arXiv subjects

Mikito Koga

Publications and source records attributed to Mikito Koga.

At least 19 recordsLinked to original sources

arXiv2026

Novel Magnetoacoustic Resonance Technique for Exploring Hidden Quadrupoles in a Crystal Field Quartet

Crystal field quartets with quadrupole degrees of freedom play a crucial role in hidden ordering systems, as exemplified by CeB$_6$. We present a novel magnetoacoustic resonance technique that combines acoustically induced strain fields with a linearly polarized high-frequency microwave field to probe quadrupoles inherent in the quartet hidden behind magnetic properties. This method offers the advantage of enabling quantum quadrupole resonance transitions for large excitation energy gaps within quartet sublevels under a strong magnetic field, which cannot be achieved by acoustic experiments alone. Formulating a simultaneous single-phonon-single-photon absorption transition process using Floquet theory, we demonstrate how the transition probabilities are affected by changing the propagation direction of a bulk acoustic wave. The key result is that distinct maxima in transition probabilities, attributed to specific propagation directions, indicate a characteristic of quadrupole physics and exhibit an abrupt change owing to an induced ordered moment. This photon-assisted magnetoacoustic resonance technique will promote a broader range of applications of acoustic experiments for the study of quadrupole physics.

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arXiv2026

Theory of Magnetoacoustic Resonance to Probe Multipole Effects Due to a Crystal Field Quartet

We present a new method of acoustically driven resonance that probes octupole degrees of freedom as well as a quadrupole usually hidden by the magnetic properties of a crystal field quartet. A characteristic of the quadrupole is reflected in the anisotropic resonance transition rate, which depends on the propagation direction of a surface acoustic wave under an external magnetic field parallel to a typical crystallographic axis. The transition rate is modulated by the anisotropic Zeeman splitting associated with octupoles. We demonstrate how to obtain information about the quartet quadrupole-strain coupling and evaluate the anisotropic octupole effect quantitatively. We also discuss the applicability of our method to identifying a quadrupole order parameter using a multipole-multipole interaction model. For large excitation energy gaps under strong magnetic fields, we propose a photon-assisted magnetoacoustic resonance formulated on the basis of the Floquet theory.

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arXiv2024

Theory of Field-Angle-Resolved Magnetoacoustic Resonance in Spin-Triplet Systems for Application to Nitrogen-Vacancy Centers in Diamond

Motivated by the recent studies of acoustically driven electron spin resonance applied to diamond nitrogen-vacancy (NV) centers, we investigate the interaction of an electronic spin-triplet state with periodically time-dependent oscillating strain fields. On the basis of a lowest-lying two-level system, we show the importance of two-phonon transition probabilities controlled by rotating an applied magnetic field using the Floquet theory. In particular, we demonstrate how to evaluate coupling-strength parameters in the spin--strain interaction for the $C_{3v}$ point group considering the NV spin states. The level splitting of spin states can be adjusted by changing the field directions relative to the NV axis to obtain lower phonon resonance frequencies suitable for practical applications. Focusing on a field-rotation angle for the vanishment of a longitudinal phonon-mediated transition, we show that the magnetoacoustic resonance presented here provides useful information as a new probe of unquantified spin--strain couplings possessed by NV defects.

cond-mat.mes-hall↗

arXiv2024

Theory of Spin-Acoustic Resonance for Spin-3/2 Si Vacancy with $C_{3v}$ Site Symmetry in Silicon Carbide

Motivated by the recent acoustically driven spin resonance studies applied to silicon vacancy centers in silicon carbide, we theoretically investigate the spin--strain interaction characterized by the defect spin-$3/2$ quadrupole components coupled to strain fields. Considering the $C_{3v}$ symmetry of the vacancy site beyond the spherical approximation, we clarify the effect of a deviation from the spherical symmetry on spin resonance transition rate, which can be changed by rotating a static magnetic field. The ratios of spin--strain coupling parameters can be evaluated from the anisotropic field-direction dependence of the transition rate using a standing or traveling surface acoustic wave. We also discuss the effect of the propagation direction of the acoustic wave tilted from the crystallographic mirror plane reflecting the $C_{3v}$ symmetry. The results presented here reveal the quadrupole properties inherent in spin-3/2 states and will promote the realization of the acoustically driven strain control of spin.

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arXiv2020

Magnetoacoustic Resonance to Probe Quadrupole-Strain Coupling in a Diamond Nitrogen-Vacancy Center as a Spin-Triplet System

A theory of magnetoacoustic resonance is proposed to measure quadrupole-strain couplings in a spin-triplet state with the $C_{3v}$ point group symmetry, considering the spin-strain interaction in a diamond nitrogen-vacancy (NV) center. Based on the Floquet theory, we demonstrate how the single- and two-phonon transition probabilities depend on the change in the longitudinal and transverse quadrupole couplings, which can be controlled by rotating an applied magnetic field, around the threefold axis. The obtained quadrupole dynamics results are useful for realizing mechanical or ac strain-control of the NV spin as an alternative to the conventional magnetic control by spin resonance.

cond-mat.mes-hall↗

arXiv2020

Theory of Photon-Assisted Magnetoacoustic Resonance as a New Probe of Quadrupole Dynamics

Motivated by the recent progress of phonon-mediated control in quantum spin devices, we propose a possibility of hybrid measurement using electron paramagnetic resonance (EPR) and a surface acoustic wave (SAW). Considering quadrupole-strain (QS) couplings suggested for silicon vacancies, we present a minimum model of the two-level system to investigate a magnetoacoustic resonance (MAR) coupled to various strain modes driven by the SAW. The longitudinal and transverse QS couplings can be changed by rotating a magnetic field, which depends on a combination of the strain modes. Using the Floquet theory, we elucidate each coupling effect on the time-averaged transition probability, especially focus on a single-phonon transition process. The important result is that the longitudinal QS coupling brings about a sharp photon-assisted resonance and leads to an abrupt change in the field-angle dependent transition probability. Since this phonon transition process is always accompanied by the photon transition, the field angle for the sharp resonance peak can be detected by the EPR measurement. The hybrid EPR-MAR measurement is useful to confirm the existence of quadrupole degrees of freedom strongly coupled to elastic strains, and thus it is expected to be a complementary probe for the precise evaluation of quadrupole properties.

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arXiv2020

Analysis of Magnetoacoustic Quadrupole Resonance and Application to Probe Quadrupole Degrees of Freedom in Quantum Magnets

Motivated by the recent progress of high-frequency ultrasonic measurements, we propose a theory of magnetoacoustic resonance as a microscopic probe for quadrupole degrees of freedom hidden in magnetic materials. A local strain driven by an acoustic wave couples to electronic states of a magnetic ion through various quadrupole-strain couplings, and this provides a periodically time-dependent oscillating field. As a typical two-level system with the quadrupole, we consider a non-Kramers doublet and investigate single- and multiphonon-mediated transition processes on the basis of the Floquet theory. An analytic form of the transition probability is derived within the weak coupling theory, which helps us analyze the magnetoacoustic quadrupole resonance. We apply the theory to realistic non-Kramers doublet systems for the f2 configuration in Oh and D4h symmetries, and discuss how to identify the relevant quadrupole by controlling the quadrupole-strain coupling with an applied magnetic field in ultrasonic measurements.

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arXiv2020

Symmetry Analysis of Magnetoelectric Effects in Honeycomb Antiferromagnet Co4Nb2O9

Magnetoelectric effects in honeycomb antiferromagnet Co4Nb2O9 are investigated on the basis of symmetry analyses of Co ions in trigonal P-3c1 space group. For each Co ion, the possible spin dependence is classified by C3 point-group symmetry. This accounts for the observed main effect that an electric polarization rotates in the opposite direction at the twice speed relative to the rotation of the external magnetic field applied in the ab-plane. Inversion centers and twofold axes in the unit cell restrict the active spin-dependence of the electric polarization, which well explains the observed experimental results. Expected optical properties of quadrupolar excitation and various types of dichroism are also discussed.

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arXiv2019

Theoretical Study of Magnetoelectric Effects in Honeycomb Antiferromagnet Co4Nb2O9

The honeycomb antiferromagnet Co4Nb2O9 is known to exhibit an interesting magnetoelectric effect that the electric polarization rotates at the twice speed in the opposite direction relative to the rotation of the external magnetic field applied in the basal ab-plane. The spin-dependent electric dipole can be an origin of the magnetoelectric effect. It is described by the product of spin operators at different sites (type-I theory) or at the same site (type-II theory). We examine the electric polarization for the two cases on the basis of the symmetry analysis of the crystal structure of Co4Nb2O9, and conclude that the latter is the origin of the observed result. This paper also gives a general description of the field-induced electric polarization on honeycomb lattices with the C3 point group symmetry on the basis of the type-I theory.

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arXiv2017

Antisymmetric Spin-Orbit Coupling Effect on Kondo-Induced Electric Polarization in a Triangular Triple Quantum Dot

We study the local antisymmetric spin-orbit (ASO) coupling effect on spin, orbital, and charge degrees of freedom for the Kondo effect in a triangular triple quantum dot (TTQD). Here, one of the three QDs is coupled to a metallic lead through electron tunneling, and a local electric polarization is induced by the Kondo effect. The ASO interaction is introduced in the other two coupled QDs on the opposite side of the lead. Generally, the ASO coupling effect is very weak and not easily detectable, but it essentially causes spin and charge reconfigurations in the TTQD through the Kondo effect. Using an extended Anderson model for the TTQD Kondo system, we elucidate that the ASO coupling gives rise to a considerable reduction of the emergent electric polarization, as a consequence of the parity mixing of molecular orbitals in the triangular loop as well as the spin-up and spin-down coupling of local electrons. The latter leads to a local diamagnetic susceptibility owing to the ASO coupled spins. We also show that the Kondo-induced electric polarization can be controlled by the ASO coupling as well as by the magnetic flux penetrating through the TTQD.

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arXiv2017

Symmetry Analysis of Spin-Dependent Electric Dipole and Its Application to Magnetoelectric Effects

Spin-dependent electric dipole operators are investigated group-theoretically for the emergence of an electric dipole induced by a single spin or by two spins, where the spin dependences are completely classified up to the quadratic order. For a single spin, a product of spin operators behaves as an even-parity electric quadrupole operator, which differs from an odd-parity electric dipole. The lack of the inversion symmetry allows the even- and odd-parity mixing, which leads to the electric dipole described by the electric quadruple operators. Point-group tables are given for classification of the possible spin-dependent electric dipoles and for the qualitative analysis of multiferroic properties, such as an emergent electric dipole moment coexisting with a magnetic moment, electromagnon excitation, and directional dichroism. The results can be applied to a magnetic ion in crystals or embedded in molecules at a site without the inversion symmetry. In the presence of an inversion symmetry, the electric dipole does not appear for a single spin. This is not the case for the electric dipole induced by two spins with antisymmetric spin dependence, which is known as vector spin chirality, in the presence of the inversion center between the two spins. In the absence of the inversion center, symmetric spin-dependent electric dipoles are also relevant. The detailed analysis of various symmetries of two-spin states is applied to spin dimer systems and the related multiferroic properties.

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arXiv2016

Emergent Electric Polarization by Kondo Effect in a Triangular Triple Quantum Dot

A triangular triple quantum dot gives various Kondo effects, such as the emergence of an electric polarization accompanied by a complete compensation of spin degrees of freedom. The interplay of spin and charge of electrons in quantum dots is investigated using an Anderson model with an equilateral triangular spin cluster, in which a single electron state at one site is hybridized with a conduction band in a lead. The numerical renormalization group analysis shows how a nonzero electric polarization develops with local electrons traveling in the loop and how it depends on the hybridization strength as an experimentally controllable parameter in the Kondo effect.

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arXiv2016

SU(2)-SU(4) Kondo Crossover and Emergent Electric Polarization in a Triangular Triple Quantum Dot

We study an orbitally degenerate Kondo effect in a triangular triple quantum dot (TTQD), where the three dots are connected vertically with a single metallic lead through electron tunneling. Both spin and orbital degrees of freedom play an important role in the SU(4) Kondo effect. This is demonstrated by an equilateral TTQD Kondo system at half-filling, by Wilson's numerical renormalization group method. We show how an emergent electric polarization of the TTQD is associated with a crossover from SU(4) to SU(2) symmetry in the low-temperature state. A marked sign reversal of the electric polarization is generated by the fine-tuning of Kondo coupling with degenerate orbitals, which can be utilized to reveal orbital dynamics in the SU(4) Kondo effect.

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arXiv2013

Emergent Odd-Frequency Superconducting Order Parameter near Boundaries in Unconventional Superconductors

It was previously suggested that an odd-frequency pair amplitude exists in the vicinity of boundaries in unconventional superconductors. We develop this idea and quest for a novel superconducting order parameter with an odd-frequency dependence. For this purpose, we focus on p-wave superconductors and extend the quasi-classical theory to include the odd-frequency dependence in the order parameter. Both of the frequency and spacial dependences of the order parameter are determined self-consistently. Under a finite electron-phonon interaction, it is found that an odd-frequency order parameter is stabilized near the boundary and coexists with the even-frequency one. By analyzing the induced odd-frequency pair amplitude in terms of the superconducting quasi-particle wavefunction, it is found that the mid-gap bound state generates the emergent odd-frequency order parameter.

cond-mat.supr-con↗

arXiv2012

Strong-Coupling Superconductivity with Mixed Even- and Odd-Frequency Pairing

We investigate general structure of Landau free energy for stabilizing a novel superconducting state with both even-frequency and odd-frequency components in gap function. On the basis of the Luttinger-Ward functional, we elucidate an emergent mixing between different "parities" in time. The simplest case of the conventional s-wave singlet mixed with the odd-frequency triplet state under broken time-reversal symmetry is examined to demonstrate the anomalous structure of the free-energy functional. The induced odd-frequency component alters behaviors of physical quantities from those obtained by neglecting the odd-frequency component. The novel mixed state may also be relevant to strong-coupling superconductivity coexisting with ferromagnetism.

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arXiv2012

Coexistence of Even- and Odd-Frequency Superconductivities Under Broken Time-Reversal Symmetry

A novel superconducting state under the broken time-reversal symmetry is studied in conventional phonon-mediated superconductors. By solving the Eliashberg equation self-consistently with the mass renormalization effect, it is found that the even- and odd-frequency components of the order parameter coexist in the bulk system as a consequence of the broken time-reversal symmetry. This finding would direct more attention to the odd-frequency pairing that affects physical quantities, especially in strong coupling superconductors.

cond-mat.supr-con↗

arXiv2011

Effects of Impurities with Singlet-Triplet Configuration on Multiband Superconductors

Roles of multipole degrees of freedom in multiband superconductors are investigated in a case of impurities whose low-lying states consist of singlet ground and triplet excited states, which is related to the experimental fact that the transition temperature $T_{\rm c}$ is increased by Pr substitution for La in LaOs$_4$Sb$_{12}$. The most important contribution to the $T_{\rm c}$ increase comes from the inelastic interband scattering of electrons coupled to quadrupole or octupole moments of impurities. It is found that a magnetic field modifies an effective pairing interaction and the scattering anisotropy appears in the field-orientation dependence of the upper critical field $H_{{\rm c}2}$ in the vicinity of $T_{\rm c}$, although a uniaxial anisotropic field is required for experimental detection. This would be proof that the Pr internal degrees of freedom are relevant to the stability of superconductivity in (La$_{1-x}$Pr$_x$)Os$_4$Sb$_{12}$.

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arXiv2010

Effect of Impurities with Internal Structure on Multiband Superconductors - Possible Enhancement of Transition Temperature -

We study inelastic (dynamical) impurity scattering effects in two-band superconductors with the same ($s_{++}$ wave) or different ($s_\pm$ wave) sign order parameters. We focus on the enhancement of the superconducting transition temperature $T_{\rm c}$ by magnetic interband scattering with the interchange of crystal-field singlet ground and multiplet excited states. Either the $s_{++}$-wave or $s_\pm$-wave state is favored by the impurity-mediated pairing, which depends on the magnetic and nonmagnetic scattering strengths derived from the hybridization of the impurity states with the conduction bands. The details are examined for the singlet-triplet configuration that is suggestive of Pr impurities in the skutterudite superconductor LaOs$_4$Sb$_{12}$.

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