condensed matter physics. By stepping beyond traditional optics and electrical characterizations, the laser-based space-time resolved probing demonstrated here opens avenues for exploring other exotic quasiparticles and emergent phases that lack straightforward charge signatures but play pivotal roles in emergent material behaviors.
Chenhao Jin, the lead principal investigator of the study, emphasized that these discoveries deepen the understanding of the interactions governing correlated electrons in moiré structures. Unveiling the dynamical properties of intervalley coherent states and their associated Goldstone excitations provides crucial insights into how quantum materials respond to external perturbations. These insights are critical for designing materials with tailored functionalities, ultimately enabling control over novel quantum phases with minimal energy input.
The conceptual breakthrough of observing supercurrent analogs in spin-valley superfluids—where the collective modes facilitate lossless polarization transport while the charge carriers themselves remain immobile—challenges conventional wisdom about transport and coherence in condensed matter systems. It suggests intriguing possibilities for engineering superfluid-like phenomena in solid-state devices and fosters the integration of valleytronics with quantum fluid dynamics.
Finally, the success of this work paves the way for future explorations of correlated quantum phenomena using ultrafast, real-space imaging in a variety of two-dimensional and moiré materials. The methods and insights established here are expected to accelerate discovery across fields as diverse as superconductivity, magnetism, and topological quantum matter, positioning ultrafast optics at the forefront of quantum material research.
Subject of Research: The dynamic behavior of Goldstone modes and intervalley coherent states in twisted tungsten diselenide (WSe2) moiré superlattices.
Article Title: First Observation of Goldstone Modes in Twisted Tungsten Diselenide Superlattices Reveals Dissipationless Valley Transport
News Publication Date: Not provided
Web References:
– Nature Physics paper: https://www.nature.com/articles/s41567-026-03280-w
– DOI: http://dx.doi.org/10.17605/OSF.IO/X6NJ2
References:
– Research conducted by Yi Guo, Chenxin Qin, Fanzhao Yin, Samuel L. Brantley, Youngjoon Choi, Junhang Qi, Jinfei Zhou, Zihan Zhang, Andrea F. Young (UCSB), Taige Wang (UC Berkeley), Melike Erdi, Seth Ariel Tongay (ASU), Liang Fu (MIT), Kenji Watanabe, Takashi Taniguch (NIMS Japan), Shu Zhang (OIST Japan).
Image Credits: Not specified
Keywords: Goldstone modes, twisted bilayer, tungsten diselenide, transition metal dichalcogenides, moiré superlattices, intervalley coherence, ultrafast optics, condensed matter physics, valley polarization, spin-valley superfluid, quantum materials, two-dimensional materials
