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Ryotaro Arita

Publications and source records attributed to Ryotaro Arita.

At least 19 recordsLinked to original sources
arXiv2026

Spontaneous topological diode effect induced by non-coplanar antiferromagnetic order

In non-centrosymmetric bulk materials, electrons flowing in the opposite directions often exhibit different properties. This diode-like effect (i.e., non-reciprocal electron transport) typically originates from relativistic spin-orbit coupling, and its amplitude scales with external magnetic field or magnetization in ferromagnets. Although such rectification is usually absent in antiferromagnets at zero field, recent theories predict that non-coplanar antiferromagnetic order can host large non-reciprocity even without spin-orbit coupling, magnetization or external magnetic fields. This phenomenon - the spontaneous topological diode effect - can potentially be used for efficient electrical readout of antiferromagnetic domains, while it has not been verified for real materials. Here, we report the observation of this effect in a chiral-lattice non-coplanar antiferromagnet CoTa3S6. Nonlinear resistivity measurements reveal the non reciprocal nature of electron transport along the [001] direction at zero field. The observed non-reciprocity exceeds that in typical ferromagnets, whose amplitude is governed by antiferromagnetic order parameter rather than magnetization. First-principles calculations show that the non-coplanar antiferromagnetic order induces asymmetric band dispersions even without spin-orbit coupling, establishing the non-relativistic origin of the observed response. The present findings demonstrate a new mechanism for efficient electrical readout of antiferromagnetic domains, and highlight non-coplanar antiferromagnets as a promising material platform for realizing ferromagnet-like functional responses without relying on spin-orbit coupling or magnetization.

cond-mat.mtrl-sci↗
arXiv2026

Transverse magneto-gyrotropy driven by spin-induced ferroaxiality

Photon chirality is inherently sensitive to structural handedness and magnetism, which break spatial inversion and time-reversal symmetries, respectively. Ferroaxial order, by contrast, hosts two-dimensional chirality while preserving both symmetries, thereby precluding its direct coupling to photon chirality. Intertwining ferroaxiality with magnetic order, however, may lift such constraints and enable unconventional coupling to photon chirality. Nevertheless, the optical response arising from such intertwined order has yet to be demonstrated experimentally. Here, we demonstrate transverse magneto-gyrotropy arising from spin-driven ferroaxiality in the Weyl magnet EuCd2Sb2. Whereas the conventional polar magneto-optical Kerr effect is governed by magnetization parallel to the light-propagation direction, we observe a gyrotropic response associated with magnetization transverse to the light-propagation direction. The optical rotation exhibits a characteristic 120-degree periodicity upon rotation of the in-plane magnetic field, reflecting the threefold symmetry of the emergent ferroaxial state and revealing the ferroaxial origin of the transverse gyrotropic response. This coupling to photon chirality enables vectorial control of the gyrotropic response through the magnetic field direction. Our findings establish an interaction between photon chirality and transverse magnetization enabled by intertwined ferroaxial and magnetic orders, demonstrating a novel form of multiferroicity with functional light-matter interaction.

cond-mat.mtrl-sci↗
arXiv2026

Superconducting landscape of the strongly correlated bilayer Hubbard model

Coupling two strongly correlated layers opens a distinct route to repulsion-driven superconductivity. Yet the same interlayer coupling makes the optimization of superconductivity highly nontrivial. Using the dynamical vertex approximation, we determine the superconducting landscape in the enlarged parameter space spanned by filling and interlayer hopping. We identify an optimal regime at intermediate interlayer hopping and moderate doping away from half filling, where the system exhibits high superconducting transition temperatures ($T_{\rm c}$) with an interband $s_{\pm}$-wave gap structure. Toward half filling, however, strong self-energy effects can suppress the superconducting tendency by reducing the low-energy spectral weight. We further show that a Fermi-surface distortion induced by longer-range hoppings can weaken this suppression and enhance $T_{\rm c}$. Our results establish how filling, interlayer hopping, and Fermi-surface geometry jointly optimize the superconducting regime in the strongly correlated bilayer Hubbard model.

cond-mat.supr-con↗
arXiv2026

Real-space determination of orbital states driving successive phase transitions in FeV2O4

Direct experimental access to orbital states in strongly correlated materials remains a major challenge, despite their central role in driving coupled structural and magnetic phase transitions. In systems where electronic correlations, electron-lattice coupling, and relativistic spin-orbit interactions compete on comparable energy scales, even first-principles calculations often yield multiple metastable solutions, hindering the unambiguous identification of the ground state. Here, we demonstrate that the orbital states of the spinel oxide FeV2O4, which possesses active orbital degrees of freedom on both Fe and V ions, are uniquely resolved by combining valence electron density (VED) analysis based on state-of-the-art synchrotron x-ray diffraction with spin-polarized density-functional-theory calculations. Our results reveal that temperature-dependent rearrangements of orbital occupations drive successive structural transitions that accompany collinear and noncoplanar ferrimagnetic orders, establishing a direct correspondence between orbital anisotropy and spin structure. More broadly, this work shows that experimentally determined VED provides a decisive real-space constraint on competing theoretical solutions, offering a powerful and broadly applicable framework for elucidating the microscopic mechanisms of complex phase transitions in strongly correlated electron systems.

cond-mat.str-el↗
arXiv2026

Observation of complex orbital order in MnV2O4

Orbital ordering in vanadium spinel oxides with a geometrically frustrated pyrochlore structure has been a subject of controversy, owing to competing theoretical models and the absence of direct experimental evidence. Here we combine high-precision single-crystal synchrotron x-ray diffraction with core differential fourier synthesis to visualize the valence electron density (VED) of the orbital-ordered ground state in real space. By carefully investigating multiple-scattering artifacts, we identify the low-temperature structure as belonging to the I41/amd space group. The reconstructed VED around the V sites reveals an orbital-ordered state distinct from both previously proposed real- and complex-orbital models. Our results resolve the long-standing controversy in MnV2O4 and establish a route to identifying orbital states in frustrated spin-orbital systems.

cond-mat.str-el↗
arXiv2026

Intrinsic longitudinal spin-conductivity from ferroaxial spin-symmetry breaking

The interplay of antiferromagnetic order, momentum-dependent Bloch spin-splitting, time-reversal (T), and parity (P) symmetries in non-relativistic systems has emerged as a central theme for spintronics. Two well-known examples are P-preserving and T-violating altermagnets and P-violating and T-preserving odd-parity magnets. Here we examine a class of coplanar AFMs with a translation invariant vector spin chiral (VSC) order that preserves both P and T symmetries but breaks the non-relativistic spin-rotational invariance. Naively, such a VSC order is not expected to exhibit unusual phenomena. Here we show that the spin-rotational symmetry breaking generated by this VSC order can take on multiple forms, including ferroaxial symmetry breaking. We further show that these states allow for non-relativistic and non-dissipative purely longitudinal (or purely transverse) spin-conductivities. These states also allow the generation of non-relativistic altermagnetic spin-splittings through circularly polarized light. We identify 16 candidate materials in the Magndata database for which our theory is applicable and provide effective microscopic models and DFT-based results that highlight the large emergent spin conductivities.

cond-mat.str-el↗
arXiv2026

Microscopic calculation of coherence lengths and magnetic penetration depth in multiband superconductors

We present an extended Ginzburg-Landau (GL) method for calculating the superconducting coherence length and magnetic penetration depth at temperatures well below the transition temperature $T_{\mathrm c}$. In contrast to conventional GL theory, which expands the free energy in both the order parameters and their gradients, our method applies a perturbative expansion only to the covariant-gradient terms, while retaining the full dependence on the superconducting order parameters. The coefficients of these terms are determined from finite differences of microscopic free energies evaluated at small imposed pair momenta. The method applies to both single-band and multiband superconductors and therefore provides a framework for incorporating more realistic electronic structures. For the models examined here, the extended GL method agrees well with real-space Bogoliubov-de Gennes (BdG) calculations over a wide temperature range, while requiring substantially less computational effort.

cond-mat.supr-con↗
arXiv2026

Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic investigation of superconductivity in compressively strained La$_2$LnNi$_2$O$_7$ films (Ln = lanthanides) at ambient and high pressures. The normal-state resistivity at ambient pressure, revealed by suppressing superconductivity with magnetic fields of 59 T, tends toward $T^2$ behaviour. Under high pressure in a cubic-anvil cell, $T_{\mathrm{c}}$ was enhanced from 41-42 K at ambient pressure to 67-73 K at 16 GPa. On the other hand, lattice compression induced by Ln substitution, which may mimic effects of pressure, lowers $T_{\mathrm{c}}$. In both cases, $T_{\mathrm{c}}$ correlates with the evolution of normal-state transport between $T^2$ and $T$-linear behaviour, offering insight into the interplay between lattice structure and superconductivity in bilayer nickelates.

cond-mat.supr-con↗
arXiv2026

Observation of in-plane anomalous Nernst effect

The Nernst effect, which enables the conversion of a heat current into a transverse voltage under magnetic field or spin magnetization, holds significant promise for energy harvesting and thermal management in future electronics. However, the conventional Nernst effect is fundamentally constrained by the orthogonality requirement that the applied field or spontaneous magnetization must be perpendicular to the plane defined by the temperature gradient and the induced voltage. Here we report that symmetry-tailored ultrathin films of a prototypical ferromagnetic oxide exhibit anomalous Nernst effect arising from intrinsic coupling to spontaneous in-plane spin magnetization. Systematic magnetothermoelectric measurements under spherical rotations of the magnetic field reveal that a pronounced Nernst signal, comparable in magnitude to the out-of-plane response, emerges robustly associated with out-of-plane orbital magnetization. Our findings demonstrate that the anomalous Nernst effect is no longer limited by the orthogonality condition, opening new opportunities for more flexible designs of magnetothermoelectric materials and devices.

cond-mat.str-el↗
arXiv2026

Quadrupolar phase transition in superconducting lanthanum hydride

Lanthanum hydride (LaH$_{10}$) has been widely studied for its high superconducting critical temperature of 250 K at about 170 GPa pressure. Although the structural ${R\bar{3}m}$-to-${Fm\bar{3}m}$ transition under pressure connected to the emergence of the superconducting phase in this material is broadly understood, the detailed characterization of its nature and its order parameter are still missing. By applying the cluster multipole moment analysis to the hydrogen sublattice, we reveal that this transition is triggered by a quadrupolar $T_{2g}$ order parameter, and we provide evidence for its weak first-order nature. By performing path integral molecular dynamics coupled to a message-passing atomic cluster expansion (MACE) neural network potential, trained on Perdew-Burke-Ernzerhof (PBE) density functional theory configurations, we show that the collapse of the order parameter at the transition is simultaneously associated with the discontinuous softening of the optical $T_{2g}$ phonons. Their symmetry lets them carry a non-negligible electron-phonon coupling in LaH$_{10}$, while the weak first-order nature of the transition makes them soft. The presence of structural instabilities with low-frequency quadrupolar distortions can be a key ingredient to enhance superconductivity in superhydrides and provides guidance for the discovery of new high-$T_c$ superconductors in hydrogen-rich compounds.

cond-mat.supr-con↗
arXiv2026

Quantum-geometry-driven exact ferromagnetic ground state in a nearly flat band

We construct a Hubbard model with a nearly flat band whose quantum geometry can be tuned independently of the energy dispersion and the Coulomb interaction. We show that, when the nearly flat band is half-filled, the exact ground state of the model exhibits ferromagnetism and that this ferromagnetism is stabilized by the quantum metric through the spin stiffness. Furthermore, we demonstrate that tuning the quantum geometry alone drives a magnetic phase transition. Our nonperturbative results without resorting to mean-field approximations reveal the quantum-geometric origin of ferromagnetism and the underlying many-body physics in dispersive-band systems.

cond-mat.str-el↗
arXiv2026

First-principles calculation of coherence length and penetration depth based on density functional theory for superconductors

We develop a first-principles framework for evaluating the fundamental length scales of superconductivity, namely the coherence length $ξ_0$ and the magnetic penetration depth $λ_\mathrm{L}$, within superconducting density functional theory (SCDFT). By incorporating finite-momentum Cooper pairs, we formulate a microscopic scheme that enables a consistent and parameter-free determination of $ξ_0$, $λ_\mathrm{L}$, and the superconducting transition temperature $T_\mathrm{c}$ on the same theoretical footing. Applying the method to representative elemental superconductors, the A15 compound V$_3$Si, and H$_3$S under high pressure, we obtain results in good agreement with available experimental and reproduce the type-I/type-II classification across all materials studied. The unified access to $ξ_0$ and $λ_\mathrm{L}$ further allows us to construct the Uemura plot entirely from first principles, showing that higher-$T_\mathrm{c}$ systems are characterized by the simultaneous realization of strong pairing and large phase stiffness. Our results establish a predictive first-principles route to superconducting length scales and provide a microscopic interpretation of empirical correlations in superconductivity.

cond-mat.supr-con↗
arXiv2026

Self-energy pole optimization of superconductivity in the bilayer Hubbard model

We study the real-frequency structure of the self-energy in the bilayer Hubbard model, using the dynamical cluster approximation. At half filling, the Mott insulator-band insulator (MI-BI) crossover involves a rearrangement of self-energy poles between the bonding and antibonding bands; these poles cross as the interlayer hopping $t_{\perp}$ is varied. Upon doping, this pole structure produces a band-selective pseudogap and enhances $s^{\pm}$-wave superconductivity. The order parameter is maximized near the MI-BI boundary, where low-energy anomalous self-energy poles develop simultaneously in both bands and cooperatively enhance the pairing. We further show that these self-energy poles can be interpreted as emergent fermionic excitations, offering an enhanced-pairing mechanism in common with the single-layer Hubbard model. The controllability of these poles through $t_{\perp}$ makes the bilayer system an unconventional platform for optimizing strongly correlated superconductivity.

cond-mat.str-el↗
arXiv2026

Tunnel magnetoresistance effect with a Cr-doped $\mathrm{RuO_{2}}(110)$ altermagnet

Antiferromagnets can have a finite spin-polarization in the momentum space when their magnetic structure breaks the macroscopic time-reversal symmetry. This spin-polarization can produce a spin-polarized electric current even in antiferromagnets with vanishingly small net magnetizaton, which supports the antiferromagentic tunnel magnetoresistance (TMR) effect. In this paper, using first-principles calculations, we study the TMR effect with a doped altermagnet $\mathrm{Ru}_{1-x}\mathrm{Cr}_{x}\mathrm{O}_{2}$ with $(110)$ orientation, whose collinear antiferromagnetic structure breaks the time-reversal symmetry macroscopically. The momentum-dependent spin-polarization combined with the $(110)$ crystal orientation makes the electric current spin-polarized through bulk $\mathrm{Ru}_{1-x}\mathrm{Cr}_{x}\mathrm{O}_{2}(110)$. We further calculate the TMR effect in the $\mathrm{Ru}_{1-x}\mathrm{Cr}_{x}\mathrm{O}_{2}(110)/\mathrm{TiO_{2}}(110)/\mathrm{Ru}_{1-x}\mathrm{Cr}_{x}\mathrm{O}_{2}(110)$ tunnel junction and show that a finite TMR effect emerges. Based on the analysis of the tunneling transport, the TMR effect is attributed to the spin polarized tunneling transport with momentum dependence and the interfacial magnetic structures, as well as the spin-polarized electric current in a bulk form of $\mathrm{Ru}_{1-x}\mathrm{Cr}_{x}\mathrm{O}_{2}(110)$.

cond-mat.mtrl-sci↗
arXiv2026

Emergent toroidal induction in a polar Weyl ferromagnet

Spin-orbit coupling (SOC) underpins modern spintronics by enabling the electrical generation of spin torques. Its reciprocal counterpart, in which magnetization dynamics produce electromotive forces through a spin-dependent Berry phase, is known as emergent electromagnetic induction (EEMI). However, this effect has previously been observed only in magnetic textures with spatial gradients, such as domain walls, helices, and skyrmions. Here, we demonstrate that even a spatially uniform ferromagnet can host EEMI through a previously unrecognized Berry-phase mechanism inherent to noncentrosymmetric conductors. In the polar Weyl ferromagnet PrAlGe, an applied alternating current generates spin-orbit torques that drive collective magnetization dynamics. The resulting emergent toroidal moment (T = P \times M), where (P) is the crystal's polar axis and (M) is the net magnetization, acts as a gauge potential whose time derivative (dT/dt) induces a Hall voltage. This contribution appears specifically in the out-of-phase component of the AC Hall response and scales linearly with frequency, providing direct evidence for EEMI. First-principles calculations further reveal that this toroidal vector encodes the collective motion of Weyl nodes in momentum space. These findings establish "emergent toroidal induction" as a new manifestation of spin-orbit entanglement, unifying Berry phase, topology, and spin dynamics while opening a pathway toward intrinsic and energy-efficient spin-charge interconversion.

cond-mat.mtrl-sci↗
arXiv2026

Interpretable machine learning of magnetic transition temperature in Heusler magnets via hierarchical dependence extraction

We employ interpretable machine learning to analyze the material dependence of the magnetic transition temperature $T_c$ in ferromagnetic and ferrimagnetic Heusler compounds. For over 200 candidate materials with the same $F\overline{4}3m$ crystal structure but different chemical formulae and lattice constants, we consider both experimental $T_c$ and those computed via classical Monte Carlo simulations using magnetic interactions derived from ab initio calculations. We use the hierarchical dependence extraction (HDE) procedure [Morée and Arita, Phys. Rev. B 110, 014502 (2024)] to determine how $T_c$ depends on chemical composition and magnetic moments, from leading to higher-order effects, and use these dependencies to construct an explicit expression for $T_c$. Our results show that the HDE framework predicts $T_c$ with accuracy comparable to other machine-learning approaches such as neural network and random forest algorithms while remaining fully interpretable. $T_c$ is primarily governed by the proportions of Fe, Co, and Mn, increasing systematically with their concentration. These findings clarify how chemical composition and magnetic moments influence $T_c$ in collinear Heusler alloys and support the use of the HDE for computationally guided discovery of new functional materials with tailored $T_c$ values.

cond-mat.mtrl-sci↗
arXiv2026

Ab initio study of magnetoresistance effect in $\mathrm{Mn_{3}Sn}/\mathrm{MgO}/\mathrm{Mn_{3}Sn}$ antiferromagnetic tunnel junction

The antiferromagnets with the time-reversal symmetry broken magnetic structures possess a finite spin splitting in the momentum space, and may contribute to a realization of a finite tunnel magnetoresistance (TMR) effect even with magnets with zero net spin polarization. In this paper, we study the TMR effect with the noncollinear antiferromagnet $\mathrm{Mn_{3}Sn}$ whose inverse $120^{\circ}$ antiferromagnetic order breaks the time-reversal symmetry. In particular, we employ the representative barrier material $\mathrm{MgO}$ as the tunnel insulator, and calculate the TMR effect in the $\mathrm{Mn_{3}Sn}(01\bar{1}0)/\mathrm{MgO}(110)/\mathrm{Mn_{3}Sn}$ magnetic tunnel junctions (MTJs), which has an optimal geometry for the spin-orbit torque switching of the magnetic configurations. We show that a finite TMR ratio reaching $\gtrsim 1000\%$ appears in the $\mathrm{Mn_{3}Sn}/\mathrm{MgO}/\mathrm{Mn_{3}Sn}$ MTJs, which is due to the spin splitting properties of $\mathrm{Mn_{3}Sn}$ in the momentum space combined with the screening effect of $\mathrm{MgO}$.

cond-mat.mes-hall↗
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↗