Searcharxiv⌕ Search

arXiv subjects

Ryotaro Arita

Publications and source records attributed to Ryotaro Arita.

At least 19 recordsLinked to original sources
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

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

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

Pressure-induced superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ films

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity in La$_3$Ni$_2$O$_7$ under high pressure has led to a rapid expansion of the $T_{\mathrm{c}}$ range through lanthanide $Ln$ substitution, and to ambient-pressure superconductivity in strained thin films, yet the exploration of new bilayer nickelates remains strongly constrained by thermodynamic stability. Beyond the difficulty of synthesis of bulk single-crystals, here we report on the pressure-induced high-$T_{\mathrm{c}}$ superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ thin films. While the Pr$_3$Ni$_2$O$_7$ films exhibit insulating behaviour at ambient pressure regardless of ozone-annealing treatment, they show $T$-linear metallic transport and superconductivity reaching an onset $T_{\mathrm{c}}$ of 66 K and zero-resistance at nearly 40 K at 22 GPa. Furthermore, Nd$_3$Ni$_2$O$_7$, with the smaller rare-earth ion Nd, can also be stabilized, however, superconductivity is not observed in the measured pressure range. Epitaxial stabilization enables us to examine the dependence of $T_{\mathrm{c}}$ and the critical pressure $P_{\mathrm{c}}$ for superconductivity on the $Ln$ ion in $Ln_3$Ni$_2$O$_7$ ($Ln$ = La, Pr, Nd). These results suggest that a higher $P_{\mathrm{c}}$ is required for smaller $Ln$ ions, consistent with trends observed in bulk studies of $Ln$ substitution. This study demonstrates that epitaxial stabilization is a powerful technique to further expand the family of superconducting bilayer nickelates.

cond-mat.supr-con↗
arXiv2026

Carrier-density dependence of magnetotransport in correlated Dirac semimetal CaIrO$_3$

We report the carrier density dependence of the magnetotransport property in the correlated Dirac semimetal CaIrO$_3$. In the dilute carrier density region ($n_{\rm H}$ $\sim 2.2 \times 10^{16} \,$$\rm{cm}^{-3}$) at $2 \, \mathrm{K}$, the mobility exceeds $1.0 \times 10^{5} \,$$\rm{cm}^{2}/\rm{Vs}$ at $2 \, \mathrm{K}$, and the transverse magnetoresistance (MR) reaches $2,000 \,$\% at $12 \, \mathrm{T}$. The analysis of quantum oscillations and Hall conductivity shows that the Fermi velocity is nearly independent of the cross-sectional area of the Fermi surface, or equivalently the carrier density, supporting a $k$-linear dispersion of the Dirac node. The field dependence of magnetoresistivity is nearly $B$-linear in the moderate carrier density region ($n_\mathrm{H} \geq 4 \times 10^{16}\,$cm$^{-3}$), but scales with $B^{\alpha}$ ($\alpha > 2$) in the lower carrier density region. The variation of magnetoresistivity is likely affected by the enhanced long-range Coulomb interaction in the quantum limit, where Dirac electrons are subject to the magnetic confinement.

cond-mat.mtrl-sci↗
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

Electric toroidal octupolar symmetry in pyrite FeS$_2$ probed by Raman optical activity

We report Raman optical activity in pyrite FeS$_2$, which hosts an electric toroidal octupolar symmetry. A clear and reproducible sign reversal of the circular intensity difference is observed between neighboring $\{111\}$ faces under cross-circular polarization. The signal appears only for the doubly degenerate $E_g$ phonon mode and is absent for other modes, consistent with symmetry analysis. First-principles calculations reproduce these features, establishing Raman optical activity as a probe of higher-rank axial multipolar symmetry.

cond-mat.mtrl-sci↗
arXiv2026

Parity and time-reversal invariant Ising spin ordering

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. These both exhibit an Ising, or uniaxial, Bloch spin-splitting. Here we introduce a new class of coplanar AFMs that generate a P and T symmetric, translation-invariant Ising spin order in real space. Naively, such AFMs are not expected to exhibit unusual phenomena. Here we show that the spin-rotational symmetry breaking generated by these AFMs allows: pure non-relativistic longitudinal (or transverse) spin-conductivities, the generation of non-relativistic altermagnetic spin-splittings through circularly polarized light, and the generation of non-relativistic odd-parity spin-splittings through parity symmetry breaking, by, for example, applied electric fields. We identify 16 candidate materials in the Magndata database for which our theory applies and provide effective microscopic models and DFT-based results that highlight the large emergent responses.

cond-mat.str-el↗
arXiv2026

Ferroaxial magnets: time-reversal-even mirror symmetry violation from spin order

We investigate ferroaxial magnets, a new class of spin-order-driven multiferroic magnets in which magnetic ordering induces mirror-symmetry breaking while preserving both time-reversal and spatial-inversion symmetries. These systems exhibit a ferromagnet-like axial anisotropy that allows optical control of the ferroaxial polarization, while their macroscopic time-reversal symmetry makes them attractive for antiferromagnetic spintronics. Using spin crystallographic group analysis, we identify the candidate materials and the nonrelativistic ferroaxial nature stemming from the strong exchange splitting of magnets. Furthermore, a symmetry-based identification shows magnetic materials that host ferroaxial order and metallic conductivity, realizing the ferroaxial metal state that undergoes a ferroaxial phase transition while remaining metallic. As a direct probe for the ferroaxial metal, we propose a third-order nonlinear Hall effect originating from the transverse coupling between the electric field and Berry curvature dipole mediated by the ferroaxial anisotropy. Our results establish ferroaxial magnets as a platform for nonrelativistic multiferroicity and spintronic applications.

cond-mat.mtrl-sci↗
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 $\xi_0$ and the magnetic penetration depth $\lambda_\mathrm{L}$, 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 $\xi_0$, $\lambda_\mathrm{L}$, 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 $\xi_0$ and $\lambda_\mathrm{L}$ 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

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

Plasmon assisted superconductivity in LiTi$_2$O$_4$

We combine $GW$ plus extended dynamical mean field theory ($GW$+EDMFT) with the density functional theory for superconductors (SCDFT) framework to study the electronic properties of LiTi$_2$O$_4$. Excellent agreement with experiment is obtained for the density of states, mass enhancement, Sommerfeld coefficient and superconducting $T_c$, if the dynamical nature of the screened Coulomb interaction is taken into account. Our results show that the coupling to collective charge fluctuations (plasmons) plays an important role in the pairing mechanism and explains the remarkably high $T_c$ of this moderately correlated spinel compound.

cond-mat.supr-con↗
arXiv2026

Systematic Magnetic Structure Generation Based on Oriented Spin Space Groups: Formulation, Applications, and High-Throughput First-Principles Calculations

We propose a framework for generating magnetic structures, inspired by the concept of oriented spin space groups (SSGs): magnetic structures are first generated as totally symmetric representations of an SSG and are then rotated such that they belong to the maximal magnetic space group of the SSG, which we term spin-symmetry-adapted (SSA) structures and oriented SSA structures, respectively. This is a natural framework to enforce fixed magnetic moment magnitudes on the symmetry-equivalent sites as well as to exploit the spin-orbit coupling (SOC)-induced hierarchy of energy scales. To examine the present scheme, we analyze the MAGNDATA database and find that 77% of the reported structures are reproducible at the SSG level, among which 82% are fully reproduced within the oriented SSG scheme, regardless of their spin-only group types or propagation vectors. To quantitatively assess computational and predictive performance, we perform spin density functional theory calculations for 283 materials, first carrying out self-consistent calculations for SSA structures without SOC, followed by fixed-charge calculations including SOC for the descendant oriented SSA structures. The experimental magnetic structures are reproduced as energetically most stable in 82% of cases at the SSG level without SOC and in 76% of cases at the oriented SSG level with SOC, showing that the fixed-charge scheme enables accurate evaluation of SOC-induced energy differences at low computational cost. The characteristic energy scale among oriented SSA structures is only $\sim$0.29 meV per magnetic atom, about 300 times smaller than that of distinct SSA structures. These results demonstrate that oriented SSG-based enumeration, combined with the two-step calculations for SSA and oriented SSA structures, provides an efficient and robust route for large-scale magnetic-structure prediction.

cond-mat.mtrl-sci↗