Yasuhiro Niimi

How collective electronic order responds to strain remains a central question in quantum materials. Here, the authors find that charge density wave (CDW) transitions in NbSe${}_{3}$ and 2H-TaSe${}_{2}$ strongly modulate the acoustoelectric current driven by surface acoustic waves. A simple phenomenological model based on strain-dependent conductivity captures the essential behavior. The work demonstrates acoustoelectric transport as a sensitive probe of CDW physics and opens new avenues for strain-based control of electronic states in van der Waals materials.

We investigated spin transport properties of a metallic van der Waals ferromagnet Fe ₃ GeTe ₂ using Cu-based nonlocal spin valves. Although the bulk Curie temperature of Fe ₃ GeTe ₂ is approximately 230 K, distinct spin polarization was unexpectedly observed even at room temperature. We attribute this anomaly to a magnetically modified surface induced by Ar-ion milling during device fabrication. These findings indicate that standard surface treatments can irreversibly alter the surface magnetism of delicate van der Waals materials, complicating the accurate quantitative analysis of their intrinsic spin transport properties.

Recent developments in nanotechnology have enabled us to investigate the dynamic properties of van der Waals materials on a piezoelectric substrate. Here, we report on the dynamics of charge density wave in NbSe_{3} nanowires induced by surface acoustic waves (SAWs). Clear peaks in the differential resistance were observed at the resonant frequency of the SAW device. These peaks, known as "Shapiro steps," are typically observed by applying an rf current to NbSe_{3} nanowires. We found that the Shapiro steps induced by SAWs show several features distinct from the ones induced by an rf current. Our detailed study revealed that a strain induced by SAWs plays a significant role in the Shapiro steps. The result clearly demonstrates the importance of the strain in charge density wave materials and paves the way for strain-induced device applications.

Recent development in nanotechnology has enabled us to investigate the dynamic properties of van der Waals materials on a piezoelectric substrate. Here we report on the dynamics of charge density wave (CDW) in NbSe$_{3}$ nanowires induced by surface acoustic waves (SAWs). Clear peaks in the differential resistance were observed at the resonant frequency of the SAW device. These peaks known as Shapiro steps are typically observed by applying an rf current to NbSe$_{3}$ nanowires. We found that the Shapiro steps induced by SAWs show several distinct features from the ones induced by an rf current. Our detailed study revealed that a strain induced by SAWs plays a significant role in the Shapiro steps. The result clearly demonstrates the importance of the strain in CDW materials and paves the way for strain-induced device applications.

Topological superconductors are one of the intriguing material groups from the viewpoint of not only condensed matter physics but also industrial application such as quantum computers based on Majorana fermion. For the real application, developments of the thin-film topological superconductors are highly desirable. Bi/Ni bilayer is a possible candidate for thin-film chiral superconductors where the time-reversal symmetry is broken. Here we report the phase shift of resistance oscillations by half flux quantum in a ring-shaped device of epitaxial Bi/Ni bilayer induced by a small magnetic field through the ring. The half quantum fluxoid can be a decisive evidence for unconventional superconductors where the superconducting order parameter has an internal degree of freedom. The present result provides a functional operating principle for quantum devices where the phase of the supercurrent can be shifted by πwith a small magnetic field, based on the internal degree of freedom possessed by topological superconductivity.