🔬 mesoscale physics

Significant modulation of acoustoelectric current associated with charge density wave transitions

This study demonstrates that charge density wave transitions in NbSe $_3$ and 2H-TaSe $_2$ significantly modulate the polarity and magnitude of acoustoelectric currents induced by surface acoustic waves, a phenomenon explained by a strain-modified conductivity model and offering a powerful tool for advancing straintronics in van der Waals materials.

Original authors: Natsumi Nikaido, Takuya Kawada, Koji Fujiwara, Jihoon Park, Nan Jiang, Kouta Kondou, Shintaro Takada, Yasuhiro Niimi

Published 2026-08-31

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CC BY 4.0

Original authors: Natsumi Nikaido, Takuya Kawada, Koji Fujiwara, Jihoon Park, Nan Jiang, Kouta Kondou, Shintaro Takada, Yasuhiro Niimi

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 world where electricity can be generated not by batteries or solar panels, but by the gentle, invisible ripples of sound traveling across a solid surface. This is the realm of acoustoelectricity, a phenomenon where sound waves, specifically those too high-pitched for human ears to hear, interact with the flow of electrons in a material. When these sound waves, known as surface acoustic waves, travel along a special crystal, they create a tiny electric current. For decades, scientists have used this effect to build devices for communication and sensing. However, a new frontier has opened up with the discovery of van der Waals materials, a class of crystals that can be peeled apart into sheets as thin as a single atom. These ultra-thin sheets can host exotic states of matter where electrons move in synchronized patterns, creating what are called charge density waves. The question researchers have been asking is whether these sound waves can do more than just push electrons along; can they actually change the very nature of how these electrons move, especially when the material is in one of these special, synchronized states?

A team of physicists at the University of Osaka and the University of Tokyo set out to answer this by listening to the electrical response of two specific materials: niobium triselenide and a form of tantalum diselenide. These materials are famous for undergoing a phase transition, a change in their internal structure where the electrons lock into a periodic pattern, much like a crowd of people suddenly deciding to march in perfect step rather than walking randomly. The researchers placed thin flakes of these materials onto a piezoelectric substrate, a type of crystal that turns electrical signals into mechanical vibrations. They then sent a high-frequency sound wave across the surface of this crystal, which in turn sent a ripple through the thin material sitting on top. By measuring the voltage that appeared across the material, they could see exactly how the sound wave was driving the electric current.

What they found was a dramatic and unexpected shift in behavior. As they cooled the materials down to temperatures where the electrons began to march in step, the electric current generated by the sound wave did not just get stronger; it flipped its direction. In the normal state, the current flowed one way, but once the material entered its synchronized charge density wave state, the current reversed and flowed the opposite way. This reversal happened at specific temperatures where the phase transitions occurred, acting like a clear signal that the sound wave was interacting with the new, ordered state of the electrons. Furthermore, the researchers discovered that the direction of the current depended entirely on which way the sound wave was traveling relative to the crystal's internal structure. If they sent the sound wave along one axis of the crystal, the current flowed one way; if they rotated the setup and sent the sound wave along a perpendicular axis, the current flipped again.

To understand why this was happening, the team proposed a model based on how the material's ability to conduct electricity changes when it is squeezed or stretched. The sound wave traveling through the crystal creates a tiny, rhythmic stretching and compressing of the material, a dynamic strain. In these special materials, this stretching seems to alter how easily the electrons can move, and this change is different depending on the temperature and the direction of the wave. The researchers suggest that the electric current is generated by a rectification process, where the combination of the sound-induced electric field and the mechanical strain creates a net flow of charge. Their calculations showed that the effect of this strain on the material's conductivity was massive, far larger than what has been seen in previous studies using static, non-moving pressure. This suggests that the rapid, vibrating nature of the sound wave is key to unlocking this strong response.

The study also ruled out several other possibilities. The researchers confirmed that the effect was not caused by the material heating up, as the temperature changes from the tiny currents were negligible. They also verified that the sound wave was not strong enough to break the synchronized electron pattern, meaning the effect was a subtle modulation of the existing state rather than a destruction of it. By comparing their results with a simple metal film that does not have these special electron patterns, they showed that the dramatic flipping and enhancement of the current were unique to the materials with charge density waves. This work provides a powerful new tool for exploring how sound and electricity interact in the thinnest possible materials, opening the door to a field of research where mechanical strain is used to control electronic properties, potentially leading to new types of sensors and electronic devices that operate on the principles of sound and vibration.

Problem Statement
Surface acoustic waves (SAWs) serve as a versatile tool for probing electronic states in various materials, yet characterizing SAW-electron interactions in microscale van der Waals (vdW) flakes presents significant challenges. Conventional SAW transmission measurements are often ineffective because the lateral dimensions of exfoliated vdW flakes are typically orders of magnitude smaller than the SAW propagation region, resulting in minimal interaction. While acoustoelectric (AE) current measurements offer an alternative probe for microscale samples, their application to materials undergoing charge density wave (CDW) transitions remains underexplored. Specifically, the dynamic coupling between SAW-induced strain and CDW states, and how this interaction modulates AE currents, requires systematic investigation.

Methodology
The study investigated AE currents in two prototypical CDW materials: quasi-one-dimensional NbSe $_3$ and two-dimensional 2H-TaSe $_2$ . Thin flakes of these materials were mechanically exfoliated onto 128 $^\circ$ Y-cut LiNbO $_3$ piezoelectric substrates. Device structures included interdigital transducers (IDTs) for SAW generation and four-terminal contacts for electrical measurements.

Key experimental parameters included:

SAW Generation: RF signals were applied to IDTs to excite SAWs, with transmission coefficients ( $S_{21}$ ) characterized via vector network analysis.
Measurement Geometry: Experiments compared SAW propagation along the substrate's crystallographic X-axis versus the X + 90 $^\circ$ axis to isolate directional dependencies.
Conditions: Measurements were conducted across a temperature range (50 K to 190 K) to capture CDW phase transitions. AE voltage ( $V_{AE}$ ) was measured using lock-in detection with amplitude-modulated RF signals to enhance signal-to-noise ratios.
Controls: Metallic Ta thin films were measured as non-CDW controls to distinguish CDW-specific effects from general acoustoelectric phenomena.
Data Processing: The AE current ( $\tilde{I}_{AE}$ ) was derived by normalizing the measured AE voltage by the film resistance ( $R$ ) and SAW power ( $P_{SAW}$ ).

Key Contributions and Results

CDW-Induced Modulation of AE Current:
- In NbSe $_3$ , the AE current exhibited a distinct peak at the first CDW transition temperature ( $T_1 \approx 140$ K) and a more pronounced increase below the second transition ( $T_2 \approx 60$ K). Notably, the polarity of the AE current reversed specifically around $T_1$ , remaining negative in other temperature regions.
- In 2H-TaSe $_2$ , the AE current increased below the incommensurate CDW transition ( $\approx 120$ K) and showed a broad peak near the commensurate transition ( $\approx 90$ K). The sign of the AE current was positive in the temperature range from 115 K to 65 K, while it became negative outside this range.
- In contrast, control Ta devices showed no temperature-dependent polarity reversal or significant enhancement, confirming that these effects are intrinsic to the CDW states.
Propagation Direction Dependence:
- The polarity of the AE current was found to depend on the SAW propagation direction relative to the substrate's crystallographic axes. The AE current in the NbSe $_3$ device with propagation along the X-axis exhibited opposite polarity to that observed in the device with propagation along the X + 90 $^\circ$ axis over almost the entire temperature range.
- This observation indicates that both the electric field and the strain field components of the SAW contribute to the AE current generation, a nuance not fully captured by conventional models.
Phenomenological Modeling:
- The authors proposed a model where the AE current arises from the rectification of the SAW-induced electric field ( $E_x$ ) via strain-induced modulation of electrical conductivity ( $\sigma$ ).
- The model assumes $\sigma(\varepsilon_{xx}) \approx \sigma_0 + (\partial\sigma/\partial\varepsilon_{xx})\varepsilon_{xx}$ , where $\varepsilon_{xx}$ is the longitudinal strain.
- The time-averaged current is proportional to $(\partial\sigma/\partial\varepsilon_{xx}) \cos \phi$ , where $\phi$ is the phase difference between the strain and electric field.
- This model qualitatively reproduces the experimental observations, including the sign reversal (attributed to the anisotropic piezoelectric constants of LiNbO $_3$ affecting $\cos \phi$ ) and the magnitude enhancement near CDW transitions (attributed to a large strain sensitivity of conductivity, $\partial\sigma/\partial\varepsilon_{xx}$ ).
Quantitative Findings:
- The estimated strain-induced conductivity modulation factor, $(1/\sigma_0)(\partial\sigma/\partial\varepsilon_{xx})$ , was found to be orders of magnitude larger for NbSe $_3$ and 2H-TaSe $_2$ (reaching $10^3$ – $10^4$ and $10^2$ , respectively) compared to metallic Ta ( $\approx -2$ ).
- The magnitude of this modulation in the current study was significantly larger than values reported in previous studies on bulk NbSe $_3$ under static strain, suggesting that dynamic (AC) strain or thin-film effects play a critical role.

Significance
The paper claims that these results provide a powerful probe for exploring SAW-electron interactions specifically within van der Waals materials. By demonstrating that AE currents are highly sensitive to CDW transitions and SAW propagation orientation, the study highlights the potential of these systems for advancing the emerging field of "straintronics." The work establishes that dynamic strain can significantly modulate the transport properties of CDW materials, offering a new avenue for controlling electronic states in low-dimensional systems. The authors note that while their phenomenological model accounts for the observed trends, a complete theoretical explanation for the temperature and propagation-direction dependencies remains an open issue for future research.

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