Quantization of electric current by surface acoustic waves — University of Osaka discovery raises hopes for new strain-based devices

A research group including Graduate Student Koji Fujiwara (at the time of research), Associate Professor Shintaro Takada, and Professor Yasuhiro Niimi from the Department of Physics, Graduate School of Science at the University of Osaka has discovered that applying strain to NbSe₃, a one-dimensional chain compound which exhibits a charge density wave state, through the use of surface acoustic waves (SAWs) —a type of sound wave—causes plateau structures indicating quantization to appear in current-voltage characteristics. Their findings were published in Physical Review Letters.

Within quantum mechanics, there is a well-known phenomenon called Shapiro steps, where voltage is precisely quantized using combinations of fundamental constants, such as Planck's constant and the elementary charge. Owing to robustness and high precision, Shapiro steps are used as a voltage standard. The quantum Hall effect also showed that quantized resistance can similarly be described using Planck's constant and the elementary charge.

On the other hand, while various studies on current standards have progressed, no standards have been established to date.

The research group used adhesive tape to mechanically exfoliate NbSe₃, a one-dimensional chain compound that exhibits charge density wave transitions and cleaved it (split it in a specific direction) to fabricate thin wires. They transferred these thin wires onto LiNbO₃, a piezoelectric substrate, and applied an AC voltage of approximately 300 MHz to interdigital electrodes prepared on the substrate, thereby measuring DC current-voltage characteristics while irradiating the NbSe₃ thin wires with SAWs.

Charge density waves are pinned to defects in the crystal up to a certain threshold current, but they become unpinned when above this threshold and begin to move collectively. In this study, the researchers discovered that when SAWs are applied, the current from the collectively moving charge density waves exhibits plateau structures at integer multiples of a certain value. Furthermore, they found that these plateau structures decay rapidly when the power of the applied SAWs is increased. This result diverges significantly from previous findings obtained when an AC electric field was applied directly to NbSe₃ samples.

Subsequent experiments that varied the frequency and experiments that applied pulsed SAWs identified that the quantized current values are caused by strain induced in the NbSe₃ thin wires by the SAWs.

Further research progression on quantized current utilizing strain effects is expected through the irradiation of one-dimensional chain compounds exhibiting charge density wave states with acoustic waves. Additionally, this opens the way to applications in strain-induced quantum devices, raising hopes for the fabrication of new types of devices.

Niimi commented: "This research is based on results our group reported five years ago, showing that negative resistance appears when SAWs are applied to superconducting thin films exhibiting charge density waves. Since negative resistance was not observed in ordinary superconducting thin films, I believed there was some relationship between charge density waves and SAWs. I think this achievement is the result of steady research focusing on one-dimensional chain compounds that exhibit only charge density waves."

Journal Information
Publication: Physical Review Letters
Title: Observation of Shapiro Steps in the Charge Density Wave State Induced by Strain on a Piezoelectric Substrate
DOI: 10.1103/p1nh-f6gn