arXiv subjects

Yoshihiko Togawa

Publications and source records attributed to Yoshihiko Togawa.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation-induced antiparallel spin polarization near the boundaries of chiral metals

The spin response of chiral conductors to nonequilibrium electrical fluctuations remains largely unexplored. We develop a low-frequency semiclassical Boltzmann theory coupled to Gauss's law for a chiral metal with spin-orbit coupling of hedgehog type, treating impurity scattering beyond the conventional relaxation-time approximation. We first determine the quadratic response to a local ac current and its frequency dependence, and then show that zero-mean stationary current fluctuation and electric-field fluctuation near boundaries generate finite time-averaged spin polarizations in the two boundary regions of the chiral metal. The polarizations are normal to the boundaries and antiparallel: for one chirality they point inward at both boundaries, and for the other they point outward. Within this model, the dominant contribution arises from the linear Edelstein effect driven by a quadratic effective electric field localized near each boundary. This picture may provide a qualitative explanation for CISS-related spin polarization reported in the absence of an applied bias. More broadly, we expect other externally maintained stochastic drives to induce spin polarization through the same mechanism.

cond-mat.mes-hall↗

arXiv2026

Emergence of Chiral Helimagnetic Order in Chromium-intercalated Tantalum Disulfide CrTa$_3$S$_6$ Powders with Controlled Intercalation

We report a highly sensitive change in magnetic properties of a chiral Cr-intercalated transition-metal dichalcogenide Cr$_{x}$Ta$_{3}$S$_{6}$. Magnetization curves and small-angle neutron scattering data revealed that the powder samples exhibit chiral helimagnetism with a Cr intercalation quantity $x$ below 0.996, while they show ferromagnetism above 1.000. The emergence and temperature-dependent evolution of the helimagnetic period are argued in terms of sample dimensions of powders and microfabricated crystals.

cond-mat.mtrl-sci↗

arXiv2026

Circular Raman responses from angular-momentum inequivalence in CoSi

Circularly polarized Raman scattering in solids exhibits distinct phenomena such as Raman optical activity (ROA) and chiral-phonon-induced frequency splitting, whose relationship has remained unclear. Here we show that these seemingly different responses can be understood within a common framework based on the inequivalence of phonon states carrying opposite crystal angular momenta. Using helicity-resolved Raman spectroscopy of the chiral crystal CoSi, we find that ROA and frequency splitting arise from different symmetry channels, namely axial multipolar symmetry and structural chirality, respectively. First-principles calculations reproduce both effects and clarify their symmetry origins. These results establish angular-momentum inequivalence as a unifying principle of circular Raman responses and link helicity-resolved Raman spectroscopy to the angular-momentum structure of chiral phonons in topological materials.

physics.optics↗

arXiv2026

Chirality-dependent spin polarization in metals: linear and quadratic responses

We study spin polarization induced by locally injected electric currents in a metal whose spin--orbit coupling reflects its structural chirality. We reveal both spin polarization in the bulk in the linear response and antiparallel spin polarization near the interface in the quadratic response to external electric currents, and reproduce the experimentally observed correlation between the chirality of the metal and the direction of spin polarization. In particular, we elucidate that the sign of the spin polarization in the quadratic response is opposite to that expected from the bulk spin current. This sign discrepancy originates from spin polarization induced by dipole-like charge distribution appearing in the quadratic response.

cond-mat.mes-hall↗

arXiv2026

Chiral phonons in sixfold chiral CrSi$_2$: Raman spectroscopy and first-principles calculations

Chiral phonons have been identified in several chiral crystals, primarily in those with trigonal symmetry and threefold screw axes. In this study, chiral phonons in CrSi$_2$, a chiral crystal with a sixfold helical structure, were investigated. Circularly polarized Raman spectroscopy revealed a subtle splitting of doubly degenerate $E_2$ phonon modes between cross-circular polarization configurations. These observations, supported by first-principles phonon calculations, indicate the presence of chiral phonons in CrSi$_2$, expanding the scope of materials that exhibit chiral vibrational modes beyond the conventional trigonal class.

cond-mat.mtrl-sci↗

arXiv2025

Development of a Ferromagnetic Resonance Measurement System Using NanoVNA

Ferromagnetic resonance (FMR) is a fundamental technique for probing magnetization dynamics in spintronic and magnetic materials. However, conventional FMR measurements rely on broadband vector network analyzers (VNAs), whose high cost limits accessibility for small laboratories and educational environments. To overcome this barrier, we have developed a compact and low-cost FMR measurement platform - the NanoVNA-FMR system-based on a commercially available NanoVNA. The setup integrates an electromagnet and a coplanar waveguide (CPW) and is fully automated using Python scripts. This enables synchronized magnetic-field sweeping, S-parameter acquisition, and real-time visualization. The system successfully captures clear FMR spectra that exhibit systematic shifts in resonance frequency with increasing magnetic field. The results are in excellent agreement with those obtained using a conventional VNA-based FMR system, confirming the quantitative reliability of the NanoVNA approach. Additionally, a 3D-printed sample holder further reduces overall system cost. These results demonstrate that the NanoVNA-FMR system provides a practical, accurate, and accessible alternative for quantitative magnetic characterization and educational applications.

cond-mat.mtrl-sci↗

arXiv2025

Spin phase detection by spin current in a chiral helimagnet

Helimagnets, characterized by a helical arrangement of magnetic moments, possess unique internal degrees of freedom, including the spin phase, defined by the phase of the helical magnetic structure. Electrical detection of the spin phase is essential for both practical applications and fundamental research in helimagnets. Here, we demonstrate the electrical detection of the spin phase in a van der Waals nanoscale chiral helimagnet CrNb$_3$S$_6$ using nonlocal spin valve measurements. Due to the short spin diffusion length in CrNb$_3$S$_6$ ($\sim5$~nm), the surface magnetic moment direction, which corresponds to the spin phase, can be detected via spin currents. The experimentally observed magnetic field dependence of the nonlocal spin valve signal is consistent with that of the surface magnetic moment in the helical magnetic structure, as supported by micromagnetic simulations. Our results establish spin currents as a powerful tool for detecting the spin phase in helimagnets, opening avenues for utilizing the spin phase as a novel internal degree of freedom in nanoscale spintronic devices.

cond-mat.mes-hall↗

arXiv2024

Superconductivity in Ternary Germanite TaAl$_{x}$Ge$_{2-x}$ with a C40 chiral structure

We report a new family of chiral intermetallic superconductors TaAl$_{x}$Ge$_{2-x}$. The mother compound TaGe$_2$ has a C40-type chiral hexagonal crystal structure with a pair of enantiomorphic space groups of $P6{_2}22$ and $P6{_4}22$. By substituting Ge with Al, TaAl$_{x}$Ge$_{2-x}$ polycrystals with the C40 structure were synthesized with Al substitution $x$ from 0 to 0.8. Magnetic susceptibility, magnetization curves and electrical resistivity revealed that TaAl$_{x}$Ge$_{2-x}$ with $x$ of 0.2 to 0.4 was a type-II superconductor with a superconducting transition temperature $T_{\rm c}$ of 2.0 to 2.2 K. The superconductivity disappeared or was largely suppressed at $x$ less than 0.2 and more than 0.4, although all the measurements were performed at temperatures above 1.8 K. An emergence of superconductivity is discussed in terms of the lattice constants changes with the Al substitution.

cond-mat.supr-con↗

arXiv2024

Chirality-Induced Selectivity of Phonon Angular Momenta in Chiral Quartz Crystals

A generation, propagation, and transfer of phonon angular momenta are examined on thermal transport in chiral insulative and diamagnetic crystals of $α$-quartz. We found that thermally-driven phonons carry chirality-dependent angular momenta in the quartz crystals and they could be extracted from the quartz as a spin signal. Namely, chirality-induced selectivity of phonon angular momenta is realized in the chiral quartz. We argue that chiral phonons available in chiral materials could be a key element in triggering or enhancing chirality-induced spin selectivity with robust spin polarization and long-range spin transport found in various chiral materials.

cond-mat.mtrl-sci↗

arXiv2023

Spin polarization gate device based on the chirality-induced spin selectivity and robust nonlocal spin polarization

Nonlocal spin polarization phenomena are thoroughly investigated in the devices made of chiral metallic single crystals of CrNb$_3$S$_6$ and NbSi$_2$ as well as of polycrystalline NbSi$_2$. We demonstrate that simultaneous injection of charge currents in the opposite ends of the device with the nonlocal setup induces the switching behavior of spin polarization in a controllable manner. Such a nonlocal spin polarization appears regardless of the difference in the materials and device dimensions, implying that the current injection in the nonlocal configuration splits spin-dependent chemical potentials throughout the chiral crystal even though the current is injected into only a part of the crystal. We show that the proposed model of the spin dependent chemical potentials explains the experimental data successfully. The nonlocal double-injection device may offer significant potential to control the spin polarization to large areas because of the nature of long-range nonlocal spin polarization in chiral materials.

cond-mat.mtrl-sci↗

arXiv2023

Chiral phonons: circularly polarized Raman spectroscopy and $\textit{ab initio}$ calculations in a chiral crystal tellurium

Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudo-angular momenta. In circularly polarized Raman spectroscopy, the peak split of the $Γ_3$ mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudo-angular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe chiral phonons in a chiral unary crystal Te. The pseudo-angular momentum of the phonon is obtained in Te by an $\textit{ab initio}$ calculation. From this calculation, we verified the conservation law of pseudo-angular momentum in Raman scattering. From this conservation law, we determined the handedness of the chiral crystals. We also evaluated the true chirality of the phonons using a measure with symmetry similar to that of an electric toroidal monopole.

cond-mat.mtrl-sci↗

arXiv2022

Chirality-selected crystal growth and spin polarization over centimeters of transition metal disilicide crystals

We performed a chirality-controlled crystal growth of transition metal disilicide NbSi$_{2}$ and TaSi$_{2}$ by using a laser-diode-heated floating zone (LDFZ) method. The crystal chirality was evaluated in the crystals of centimeters in length by performing single crystal X-ray diffraction as well as probing a spin polarization originating from chirality-induced spin selectivity (CISS) effect. The crystals of right-handed NbSi$_{2}$ and of left-handed TaSi$_{2}$ were obtained in the conventional LDFZ crystal growth, while the left-handed NbSi$_{2}$ and right-handed TaSi$_{2}$ crystals were grown by the LDFZ method with the composition-gradient feed rods. The spin polarization via the CISS was observed over centimeters in the NbSi$_{2}$ single crystals and the sign of the CISS signals was dependent on the chirality of crystals. The correlation between the crystal chirality and CISS signals indicates that the CISS measurements work as a non-destructive method for chirality determination even in centimeter-long specimens.

cond-mat.mtrl-sci↗

arXiv2022

Truly chiral phonons in α-HgS

Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensional materials, and they were referred to as "chiral phonons" . A natural development would be the discovery of truly chiral phonons that propagate while rotating in three-dimensional materials. Here, we used circularly polarised Raman scattering and first-principles calculations to identify truly chiral phonons in chiral bulk crystals. This approach enabled us to determine the chirality of a crystal in a non-contact and non-destructive manner. In addition, we demonstrated that the law of the conservation of pseudo-angular momentum holds between circularly polarised photons and chiral phonons. These findings are expected to help develop ways for transferring the pseudo-angular momentum from photons to electron spins via the propagating chiral phonons in opto-phononic-spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Chirality-Controlled Enantiopure Crystal Growth of a Transition Metal Monosilicide by a Floating Zone Method

We performed a crystal growth to obtain chirality-controlled enantiopure crystals using a laser-diode-heated floating zone (LDFZ) method with a composition-gradient feed rod. It has been argued that the crystal handedness of $T$Si ($T$ : transition metal) is fixed depending on $T$ in the case of the ones grown by the conventional methods. We found that right-handed single crystals of CoSi and MnSi were grown from the composition gradient feed rods that consist of FeSi--CoSi and FeSi--MnSi, respectively. The obtained CoSi and MnSi crystals inherit the chirality from the seed part of FeSi, which grows in a right-handed structure, and thus have the chirality opposite to that for the crystals in the literature. The LDFZ method with the feed rods with various combinations of $T$Si compounds enables a flexible control of the chirality of $T$Si and will be useful for clarifying the interplay between the crystalline chirality and chirality-induced physical responses.

cond-mat.mtrl-sci↗

arXiv2021

Detection of chirality-induced spin polarization over millimeters in polycrystalline bulk samples of chiral disilicides NbSi$_2$ and TaSi$_2$

We report that spin polarization occurs over millimeters in polycrystalline bulk samples of chiral disilicide NbSi$_2$ and TaSi$_2$. As previously demonstrated in the experiments using single crystals of NbSi$_2$ and TaSi$_2$, electrical transport measurements allow detection of direct and inverse signals associated with the chirality-induced spin polarization even in the chiral polycrystals. The spin polarization signals also appear in nonlocal measurements, in which charge current flows only in the area millimeters away from the detection electrode. These data mean that the spin polarization phenomena occur regardless of the presence of crystalline grains in the polycrystals, indicating a robustness and resilience of the chirality-induced spin polarization. On the basis of the experimental data, we found that the sum rule holds for the spin transport signals. A distribution of handedness over the samples was determined on average in the polycrystals. While the mechanism of preserving the spin polarization over millimeters remains to be clarified, the present study may open up prospects of spin control and manipulation over macroscopic length scales using chiral materials.

cond-mat.mtrl-sci↗

arXiv2021

Thin Film Growth of Heavy Fermion Chiral Magnet YbNi3Al9

We grew thin films of a heavy fermion chiral magnet YbNi$_3$Al$_9$ by using molecular beam epitaxy. They were grown on $c$-plane sapphire substrates under ultra-high vacuum while maintaining a deposition rate at a stoichiometric ratio among Yb, Ni, and Al. The resulting thin films contain epitaxial grains with a $c$ axis parallel to the substrate surface: The YbNi$_3$Al$_9$ $c$ axis is parallel to the sapphire $b$ or $a$ axis. The temperature dependence of the resistivity exhibits a typical feature of a dense Kondo system with a broad shoulder structure at $\sim$40\,K, as well as a kink as a signature of the chiral helimagnetic ordering at 3.6\,K. These features are consistent with those previously observed in bulk samples. The shift in the kink associated with the field-induced phase transition is found in the magnetoresistance curves under a magnetic field applied in the direction perpendicular to the $c$-axis. The magnetic phase diagram well reproduces that for the bulk crystals, implying that the chiral soliton lattice phase arises under magnetic fields, even in thin films.

cond-mat.str-el↗

arXiv2019

Order and Disorder in the Magnetisation of the Chiral Crystal CrNb$_3$S$_6$

Competing magnetic anisotropies in chiral crystals with Dzyaloshinskii Moriya exchange interactions can give rise to non-trivial chiral topological magnetisation configurations with new and interesting properties. One such configuration is the magnetic soliton, where the moment continuously rotates about an axis. This magnetic system can be considered to be one dimensional and, because of this, it supports a macroscale coherent magnetisation, giving rise to a tunable chiral soliton lattice (CSL) that is of potential use in a number of applications in nanomagnetism and spintronics. In this work we characterise the transitions between the forced-ferromagnetic (F-FM) phase and the CSL one in CrNb$_3$S$_6$ using differential phase contrast imaging in a scanning transmission electron microscope, conventional Fresnel imaging, ferromagnetic resonance spectroscopy, and mean-field modelling. We find that the formation and movement of dislocations mediate the formation of CSL and F-FM regions and that these strongly influence the highly hysteretic static and dynamic properties of the system. Sample size and morphology can be used to tailor the properties of the system and, with the application of magnetic field, to locate and stabalise normally unstable dislocations and modify their dimensions and magnetic configurations in ways beyond that predicted to occur in uniform films.

cond-mat.mtrl-sci↗

arXiv2018

Chiral solitons in monoaxial chiral magnets in tilted magnetic field

We show that the stability (existence/absence) and interaction (repulsion/attraction) of chiral solitons in monoaxial chiral magnets can be varied by tilting the direction of magnetic field. We, thereby, elucidate that the condensation of attractive chiral solitons causes the discontinuous phase transition predicted by a mean field calculation. Furthermore we theoretically demonstrate that the metastable field-polarized-state destabilizes through the surface instability, which is equivalent to the vanishing surface barrier for penetration of the solitons. We experimentally measure the magnetoresistance (MR) of micrometer-sized samples in the tilted fields in demagnetization-free configuration. We corroborate the scenario that hysteresis in MR is a sign for existence of the solitons, through agreement between our theory and experiments.

cond-mat.str-el↗