🔬 mesoscale physics

Propagating Collective Spin-valley Modes in Twisted WSe2

Using a novel ultrafast imaging technique, researchers observed the space- and time-resolved propagation of neutral collective spin-valley modes, including a fast Goldstone mode and a slow amplitude mode, in twisted WSe2 moiré superlattices, providing the first direct experimental evidence of intervalley coherent states in condensed matter systems.

Original authors: 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 Ari

Published 2026-06-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: 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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a crowded dance floor where everyone is moving in perfect sync. Usually, when we study these crowds (which in physics are electrons in a material), we only watch the people who are carrying heavy bags (electric charge). But what about the dancers who are just spinning in place, holding hands with their neighbors, without carrying any bags? These "bag-free" dancers represent a mysterious type of movement called neutral collective modes. For a long time, scientists could hear the music of these dancers but couldn't actually see them moving across the floor.

This paper is like a high-speed, super-slow-motion camera that finally captures these invisible dancers in action. Here is the story of what they found, explained simply:

The Stage: Twisted WSe₂

The scientists used a special material called Twisted WSe₂. Imagine taking two sheets of a honeycomb-patterned fabric (like graphene or this material) and stacking them on top of each other, but twisting one slightly. This creates a new, giant "moiré" pattern (like the rippling effect you see when two window screens overlap). In this twisted state, the electrons get stuck in a "flatband," meaning they move very slowly and interact intensely with each other, like a crowded dance floor where everyone is bumping into neighbors.

The Mystery: The "Exotic" Dancers

In these materials, scientists predicted a special state called an Intervalley Coherent (IVC) state. Think of this as a state where electrons from two different "valleys" (two different neighborhoods in the material) decide to hold hands and form a super-coordinated group.

The Prediction: When this happens, a special "Goldstone mode" should appear. In simple terms, if you push one part of this coordinated group, the whole group should ripple through the material like a wave in a stadium crowd, without any of the individual people actually running. This wave carries "spin-valley" information (a type of internal spin) but no electric charge.
The Problem: No one had ever seen this wave move. It was like knowing a sound exists but never seeing the sound waves travel.

The Experiment: The Ultrafast Camera

The team built a new "camera" using ultrafast laser pulses.

The Pump (The Push): They hit the material with a quick flash of light (the pump) to wake up the electrons.
The Probe (The Snapshot): A split-second later, they took a picture with a second flash of light (the probe) to see where the excitement had moved.
The Trick: By changing the angle of the light and the magnetic field, they could filter out the "loud" charged dancers and only watch the "quiet" neutral dancers.

The Discovery: Two New Waves

When they watched the material, they didn't just see one thing; they saw two distinct types of waves moving out from where they hit the material:

The "Ordinary" Dancer (Diffusion): This is the standard behavior. If you drop a dye in water, it spreads out slowly and gets blurry. This is how normal spin usually moves. It's slow and messy.
The "Fast" Dancer (The Goldstone Mode): This was the big surprise. One wave zoomed across the material at about 3 kilometers per second (roughly 6,700 mph!). It moved almost like a bullet or a perfect wave packet, keeping its shape as it traveled.
- The Analogy: Imagine a superfluid (like liquid helium that flows without friction). If you heat up a spot in a superfluid, a "hole" in the fluid moves out incredibly fast because the fluid rushes to fill it. The scientists found that the fast wave in their material behaves exactly like this "superfluid" wave. It is the Goldstone mode they were looking for.
The "Slow" Dancer (The Higgs Mode): There was a second exotic wave, but it moved much slower, more like the ordinary diffusion. This is likely an "amplitude mode" (sometimes called a Higgs mode), which is like the rhythm of the dance changing rather than the direction.

Why This Matters

Seeing the Invisible: This is the first time scientists have directly imaged these neutral waves moving through a solid material. Before, they could only guess they were there based on indirect clues.
The "Superfluid" Connection: The fact that the fast wave moves so quickly and carries spin without charge suggests that the electrons in this twisted material are acting like a spin-valley superfluid. Just as superconductors conduct electricity without resistance, this material conducts "spin" without resistance.
Solving a Puzzle: This discovery helps explain why these materials sometimes become superconductors (conduct electricity with zero loss). The "dance" of these neutral waves seems to be a key ingredient in that process.

In a Nutshell

The scientists used a high-speed laser camera to watch electrons in a twisted material. They discovered that when the electrons form a special coordinated group, they create a "super-wave" that zooms across the material at incredible speeds, carrying information but no electric charge. It's like watching a stadium wave travel perfectly across a crowd without anyone actually leaving their seat, proving that these materials can act like a "superfluid" for spin.

Technical Summary: Propagating Collective Spin-valley Modes in Twisted WSe2

Problem Statement
The identification of correlated quantum phases in two-dimensional flatband systems has historically relied on charge transport measurements, which provide direct information on low-energy charge responses. However, a significant experimental gap exists in probing emergent charge-neutral modes, which are central to phenomena such as intervalley coherent (IVC) states, superfluidity, and quantum spin liquids. Specifically, the IVC state, characterized by the spontaneous breaking of valley $U(1)$ symmetry, is predicted to host a neutral Goldstone mode. While signatures of IVC states have been inferred from symmetry-broken gaps and field-tunability in graphene and transition metal dichalcogenide (TMD) systems, the defining feature of this state—the linearly dispersing neutral Goldstone mode—has remained elusive. Conventional scanning tunneling microscopy (STM) is often inapplicable to twisted TMD bilayers because the IVC state in these systems involves a coherent superposition of opposite spins in the $K$ and $K'$ valleys, creating a spin texture without appreciable charge density modulation. Furthermore, standard transport measurements lack the space-and-time resolution necessary to distinguish coexisting neutral modes from charged quasiparticles or to separate multiple neutral modes with different dynamics.

Methodology
To address these challenges, the authors employ a novel ultrafast imaging technique capable of space-and-time-resolved transport measurements of neutral modes in twisted WSe $_2$ (tWSe $_2$ ) moiré superlattices. The experimental setup utilizes a dual-gated device configuration with dual-gate electrodes to tune carrier density ( $n$ ) and electric field ( $E$ ).

Excitation: A femtosecond pump pulse (1.88 eV) creates excitations. The pump profile is engineered to be either homogeneous (to study local dynamics) or line-shaped (to investigate one-dimensional transport perpendicular to the line).
Detection: A delayed, wide-field probe pulse (1.67 eV) captures full images of the excitations without scanning.
Mode Separation: The technique selectively probes spin-valley excitations ( $S_z$ ) distinct from charge excitations. By utilizing circularly polarized (RCP/LCP) and unpolarized pump lights, and analyzing the response through a "near-cross" polarization detection configuration (sensitive to magnetic circular dichroism), the authors separate "ordinary" modes (single-particle spin-valley imbalances) from "exotic" modes (collective excitations).
Analysis: The spatial evolution of the excitations is tracked over time (nanosecond scale) to extract propagation velocities and lifetimes. The data is compared against diffusion-decay models and a minimum hydrodynamic theory describing phase (Goldstone) and amplitude (Higgs) modes in a spin-valley superfluid.

Key Contributions and Results
The study investigates tWSe $_2$ devices with twist angles of 3.5°, 3.8°, and 5.0°. The primary findings include:

Discovery of Two Propagating Collective Modes: In the vicinity of the van Hove singularity (VHS), the authors identify two distinct "exotic" modes that propagate with opposite signs of out-of-plane spin-valley polarization ( $S_z$ ) and vastly different velocities:
- Fast Mode: Propagates at approximately 3 km/s. Its velocity and non-diffusive, ballistic-like characteristics are consistent with a Goldstone mode arising from the spontaneous breaking of valley $U(1)$ symmetry.
- Slow Mode: Propagates significantly slower (diffusive behavior) and is identified as a gapped amplitude mode (analogous to a Higgs mode).
Evidence for Intervalley Coherent (IVC) States: The emergence of these modes is strictly correlated with the VHS region and disappears outside this regime. The fast mode's linear dispersion and ability to transport $S_z$ efficiently via a "spin-valley supercurrent" provide strong evidence for an IVC ground state. The observation of both phase and amplitude modes mirrors the behavior of superfluids, where the phase mode corresponds to the Goldstone mode and the amplitude mode to the gapped Higgs mode.
Temperature and Magnetic Field Dependence:
- The exotic modes are highly temperature-sensitive, disappearing at approximately 10 K (fast mode) and 20 K (slow mode), whereas the ordinary single-particle mode persists up to higher temperatures.
- The amplitude of the exotic modes exhibits non-monotonic magnetic field dependence, being suppressed at high fields ( $B_z > 2$ T) where the system transitions to a valley-polarized state. This behavior contrasts with the monotonic increase of equilibrium magnetization, ruling out simple paramagnetic responses or single-particle excitations.
Universality Across Twist Angles: The phenomenon is observed in both large-angle (5°) and intermediate-angle (3.5°–4°) twisted samples, suggesting the IVC state is a robust feature of the tWSe $_2$ phase diagram near the VHS. Notably, the spin-valley transport signatures do not show strong disruptions at the $\nu = -1$ filling, despite the presence of insulating states in electrical transport, suggesting the IVC state may coexist with or underlie other correlated phases.

Significance
The paper claims to provide the first direct imaging of the propagation of collective spin-valley modes in a condensed matter system. By resolving the space-and-time evolution of these modes, the authors successfully isolate the neutral Goldstone mode of an IVC state, a feature that has been theoretically predicted but experimentally elusive. The study demonstrates that ultrafast imaging is a powerful approach for probing charge-neutral modes that are otherwise obscured in conventional transport measurements by dominant charge responses. The identification of IVC states in tWSe $_2$ offers critical insights into the interplay between charge and spin-valley physics in moiré superlattices and suggests a potential connection between IVC order and the superconductivity observed in these systems. The methodology opens new avenues for investigating other charge-neutral phenomena, such as excitonic insulators and fractional quantum anomalous Hall states, where multiple low-energy modes coexist.