Quantized current induced by acoustic waves
Opening the way to new quantum devices that utilize strain
- When a one‑dimensional chain compound NbSe₃ is transferred onto a piezoelectric substrate and irradiated with surface acoustic waves (SAWs), a type of sound wave that propagates in solids, the current remained at a certain value for a while before changing in a step-like manner (a plateau structure indicating quantization appeared in the current-voltage characteristics).
- From experiments on frequency characteristics and pulsed SAWs irradiation, it has been elucidated that this quantized current appears when strain is applied to the one-dimensional chain compound.
- It is expected to lead to the development of new quantum devices that utilize strain.
Outlines
A research group including Koji Fujiwara (a third-year doctoral student at the time of the research), Associate Professor Shintaro Takada, and Professor Yasuhiro Niimi of the Department of Physics, Graduate School of Science, the University of Osaka, discovered that by applying strain to the one-dimensional chain compound NbSe₃, which exhibits a charge-density-wave (CDWs) state, using an acoustic wave of SAW, a plateau structure appears in the current-voltage characteristics, indicating quantization.
Strain is one of the fundamental concepts in physics. In the area of condensed matter physics, which deals with materials, strain can be electrically controlled by applying an electric field onto a piezoelectric substrate. This principle is the basis for surface acoustic waves (SAWs), which are also applied in frequency filters.
The research group cleaved (split in a specific direction) the one-dimensional chain compound NbSe₃ using a mechanical exfoliation method with scotch tapes to process it into thin wires, then transferred them onto a piezoelectric substrate, LiNbO₃. Then, by applying an AC voltage to the interdigitated electrode (IDE) set on the LiNbO₃ substrate, SAWs were irradiated onto the NbSe₃ thin wire (Fig. 1, left). Below a certain temperature, NbSe₃ exhibits a charge density waves (CDWs) state, in which the crystal disforms and the electrons become ordered. By irradiating this state with SAWs, the researchers discovered that a plateau structure, indicating quantization, appears in the current-voltage characteristics (Fig. 1, right).
Furthermore, the research group clarified that this quantized current is induced by strain in the one-dimensional chain compound. This result advances research into strain-induced quantized currents and paves the way for device applications using strain induction.
Fig. 1 (Left) Optical microscope image of NbSe₃ thin wires transferred onto a piezoelectric substrate LiNbO₃, and the IDEs used to irradiate the NbSe₂ thin wires with SAWs. (Right) Relationship between the CDW current ICDW and the applied DC voltage Vdc. The measurements were conducted at 45 K, below the CDW transition temperature. Plateau structures (n = 1, 2, 3) associated with quantization were observed in the ICDW.
Credit: Yasuhiro Niimi
Research Background
One of the most interesting phenomena in quantum mechanics is the precise quantization of voltage by a combination of fundamental constants such as Planck constant and elementary charge, a phenomenon known as the Shapiro step. The Shapiro step is used as a voltage standard due to its robustness and high accuracy.
In addition, regarding resistance, it is known that quantized resistance can also be described by Planck constant and elementary charge due to a phenomenon known as the Quantum Hall Effect, and the phenomenon of quantization is known for voltage and resistance.
On the other hand, although various studies have been conducted on current standards, none has been established to date.
Research Contents
The research group has created thin wires by mechanically exfoliating NbSe₃, a one-dimensional chain compound that exhibits a CDW transition. The thin wire was transferred onto a piezoelectric substrate LiNbO₃, and an AC voltage of approximately 300 MHz was applied to IDE set on the substrate. By irradiating the NbSe₃ thin wire SAWs, the DC current-voltage characteristics were measured (Fig. 1, left).
The charge density waves (CDWs) are pinned to defects in the crystal up to a certain threshold voltage, but above that threshold voltage they become unpinned and start moving all at once. In this study, the research group discovered that when SAWs are irradiated, the current generated by the CDWs that start moving simultaneously have a plateau structure at an integer multiple of a certain value (Fig. 1, right). Furthermore, the research group found that this plateau structure rapidly decays when the power of the applied SAWs is increased (Fig. 2, left). This result is significantly different from previous results of when an AC electric field was applied directly to a NbSe₂ sample (Fig. 2, right).
Subsequent experiments in which the frequency was changed and application of pulsed SAWs revealed that the quantized current value was caused by strain induced in the NbSe₃ thin wire by SAWs.
Fig. 2 Dependence of Shapiro step width on (a) SAW power PSAW and (b) AC electric field power Pac. The measurements were conducted at 45 K, below the CDW transition temperature.
Credit: Yasuhiro Niimi
Social Impact of Research
By irradiating one-dimensional chain compounds exhibiting a CDW state with sound waves, research into quantized currents utilizing strain effects will be expected to progress further. It will also open the way to applications in strain-induced quantum devices, which is expected to lead to the creation of new types of devices.
Notes
The article, “Observation of Shapiro Steps in the Charge Density Wave State Induced by Strain on a Piezoelectric Substrate,” was published in American scientific journal of Physical Review Letters at DOI: 10.1103/p1nh-f6gn.
