arXiv subjects

Takuya Kawada

Publications and source records attributed to Takuya Kawada.

15 recordsLinked to original sources

arXiv2026

Significant modulation of acoustoelectric current associated with charge density wave transitions

We studied acoustoelectric (AE) currents in materials that undergo charge density wave (CDW) transitions, induced by a surface acoustic wave (SAW) on a piezoelectric substrate. The polarity and magnitude of the AE current in NbSe$_3$ and 2H-TaSe$_2$ were modulated due to their CDW transitions. We also found that the sign of the AE current depends on the SAW propagation direction with respect to the crystalline axis of the substrate.A phenomenological model assuming strain-modified conductivity can qualitatively account for the significant modulation of the AE current associated with the CDW transition, as well as the SAW propagation orientation dependence. The present results offer a powerful probe for exploring SAW-electron interactions in van der Waals materials, thereby highlighting their potential for advancing the emerging field of straintronics.

cond-mat.mes-hall↗

arXiv2026

Electrical detection of spin-flip transition in metal/\ce{Na_5Co_{15.5}Te_6O_{36}} heterostructure

We report on the longitudinal magnetoresistance (MR) in thin metal films on an Ising-type antiferromagnetic insulator, \ce{Na_5Co_{15.5}Te_6O_{36}} (\ce{NCTO}). Steep changes in the MR spectra with hysteresis were observed at spin-flip transitions driven by magnetic fields applied along the easy axis of the \ce{NCTO} crystal. The MR jumps almost follow step-like changes in magnetization at the spin-flip transition. At very low temperatures where \ce{Co} moments are partially frozen, the MR anomalies exhibit a tunnel-magnetoresistance-like shape. The observed MR anomalies at the spin-flip transition are attributed to strain effects via magnetostriction upon the magnetic-structure change of the \ce{Co} nets in \ce{NCTO}, because similar MR jumps are observed in \ce{Pt/NCTO}, \ce{Pt/SiO_x/NCTO}, and \ce{Cu/NCTO} heterostructures. Interestingly, we found that the high-field slopes of the MR spectra show opposite signs between \ce{Pt/NCTO} and \ce{Cu/NCTO} at low temperatures. Because similar MR spectra are observed for \ce{Pt/SiO_x/NCTO} in which the interface magnetic interaction is negligible, the weak antilocalization due to the strong spin-orbit interaction of the Pt films is likely to contribute to the low-temperature MR.

cond-mat.str-el↗

arXiv2026

Hydrogenation-induced gigantic resistance decrease of palladium films deposited by high pressure magnetron sputtering

We demonstrate a pronounced decrease in the electrical resistance of highly disordered palladium (Pd) films deposited under a high working Ar pressure using a compact film coating system. The resulting resistance change ratio of up to $1/335$ is predominant among those reported previously. Film characterization suggests two primary mechanisms responsible for this significant resistance reduction: atomic force microscopy observation indicates improved electrical contacts among Pd grains, and X-ray diffraction measurement demonstrates hydrogenation-induced crystallization of Pd. These findings offer a simple scheme to enhance hydrogen sensor performance and can contribute to a more comprehensive understanding of the hydrogenation process in Pd.

cond-mat.mtrl-sci↗

arXiv2025

Observation of Shapiro Steps in the Charge Density Wave State Induced by Strain on a Piezoelectric Substrate

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.

cond-mat.mes-hall↗

arXiv2025

Nonlinear planar Hall effect from superconducting vortex motion

We report the nonreciprocal charge transport along the longitudinal and transverse directions in the vortex flow regime of FeSe superconducting films. Clear nonreciprocal signals under an inplane magnetic field reveals symmetry breaking at the film surfaces since the crystal structure of FeSe is centrosymmetric. Although the symmetry in such polar superconductors allows the nonreciprocal transverse response under a magnetic field parallel to the electric current, its observation is physically counterintuitive because vortex motion is not expected in this configuration. We propose that thermally excited (anti)vortices due to the two-dimensional nature of FeSe give rise to the nonreciprocal transverse signals when the mirror symmetry is broken by the inplane magnetic field.

cond-mat.supr-con↗

arXiv2025

Model calculations of the strains associated with surface acoustic waves

Magnon-phonon coupling has garnered increasing interest in condensed matter physics due to its fertile physics and potential applications in devices with novel functionalities. Surface acoustic waves (SAWs) are commonly employed as a source of coherent acoustic phonons. The strain associated with SAWs couples to magnetization of magnetic materials via magnetoelastic coupling and/or spin-rotation coupling. A typical SAW device is formed on a piezoelectric substrate with anisotropic crystal structure. Since the form of strain depends on the material parameters and structure of the SAW device, it is of vital importance to understand its character. In this paper, we present a comprehensive methodology to numerically calculate the SAW velocity, SAW excitation efficiency, lattice displacement and all strain components associated with SAW. LiNbO$_3$ is used as a prototypical material system. All quantities depend on the SAW propagation direction with respect to the crystalline axis and on the electrical boundary conditions. In contrast to non-piezoelectric isotropic media, we find that all shear strain components can be induced in LiNbO$_3$, with their amplitude and relative phase (with respect to the longitudinal strain) dependent on the propagation direction and the boundary conditions at the LiNbO$_3$ surface. These results offer a robust foundation for analyzing strain-driven magnon-phonon coupling mechanisms and contribute to designing strain-engineered functional magnonic and phononic devices.

cond-mat.mtrl-sci↗

arXiv2024

Spin current generation by acousto-electric evanescent wave

We experimentally demonstrate that a spin current can be induced by the acousto-electric evanescent wave, an electric field associated with surface acoustic waves (SAWs) that decay along the surface normal. A previous study showed that a magnetic-field-dependent dc voltage (acoustic voltage) emerges in heavy metal (HM)/ferromagnet (FM) bilayers under excitation of SAWs. The effect, referred to as the acoustic spin Hall effect, was understood by assuming a SAW-induced ac spin current rectified by the oscillation of the FM layer magnetization and the inverse spin Hall effect. However, the mechanism of the spin current generation remained unidentified. Here we measure the acoustic voltage as a function of the SAW propagation direction relative to the crystalline orientation of a LiNbO$_3$ substrate. We find that the magnetic field angle dependence of the acoustic voltage exhibits a phase shift depending on the SAW propagation direction. The result is consistently explained in terms of the acousto-electric evanescent wave generating the spin current in HM layer via the spin Hall effect, thus clarifies the origin of the acoustic spin Hall effect.

cond-mat.mes-hall↗

arXiv2024

Electromagnetic evanescent field associated with surface acoustic wave: Response of metallic thin films

Surface acoustic waves (SAWs), coherent vibrational modes localized at solid surfaces, have been employed to manipulate and detect electronic and magnetic states in condensed-matter systems via strain. SAWs are commonly excited in a piezoelectric material, often the substrate. In such systems, SAWs not only generate strain but also electric field at the surface. Conventional analysis of the electric field accompanying the SAW invokes the electrostatic approximation, which may fall short in fully capturing its essential characteristics by neglecting the effect of the magnetic field. Here we study the electric and magnetic fields associated with SAWs without introducing the electrostatic approximation. The plane wave solution takes the form of an evanescent field that decays along the surface normal with a phase velocity equal to the speed of sound. If a metallic film is placed on the piezoelectric substrate, a time- and space-varying electric field permeates into the film with a decay length along the film normal defined by the skin depth and the SAW wavelength. For films with high conductivity, the phase of the electric field varies along the film normal. The emergence of the evanescent field is a direct consequence of dropping the electrostatic approximation, providing a simple but critical physical interpretation of the SAW-induced electromagnetic field.

cond-mat.mes-hall↗

arXiv2024

Acoustoelectric non-local spin wave power detector for studying magnon-phonon coupling

We have developed a simple detection scheme to study spin waves excited by surface acoustic wave (SAW) in ferromagnetic thin films. Metallic antennas made of Ta and a ferromagnetic element are placed along the SAW propagation path. The SAW excites spin waves in the ferromagnetic element and induces acoustoelectric current in the antennas, which are detected as a DC voltage. The DC voltage takes an extremum at the spin wave resonance condition, which demonstrates that the antenna acts as a non-local spin wave detector. The antennas placed before and after the ferromagnetic element along the SAW propagation path can probe spin wave excitation from reflected and transmitted SAWs, respectively. Interestingly, we find characteristics of spin wave excitations that are different for the reflected and transmitted SAWs: the former excites spin waves with larger frequency with broader linewidth compared to the latter. The results show that the antennas act as a non-local spin wave power detector and can be used to map out the spin wave spectra in a unique way, providing insights into the magnon-phonon coupling in magnetic nanostructures fabricated on phononic SAW devices.

cond-mat.mes-hall↗

arXiv2023

Magnon-phonon coupling of synthetic antiferromagnets in a surface acoustic wave cavity resonator

We use a surface acoustic wave (SAW) cavity resonator to study the coupling of acoustic magnons in a synthetic antiferromagnet (SAF) and the phonons carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer and the scattering matrix of the SAW resonator is studied to assess the coupling. We find that the spectral linewidth of the SAW resonator is modulated when the frequency of the excited magnons approaches the SAW resonance frequency. Moreover, the linewidth modulation varies with the magnitude and orientation of the external magnetic field. Such change in the spectral linewidth can be well reproduced using macrospin-like model calculations. From the model analyses, we estimate the magnon-phonon coupling strength to be $\sim$15.6 MHz at a SAW resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity is $\sim$0.66. As the spectral shape hardly changes in a CoFeB single layer reference sample under the same experimental condition, these results show that SAF provides an ideal platform to study magnon-phonon coupling in a SAW cavity resonator.

cond-mat.mes-hall↗

arXiv2023

On-chip all-electrical determination of the magnetoelastic coupling constant of magnetic heterostructures

We have developed an approach to determine the magnetoelastic coupling constant of magnetic layers in thin film heterostructures. The film is formed on a piezoelectric substrate between two interdigital transducers (IDT), a platform often used to construct a surface acoustic wave device. With the substrate piezoelectricity, strain is induced into the film by applying a dc voltage to the IDTs. The strain causes changes in the magnetization direction of the magnetic layer, which is probed by measuring changes, if any, in the transverse resistance of the heterostructure. We find the extracted magnetoelastic coupling constant of the magnetic layer (CoFeB) depends on the film stacking. Such change can be accounted for provided that the elastic properties of the layers that constitute the heterostructures are taken into account. The on-chip all-electrical approach described here provides a versatile means to quantitatively assess the magnetoelastic coupling constant of thin film heterostructures.

cond-mat.mes-hall↗

arXiv2023

Helicity resolved Raman spectroscopy of mono- and a few-layers thick PtSe$_2$

We studied helicity resolved Raman scattering in PtSe$_2$ flakes with different thicknesses. The peak amplitude of helicity-switched Raman scattering is significantly larger than that of helicity-conserved scattering for the in-plane $E_{g}$ mode, consistent with the Raman tensor analyses and conservation law of angular momentum. The peak amplitude of the helicity-switched $E_{g}$ mode is larger for the thinner flakes. In addition, we find Raman peaks near the energy levels of infrared (IR)-active $E_u$ and $A_{2u}$ modes, only for monolayer and a few-layers thick flakes. Interestingly, these peaks manifest themselves only for helicity-switched Raman scattering; they are nearly absent for helicity-conserved scattering.

cond-mat.mes-hall↗

arXiv2022

Large surface acoustic wave nonreciprocity in synthetic antiferromagnets

We have studied the transmission of surface acoustic waves (SAWs) in ferromagnetic/non-magnetic/ferromagnetic tryilayers. The SAW scattering matrix is studied for devices with various non-magnetic spacer thickness, which defines the strength of the interlayer exchange coupling. We find the SAW transmission amplitude depends on their propagation direction when the two ferromagnetic layers are coupled antiferromagnetically. The degree of such SAW nonreciprocity increases with increasing exchange coupling strength and reaches 37 dB/mm for a device with the thinnest spacer layer. These results show the potential of interlayer exchange coupled synthetic antiferromagnets for viable acoustic nonreciprocal transmission devices, such as circulators and isolators.

cond-mat.mes-hall↗

arXiv2020

Acoustic spin Hall effect in strong spin-orbit metals

We report on the observation of the acoustic spin Hall effect that facilitates lattice motion induced spin current via spin orbit interaction (SOI). Under excitation of surface acoustic wave (SAW), we find a spin current flows orthogonal to the propagation direction of a surface acoustic wave (SAW) in non-magnetic metals. The acoustic spin Hall effect manifests itself in a field-dependent acoustic voltage in non-magnetic metal (NM)/ferromagnetic metal (FM) bilayers. The acoustic voltage takes a maximum when the NM layer thickness is close to its spin diffusion length, vanishes for NM layers with weak SOI and increases linearly with the SAW frequency. To account for these results, we find the spin current must scale with the SOI and the time derivative of the lattice displacement. Such form of spin current can be derived from a Berry electric field associated with time varying Berry curvature and/or an unconventional spin-lattice interaction mediated by SOI. These results, which imply the strong coupling of electron spins with rotating lattices via the SOI, show the potential of lattice dynamics to supply spin current in strong spin orbit metals.

cond-mat.mes-hall↗

arXiv2019

Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films

The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO$_3$ substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.

cond-mat.mes-hall↗