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Kazushi Aoyama

Publications and source records attributed to Kazushi Aoyama.

At least 19 recordsLinked to original sources
arXiv2026

Magnetic field effects on spin-split band and magnon transport in altermagnets and emergent compensated ferrimagnets

In altermagnets and fully compensated ferrimagnets, not only the electron band but also the magnon band exhibits spin splitting without net magnetization, which enables thermal activation of the magnon spin current. Here, we theoretically investigate magnetic field effects on the magnon properties of these antiferromagnets in the presence of a weak easy-axis anisotropy which makes the collinear states robust against the magnetic field. For the altermagnet and compensated ferrimagnet, we analyze a 2 sublattice order in the $J_1$-$J_2$-$J_2^\prime$ model on the square lattice and a triple-${\bf Q}$ 12-sublattice order in the $J_1$-$J_3$ model on the kagome lattice, respectively, each accompanied by $d$-wave and $s$-wave spin splitting at zero field. It is shown that for positive (negative) magnetic field $H$ whose energy scale is smaller than the anisotropy gap, the up- and down-spin magnon bands are shifted to lower (higher) and higher (lower) energies, respectively, similarly to the Zeeman coupling in electron systems. In the altermagnet, with increasing field, the $d$-wave splitting tends to be deformed into the $s$-wave one, which is reflected as the change in the direction of the spin current generated by thermal gradient. In the compensated ferrimagnet, the $s$-wave nature, i.e., the population imbalance between the up- and down-spin magnons at $H=0$, can induce an asymmetric field dependence of the longitudinal spin and thermal conductivities in a field-sweep process.

cond-mat.str-el↗
arXiv2026

Spatiotemporal Structures of Parametrically Driven Nonlinear Lattices

We theoretically investigate the effects of parametric driving on the one-dimensional Frenkel-Kontorova model, a nonlinear many-body lattice system. It is numerically found that a parametric vibration induces spatiotemporal ordering characterized by the subharmonic frequency $ω_{\mathrm{ex}}/2$ and wavenumber $k$ which takes nontrivial values depending on the vibration strength $P_{\mathrm{ac}}$ and frequency $ω_{\mathrm{ex}}$. With increasing $P_{\mathrm{ac}}$, $k$ gradually increases and discontinuously jumps up to $k=π$. Based on an extended linear stability analysis, we show that the former $k$ resonance (the latter $π$ resonance) corresponds to the unstable (stable) mode and that the $k$ resonance is made possible by the interplay between the parametric amplification and the collective fluctuation developing through the nonlinear effect.

cond-mat.stat-mech↗
arXiv2026

The second altermagnet candidate in organic conductors: $κ$-(BEDT-TTF)$_2$$m$-HOOCC$_6$H$_4$SO$_3$

We have developed a novel BEDT-TTF-based organic conductor, $κ$-(BEDT-TTF)$_2 m$-HOOCC$_6$H$_4$SO$_3$ ($κ$-$m$-SBA), and propose it as a candidate for altermagnet. Tight-binding band calculations of $κ$-$m$-SBA provide a $t'/t$ of 1.01 at 100 K, indicating that the spin structure is closely aligned to an equilateral triangle ($t'/t= 1$). While most $κ$-type BEDT-TTF-based salts become spin liquids due to the spin frustration caused by the triangular lattice, $κ$-$m$-SBA surprisingly shows a weak ferromagnetic transition at $T_\mathrm{N} = 14$ K due to a canted antiferromagnetic (AFM) spin structure. Until recently, $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($κ$-Cl) was the only $κ$-type organic conductor known to exhibit this order, and it is also recognized as the first candidate for altermagnetism in organic conductors. This was theoretically predicted by Naka et al. in 2019, who demonstrated that $κ$-type organic conductors can be candidates for altermagnetism if they display such order. Consequently, $κ$-$m$-SBA can be considered the second candidate for altermagnetism in organic conductors. Furthermore, numerical calculations demonstrate a characteristic of altermagnets in $κ$-$m$-SBA, namely spin splitting of energy bands.

cond-mat.mtrl-sci↗
arXiv2026

Triple-${\bf Q}$ collinear state with compensated ferrimagnetic nature on frustrated kagome lattice

Spin-selective band splitting without net magnetization and spin-orbit couplings serves for a next-generation spin-current generator, and its typical platforms are altermagnets and compensated ferrimagnets as well, where the existence of a crystal asymmetry or nonequivalent sites is essential. Here, we theoretically demonstrate that such a splitting can be realized in a triple-{\bf Q} 12-sublattice state emerging in a $J_3$-dominant kagome antiferromagnet, without the help of the crystal asymmetry. Reflecting the multi-sublattice nature, a local magnetization reveals a fully compensated ferrimagnetic pattern in units of a triangle plaquette, leading to $s$-wave-type spin splittings in magnon and electron bands. This enables an atiferromagnetic spin Seebeck effect at zero field in insulating systems and filling-controlled polarized states in metallic systems, highlighting the potential of frustrated magnets to realize novel spintronics functionalities.

cond-mat.str-el↗
arXiv2025

Observation of the crossover from quantum fluxoid to half-quantum fluxoid in a chiral superconducting device

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.

cond-mat.supr-con↗
arXiv2025

Spontaneous chirality selection and nonreciprocal spin wave in breathing-kagome antiferromagnets at zero field

It has been known that the spin-wave dispersion, which is usually symmetric in the momentum space with respect to ${\bf q}=0$, can be asymmetric in the presence of the Dzyaloshinskii-Moriya (DM) interaction and an applied magnetic field. Here, we theoretically demonstrate that in $J_3$-dominant classical Heisenberg antiferromagnets on the breathing kagome lattice, the asymmetric spin-wave dispersion appears in a chiral phase due to non-uniform geometric phases acquired in the spin-wave propagation processes. This points to the emergence of a nonreciprocal spin wave in the absence of both the DM interaction and the magnetic field. Reflecting the asymmetry, positive-spin-chirality and negative-spin-chirality states, either one of which is selected in the low-temperature phase by the symmetry breaking, show different spin-wave dispersions, suggesting that the two energetically-degenerate chiral states can be distinguished by the spin-wave propagation.

cond-mat.str-el↗
arXiv2025

RKKY interaction in the presence of a charge-density-wave order

Effects of a charge-density-wave (CDW) order on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction has been theoretically investigated. Assuming that the CDW with an incommensurate ordering vector ${\bf Q}_c$ is induced by a Fermi surface nesting, we show that the CDW order suppresses the conventional RKKY interaction and that it can induce a mode coupling between magnetic ordering vectors ${\bf q}_m$ and ${\bf q}_m\pm {\bf Q}_c$. This suggests that below the CDW transition temperature, the correlation between localized spins may develop at both ${\bf q}_m$ and ${\bf q}_m\pm {\bf Q}_c$. Experimental implications of our result to van der Waals materials $R$Te$_3$ are also discussed.

cond-mat.str-el↗
arXiv2024

Effects of frequency mixing on Shapiro-step formations in sliding charge-density-waves

A one-dimensional charge density wave (CDW) is driven to slide by a dc electric field, carrying an electric current. In an additional ac field with frequency $ω_{\mathrm{ex}}$, it is known that the sliding CDW can be synchronized to $ω_{\mathrm{ex}}$, leading to the occurrence of Shapiro steps in the $I$-$V$ characteristics. Motivated by a recent experiment where ac fields with two frequencies $ω_{\mathrm{ex}}$ and $ω_{\mathrm{ex}}^{\prime}$ are simultaneously applied, we theoretically investigate the effects of frequency mixing on the Shapiro-step formation. Based on the Fukuyama-Lee-Rice model, we show that in addition to the main steps induced by $ω_{\mathrm{ex}}$, satellite steps characterized by $ω_{\mathrm{ex}}^{\prime}$ emerge. It is also found that with increasing the ac-field strength for $ω_{\mathrm{ex}}^{\prime}$, each step width first exhibits a damped oscillation as in the one-frequency case, and then, exhibits a non-monotonic behavior. The origin of these behaviors and the relevance to the associated experiment are also discussed.

cond-mat.str-el↗
arXiv2023

Stripe order and diode effect in two-dimensional Rashba superconductors

In two-dimensional superconductors with a Rashba-type spin-orbit coupling, it is known that an in-plane magnetic field can induce a helical superconducting (SC) state with a phase modulation $e^{i {\bf q}\cdot {\bf r}}$. Here, we theoretically investigate the stability of a stripe order, a weight-biased superposition state composed of $+{\bf q}$ and $-{\bf q}$ modes taking the form of $Δ_+ e^{i{\bf q}\cdot{\bf r}}+Δ_- e^{-i{\bf q}\cdot{\bf r}}$ with $|Δ_+|\neq|Δ_-|\neq 0$, assuming that the spin-singlet pairing channel is dominant. Based on the Ginzburg-Landau theory, we show that for both s-wave and d-wave pairing symmetries, the stripe order can appear in the high-field and low-temperature region inside the helical phase and that the transition between the helical and stripe phases is of second order. It is noteworthy that for the d-wave pairing, the stability region of the stripe phase shrinks when the in-plane field is rotated from the nodal direction to the anti-nodal direction. It is also found that the nonreciprocity of the critical current, the so-called SC diode effect, emerges not only in the helical phase but also in the stripe phase, with no clear nonreciprocity anomaly at the helical-stripe transition due to its second-order nature.

cond-mat.supr-con↗
arXiv2023

Fractal and subharmonic responses driven by surface acoustic waves during charge density wave sliding

We theoretically investigate the effects of surface acoustic waves (SAWs) on an electric-field-driven sliding motion of a one-dimensional charge density wave (CDW), which is initially pinned by impurities. By numerically analyzing an extended Fukuyama-Lee-Rice model, we show that a mechanical vibration of the SAW, which, in the model, is assumed to affect the CDW via the pinning site in the form of temporally oscillating pinning parameters, induces Shapiro steps with self-similarity, i.e., the devil's staircase, in the current-voltage characteristics. It is also found that when the SAW acts as the vibration in the pinning strength, the mechanism of the mode locking (harmonic and subharmonic responses) leading to the occurrence of the Shapiro steps is modified, and as a result, the fractal dimension and parameter dependence of the SAW-induced staircase can be considerably different from those for the conventional ac-electric-field-induced one. This suggests that an unconventional type of fractal phenomena can emerge in the SAW-induced CDW dynamics.

cond-mat.str-el↗
arXiv2023

Half-quantum flux in spin-triplet superconducting rings with bias current

Effects of a bias electric current have been theoretically investigated in a spin-triplet superconducting ring in a magnetic field. Based on the Ginzburg-Landau theory, we show that the bias current can stabilize a half-quantum-flux (HQF) state via couplings to the Zeeman field and the dipole-type spin-orbit interaction, the latter becoming active when the field is tilted from the ring axis. The emergence of the HQF state is reflected as a field-induced half-quantum-shift in the Little-Parks (LP) oscillation in the critical current. Possible relevance to recent LP experiments is also discussed.

cond-mat.supr-con↗
arXiv2023

Zero-field miniature skyrmion crystal and chiral domain state in breathing-kagome antiferromagnets

The stability of a miniature skyrmion crystal (SkX) with only a small number of spins in the magnetic unit cell has been theoretically investigated in $J_1$-$J_3$ antiferromagnets on the breathing kagome lattice with a single-ion anisotropy $D$ at zero field. It is found by means of Monte Carlo simulations that due to the breathing bond-alternation, a zero-field triple-${\bf Q}$ miniature SkX can be stabilized not only in the specific case of $D=0$ [K. Aoyama and H. Kawamura, Phys. Rev. B 105, L100407 (2022)] but also in more general situations with easy-axis ($D<0$) and easy-plane ($D>0$) anisotropies which favor triple-${\bf Q}$ collinear and noncoplanar states, respectively. Since the SkX and anti-SkX each having positive or negative chirality are energetically degenerate, the topological Hall effect of alternative sign is possible at zero field. It is also found that reflecting the chiral degeneracy, the collinear and coplanar phases preempting the SkX phase possess random domain structures consisting of positive- and negative-chirality clusters.

cond-mat.str-el↗
arXiv2022

Spin and thermal transport and critical phenomena in three-dimensional antiferromagnets

We investigate spin and thermal transport near the Néel transition temperature $T_N$ in three dimensions, by numerically analyzing the classical antiferromagnetic $XXZ$ model on the cubic lattice, where in the model, the anisotropy of the exchange interaction $Δ=J_z/J_x$ plays a role to control the universality class of the transition. It is found by means of the hybrid Monte-Carlo and spin-dynamics simulations that in the $XY$ and Heisenberg cases of $Δ\leq 1$, the longitudinal spin conductivity $σ^s_{μμ}$ exhibits a divergent enhancement on cooling toward $T_N$, while not in the Ising case of $Δ>1$. In all the three cases, the temperature dependence of the thermal conductivity $κ_{μμ}$ is featureless at $T_N$, being consistent with experimental results. The divergent enhancement of $σ^s_{μμ}$ toward $T_N$ is attributed to the spin-current relaxation time which gets longer toward $T_N$, showing a power-law divergence characteristic of critical phenomena. It is also found that in contrast to the $XY$ case where the divergence in $σ^s_{μμ}$ is rapidly suppressed below $T_N$, $σ^s_{μμ}$ likely remains divergent even below $T_N$ in the Heisenberg case, which might experimentally be observed in the ideally isotropic antiferromagnet RbMnF$_3$.

cond-mat.str-el↗
arXiv2022

Hedgehog lattice and field-induced chirality in breathing-pyrochlore Heisenberg antiferromagnets

We theoretically investigate a $J_1$-$J_3$ classical Heisenberg model on the breathing pyrochlore lattice, where the nearest-neighbor (NN) exchange interactions for small and large tetrahedra, $J_1$ and $J_1'$, take different values due to the breathing bond-alternation and $J_3$ is the third NN antiferromagnetic interaction along the bond direction. It is found by means of Monte Carlo simulations that for large $J_3$, a hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, emerges in the form of a quadruple-${\bf Q}$ state characterized by the ordering vector of ${\bf Q}=(\pm\frac{1}{2},\pm\frac{1}{2},\pm\frac{1}{2})$, being irrespective of the signs of $J_1$ and/or $J_1'$ as long as $J_1\neq J_1'$. It is also found that in an applied magnetic field, there appear six quadruple-${\bf Q}$ states depending on the values of $J_1$ and $J_1'$, among which three phases including the in-field hedgehog-lattice state exhibit nonzero total chirality $\mbox{\boldmath $χ$}^{\rm T}$ associated with the anomalous Hall effect of chirality origin. In the remaining two chiral phases, which are realized in the presence of ferromagnetic $J_1$ and/or $J_1'$, the spin structure is not topologically nontrivial, in spite of the fact that $\mbox{\boldmath $χ$}^{\rm T} \neq 0$. The role of the topological objects of the monopoles in $\mbox{\boldmath $χ$}^{\rm T}$ is also discussed.

cond-mat.str-el↗
arXiv2022

Little-Parks oscillation and ${\bf d}$-vector texture in spin-triplet superconducting rings with bias current

We theoretically investigate the critical bias current $j_c$ for a superconducting (SC) ring in a magnetic field. Based on the Ginzburg-Landau theory, we show that $j_c$ exhibits a Little-Parks (LP) oscillation as a function of the magnetic flux passing through the ring, similarly to the LP oscillation in the SC transition temperature. It is also found that for a spin-triplet SC ring, the ${\bf d}$-vector rotates to yield a larger $j_c$, forming a texture along the circumference of the ring. Experimental implications of our result are discussed.

cond-mat.supr-con↗
arXiv2022

Emergent skyrmion-based chiral order in zero-field Heisenberg antiferromagnets on the breathing kagome lattice

We show that classical Heisenberg antiferromagnets on the breathing kagome lattice can be a platform to realize a zero-field topological order of the scalar spin chirality which can be viewed as a miniature skyrmion crystal (SkX) of discrete form with a small number of spins in its magnetic unit cell. In the model, a third nearest-neighbor (NN) antiferromagnetic interaction along the bond direction $J_3$ and the breathing bond-alternation characterized by the ratio of the NN interaction for large triangles to that for small ones, $J_1'/J_1$, are essential. It is found by means of Monte Carlo simulations that a commensurate triple-${\bf Q}$ state appearing for relatively strong $J_3$ at zero field is the noncoplanar state with the SkX structure in the breathing case of $J_1'/J_1 \neq 1$, while in the uniform case of $J_1'/J_1 =1$, it is a collinear state favored by thermal fluctuations. The origin of this chiral order and experimental implications of our result are also discussed.

cond-mat.str-el↗
arXiv2021

Effects of spin-lattice coupling and a magnetic field in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

We theoretically investigate spin-lattice coupling (SLC) effects on the in-field ordering properties of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. Here, we use the two possible simplified models describing the effect of local lattice distortions on the spin ordering via the SLC, the bond-phonon and site-phonon models. It is found by means of Monte Carlo simulations that in both models, the $\frac{1}{2}$ plateau shows up in the magnetization curve being relatively robust against the breathing bond-alternation, although magnetic long-range orders (LRO's) are realized only in the site-phonon model. In the bond-phonon model, additional further neighbor interactions are necessary to induce a magnetic LRO. In the site-phonon model, it is also found that in addition to the low-field, middle-field 1/2-plateau, and high-field phases appearing on both the uniform and breathing pyrochlore lattices, various types of unconventional phases which can be viewed as LRO's in units of tetrahedron are induced by the breathing bond-alternation just below the 1/2 plateau and the saturation field. The occurrence of these tetrahedron-based orders could be attributed to the nature characteristic of the breathing pyrochlore lattice, i.e., the existence of the nonequivalent small and large tetrahedra. Experimental implications of our result are also discussed.

cond-mat.str-el↗
arXiv2020

Hedgehog-lattice spin texture in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

The hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, is known to be realized in the presence of the Dzyaloshinskii-Moriya (DM) interaction. Here, we demonstrate by means of Monte Carlo simulations that the hedgehog lattice is induced by not the DM interaction but frustration in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. In the model, the breathing bond-alternation is characterized by the ratio of the nearest-neighbor (NN) antiferromagnetic exchange interaction for large tetrahedra to that for small ones, J1'/J1. A quadruple-q state with the ordering vector of q=(\pm 1/2,\pm 1/2,\pm 1/2), which is realized for a large third NN antiferromagnetic interaction along the bond direction J3, turns out to become the hedgehog-lattice state in the breathing case of J1'/J1 <1, while in the uniform case of J1'/J1 =1, it is a collinear state favored by thermal fluctuations. It is also found that in a magnetic field, the structure of the (1/2,1/2,1/2) hedgehog lattice is changed from cubic to tetragonal, resulting in a nonzero net spin chirality which in a metalic system, should yield a characteristic topological Hall effect.

cond-mat.str-el↗