arXiv subjects

Youichi Yanase

Publications and source records attributed to Youichi Yanase.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear Edelstein effect in Rashba superconductors

We formulate a quasiclassical theory of the Edelstein effect in superconductors that incorporates both intraband and interband contributions. To describe the interband contribution, which is absent from the conventional leading-order quasiclassical formulation, we derive augmented Eilenberger equations in the presence of antisymmetric spin-orbit coupling. The intraband contribution is evaluated using multiband Eilenberger equations. We apply these formulations to supercurrent-induced surface spin magnetization in $s$-wave Rashba superconductors and investigate its dependence on temperature, distance from the surface, spin-orbit coupling strength, and supercurrent. The intraband contribution originates from a supercurrent-induced asymmetry of quasiparticles with opposite momenta and spin polarizations, whereas the interband contribution arises from the anomalous-velocity term generated by the momentum derivative of the Rashba spin-orbit potential. In the helicity basis, this anomalous-velocity term is expressed in terms of the Berry connection associated with the momentum dependence of the Rashba eigenstates. The intraband contribution increases linearly with the spin-orbit coupling strength, whereas the interband contribution exhibits a nonmonotonic dependence and is maximized when the Rashba spin splitting is comparable to the superconducting gap. Moreover, within the clean $s$-wave Rashba model considered here, we find that the nonlinear dependence of magnetization on the supercurrent arises solely from the interband contribution. Thus, although the intraband contribution dominates the linear Edelstein effect, the nonlinear Edelstein effect can serve as a useful probe of the interband contribution originating from quantum geometry.

cond-mat.supr-con↗

arXiv2026

Quantum geometry and RKKY in flat bands

Flat conduction bands quench the group velocity and thus challenge conventional, dispersion-driven pictures of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, where localized moments are coupled via an effective exchange mediated by conduction electrons. Here we show that RKKY interactions in the flat-band limit are not extinguished by the vanishing group velocity but are instead mediated by the quantum geometry of Bloch states. Starting from a microscopic RKKY derivation, we demonstrate that the Brillouin-zone-averaged quantum metric controls the long-wavelength structure of the static susceptibility, thereby determining the magnetic correlation length and the spin stiffness. As a result, the finite spatial spread of Wannier functions provides an effective long-range coupling channel even when single-particle dispersion is absent. Furthermore, we establish the general principle that the ordering temperature is governed by the quantum metric in finite and low-dimensional samples, effectively circumventing the thermodynamic-limit constraint of the Mermin-Wagner theorem. Specifically, our theoretical investigation reveals that increasing the quantum metric enhances magnetic rigidity and leads to a corresponding rise in the critical temperature within finite-sized systems.

cond-mat.str-el↗

arXiv2026

Winding charge density wave: intertwining of structural chirality and phase topology of electronic order

We propose a class of chiral charge density waves (CDWs), dubbed winding CDWs, that exhibit macroscopic chirality despite a single ordering wavevector. In screw-symmetric chiral crystals, chiral phonons drive a Peierls instability that selects a definite crystal angular momentum channel, thereby endowing the CDW with an integer azimuthal phase winding dictated by the selection rule governing electron-phonon coupling. We further extend this framework to achiral crystals with discrete rotational symmetry and demonstrate that spontaneous symmetry breaking stabilizes a winding CDW with either handedness, realizing an achiral-to-chiral phase transition. Our results reveal a fundamental link between the geometry of chiral structures and the phase topology of electronic orders.

cond-mat.str-el↗

arXiv2026

Light-Driven Intrinsic Perfect Superconducting Diode Effect

We demonstrate the perfect superconducting diode effect (SDE) -- unidirectional supercurrent with 100% diode efficiency -- in light-driven nonequilibrium systems. Although the perfect SDE is difficult to achieve in equilibrium, monochromatic light induces the perfect SDE in systems lacking inversion and time-reversal symmetries. More strikingly, multi-frequency light enables the perfect SDE even in centrosymmetric systems via dynamical symmetry breaking. Our results establish a general principle for realizing unidirectional superconducting transport based on nonequilibrium control and symmetry engineering.

cond-mat.supr-con↗

arXiv2026

Superconducting diode effect in correlated electron systems by nonreciprocal magnetism

The superconducting diode effect (SDE), characterized by a nonreciprocal critical current in superconductors, has recently been observed in strongly correlated electron systems and near quantum criticality, pointing to unconventional mechanisms beyond weak-coupling theories. Here we investigate the SDE in the Rashba-Zeeman-Hubbard model, which captures $d$-wave superconductivity in an antiferromagnetic quantum critical regime, using the Dyson-Gor'kov equation with the fluctuation exchange approximation. We show that electron correlations suppress the conventional intrinsic SDE arising from depairing currents. More importantly, a supercurrent nonreciprocally induces antiferromagnetic order, which fundamentally governs the critical current and enables perfect diode efficiency. Our results reveal a previously unrecognized correlation-driven mechanism of the SDE and establish strongly correlated superconductors as a platform for superconducting diode physics.

cond-mat.supr-con↗

arXiv2026

Electron-Hole Scattering Dichotomy and Anisotropic Warping in Quasi-Two-Dimensional Fermi Surfaces of UTe2

We present a combined experimental and theoretical study of the detailed Fermi-surface (FS) geometry of UTe2, a heavy-fermion superconductor that has recently attracted considerable attention as a promising candidate for spin-triplet pairing. Using angle-dependent magnetoresistance oscillations, a bulk- and low-energy-sensitive transport probe for quasi-two-dimensional (Q2D) electronic structures, we directly determine the in-plane FS geometry. We found that the Q2D FS exhibits a rectangular cross-sectional shape with strongly anisotropic warping, originating from the hybridization of two orthogonal quasi-one-dimensional bands. Through a quantitative comparison between experiment and theoretical calculations, we further reveal a large electron-hole scattering dichotomy: the quasiparticle lifetime on the electron FS is substantially shorter than that on the hole FS. This dichotomy is naturally explained by anisotropic, low-dimensional antiferromagnetic fluctuations, which selectively enhance scattering on the electron FS. This suggests a dominant role of the electron pockets for the emergence of superconductivity. Our results clarify a direct relation between FS geometry, magnetic fluctuations, and momentum-dependent quasiparticle lifetimes, and thus providing a crucial basis for the microscopic understanding of pairing mechanism, and impose stringent constraints on the gap symmetry of spin-triplet superconductivity in UTe2.

cond-mat.supr-con↗

arXiv2026

Field-angle dependence of magnetoresistance in UTe2

We theoretically study angle-resolved magnetoresistance under rotated magnetic field in the normal state of a spin-triplet superconductor UTe$_2$. The Wannier model derived from a GGA+$U$ calculation shows quasi-two-dimensional Fermi surfaces with warping in the $k_z$ direction, consistent with quantum oscillation measurements in the high magnetic field regime. Solving the semiclassical Boltzmann equation, we show that the Fermi surface geometry gives rise to oscillations in the magnetoresistance when the field is tilted from the $c$ axis toward the $a$ or $b$ axis. By assuming a band-dependent relaxation time, the calculated angle-resolved magnetoresistance is in good agreement with the recent transport experiment. This is direct evidence for the warped Fermi surface revealed by ordinary intraband transport. It suggests that the hole band with long relaxation time dominates electron transport. The field angle dependence of the Hall resistivity is calculated for further experimental verification.

cond-mat.supr-con↗

arXiv2026

Magnetic fluctuations driven by quantum geometry

Using quantum distance, magnetic susceptibility in the non-interacting limit can be rigorously split into two contributions: one arising solely from band dispersion, while the other stems from quantum geometric contributions. In this Letter, we apply this decomposition to two materials, LaFeAsO and Pb$_9$Cu(PO$_4$)$_6$O, and demonstrate that their dominant magnetic fluctuations originate from the geometric contribution. In LaFeAsO, stripe-type antiferromagnetic fluctuations arise primarily from quantum geometry, while in Pb$_9$Cu(PO$_4$)$_6$O the geometric term suppresses antiferromagnetic fluctuations and stabilizes ferromagnetic fluctuations. Our findings highlight the essential role of quantum geometry in governing magnetic fluctuations in multi-band systems, and provide a unique and quantitative framework to disentangle band-structure and wavefunction-geometry effects that have often been discussed collectively as multi-orbital effects.

cond-mat.str-el↗

arXiv2026

Microwave Kerr/Faraday Resonance in Two-dimensional Chiral Superconductors

We investigate the polar Kerr and Faraday effects in two-dimensional multiband chiral superconductors. We show that the clapping modes--the relative phase and amplitude oscillations between two chiral components of the superconducting order parameter--lie well within the quasiparticle excitation gap in multiband systems and dominate these magneto-optical responses in the microwave regime. The Kerr and Faraday rotation angles exhibit the resonant enhancement with sign reversals in the microwave regime as a function of the light frequency, reaching peak values on the order of 100 nrad--10 $\mu$rad in thin films of candidate chiral superconductors. These resonances are accessible in superconducting atomic layer materials and provide a generic probe of chiral superconductivity in two-dimensional systems.

cond-mat.supr-con↗

arXiv2025

Quantum geometry in correlated electron phases: from flat band to dispersive band

Quantum geometry, describing the geometric properties of the Bloch wave function in momentum space, has recently been recognized as a fundamental concept in condensed matter physics. The flat-band system offers the paradigmatic platform where quantum geometry plays the essential role in correlated electron phases. However, systems that suffer from significant effects of quantum geometry are not limited to flat-band systems; dispersive-band systems also exhibit quantum condensed phases driven by quantum geometry. In this perspective, we provide a transparent account of quantum geometry and its role in correlated electron phases, throughout flat-band and dispersive-band systems.

cond-mat.str-el↗

arXiv2025

Intrinsic spin Nernst effect in spin-triplet superconductors

We theoretically investigate the intrinsic (impurity-independent) spin Nernst effect (SNE), a spin current generation perpendicular to temperature gradients, in spin-triplet superconductors. We show that, in these systems, the SNE consists of two distinct contributions: a direct quasiparticle contribution and an indirect supercurrent contribution. The quasiparticle contribution originates from the momentum space Berry curvature generated by spin-triplet Cooper pairs. The indirect contribution arises from a compensating supercurrent that cancels the bulk thermoelectric charge current. While this contribution vanishes when the condensate has no spin-polarization in momentum space, it can be comparable in magnitude to the quasiparticle contribution in nonunitary superconductors. These results demonstrate that thermoelectric spin supercurrent must be explicitly accounted for when evaluating the SNE in nonunitary superconductors.

cond-mat.supr-con↗

arXiv2025

Spontaneous spin-selective structural phase transition in chiral crystals

In this Letter, we predict a structural phase transition unique to chiral crystals with screw symmetry. In chiral crystals, the phonon frequency renormalized by the electron-phonon coupling depends on the handedness of circular polarization. Consequently, the soft mode encoding phonon angular momentum induces spin-selective Peierls gaps in the electronic band, entailing a helical spin density wave and chiral lattice distortion. We also elucidate the chiral signatures and functional implications of collective modes. Our findings offer crucial insights into the emergence of chirality and highlight novel functional aspects of chiral materials and their design strategy.

cond-mat.str-el↗

arXiv2025

Surface acoustic wave-driven valley current generation in intervalley coherent states

Recent experiments have reported valley-gauge-symmetry-broken phases, identified as intervalley coherent (IVC) states. Exploration of anomalous responses, particularly those analogous to superconductivity, has become an urgent theoretical issue. In this study, we show that the IVC order gives rise to anomalous valley-current generation driven by surface acoustic waves (SAWs). The anomalous valley current exhibits a characteristic power-law dependence for low-frequency SAWs. Furthermore, we demonstrate by numerical analysis that the IVC order significantly enhances valley-current generation in rhombohedral graphene. These results open a pathway toward exploring exotic phenomena emerging from valley-gauge-symmetry breaking, in close analogy with gauge-symmetry breaking in superconductors.

cond-mat.mes-hall↗

arXiv2025

Quantum geometric magnetic monopole and two-phase superconductivity in CeRh$_2$As$_2$

Recent angle-resolved photoemission spectroscopy (ARPES) and density functional theory plus Hubbard $U$ (DFT+$U$) studies revealed that a heavy-fermion superconductor CeRh$_2$As$_2$ exhibits van Hove singularities and the Dirac point near the Fermi level $E_{\mathrm F}$, which are key signatures of strong-correlation effects and quantum geometry. We have constructed a two-dimensional 12-orbital \textit{Dirac-Anderson} model as an effective model for CeRh$_2$As$_2$. The band structure and Fermi-surface topology of the Dirac-Anderson model agree well with the ARPES data and the DFT+$U$ calculations. We show that the quantum geometry strongly favors magnetic-monopole fluctuations because of the Dirac point at the $M$ point. By solving the linearized \'{E}liashberg equation, we demonstrate that the $B_{1u}$ and $B_{2g}$ representations, spin-triplet states originating from the Dirac point, exhibit the leading superconducting instabilities. By comparing the random-phase approximation and the fluctuation-exchange approximation, we further demonstrate that strong-correlation effects mitigate the influence of quantum geometry. The phase diagram of CeRh$_2$As$_2$ under pressure is discussed in connection with the theoretical results.

cond-mat.supr-con↗

arXiv2025

Magnetic fluctuations and anisotropy in UTe2: a multi-orbital study based on GGA+U and RPA

Pressure-induced changes in the magnetic and superconducting properties of a spin-triplet superconductor candidate UTe$_2$ have attracted considerable interest, underscoring the need for microscopic theoretical insight. In this paper, we investigate magnetic fluctuations and their anisotropy at ambient pressure and under pressure using density functional theory (DFT) combined with the random phase approximation (RPA). For each pressure, we perform DFT+$U$ calculations for several values of the Coulomb interaction $U$, construct a 72-orbital periodic Anderson model, and calculate magnetic susceptibilities with use of the RPA. For $U = 2\mathrm{\;eV}$, the Fermi surface has a quasi-two-dimensional shape, antiferromagnetic fluctuations develop with the wave vector along the $\boldsymbol{a}^*$ axis, and the magnetic anisotropy follows $\chi^b > \chi^a > \chi^c$. The antiferromagnetic fluctuations are suppressed under pressure because of a reduced density of states at the Fermi level, while the magnetic anisotropy is weakened. In contrast, for $U = 1\mathrm{\;eV}$, where the Fermi surface is more three-dimensional, antiferromagnetic fluctuations with $\boldsymbol{Q}_2 = 0.22\,\boldsymbol{b}^*$ appear, accompanied by anisotropy $\chi^a > \chi^c > \chi^b$, consistent with experiments. Under pressure, antiferromagnetic fluctuations around $\boldsymbol{Q}_2$ are enhanced, the magnetic wave vector tilts slightly toward the $\boldsymbol{a}^*$ direction due to Fermi-surface distortion, and the magnetic anisotropy is suppressed. These results demonstrate that the pressure evolution of magnetism in UTe$_2$ is governed by the momentum-space distribution of U $5f$ states and the density of states at the Fermi level, providing a microscopic basis for understanding the magnetic and superconducting properties of UTe$_2$.

cond-mat.str-el↗

arXiv2025

Superconductivity in UTe$_2$ from local noncentrosymmetricity

Superconductivity in UTe$_{2}$ has garnered significant attention, as it is widely recognized as a promising candidate for a spin-triplet superconductor. However, the symmetry of superconductivity and the microscopic origin of spin-triplet pairing remain subjects of debate. Nevertheless, various experiments imply an intimate coupling between magnetism and superconductivity. In this paper, we analyze a multi-sublattice periodic Anderson model that incorporates a spin-orbit coupling allowed in locally noncentrosymmetric crystals to discuss magnetic fluctuations and superconductivity in UTe$_2$. Due to the sublattice-dependent spin-orbit coupling, magnetic fluctuations become anisotropic, and the spin degeneracy of superconducting states is lifted. Our calculations reveal anisotropic antiferromagnetic fluctuations along the $b$- and $c$-axes, anisotropic ferromagnetic fluctuations along the $a$-axis, and their coexistence. These can be tuned by the $f$-electron's level. Superconductivity in the $A_u$ representation is predominant for a wide range of parameters, whereas the $B_{2u}$ representation is almost degenerate and can be stabilized. The direction of the $d$-vector changes as we increase the spin-orbit coupling. We discuss the consistency between our results and several experiments.

cond-mat.supr-con↗

arXiv2025

Quasiclassical theory of vortex states in locally non-centrosymmetric superconductors: application to CeRh$_{2}$As$_{2}$

CeRh$_{2}$As$_{2}$, a heavy fermion superconductor discovered in 2021, exhibits two distinct superconducting phases under a $c$-axis magnetic field. This unconventional phase diagram has been attributed to the local inversion symmetry breaking at the Ce sites. At low magnetic fields, a conventional even-parity spin-singlet superconducting state is realized, whereas at higher fields, an odd-parity spin-singlet superconducting state, in which the order parameter alternates sign between neighboring Ce layers, becomes stabilized. In this study, we employ a quasiclassical approach to investigate the vortex states of bilayer superconductors with locally broken inversion symmetry. We calculate the local density of states (LDOS) in the vortex lattice state and find that the pairing symmetry of different superconducting states is clearly manifested in the peak structure of LDOS at the vortex core. Since LDOS is experimentally observable, our work provides a pathway for experimental verification of the superconducting parity transition in CeRh$_{2}$As$_{2}$.

cond-mat.supr-con↗

arXiv2025

Magnetic phase transitions driven by quantum geometry

We explore how the quantum geometric properties of the Bloch wave function, characterized by the Hilbert-Schmidt quantum distance, impact magnetic phases in solid-state systems. To this end, we investigate the spin susceptibility within the random phase approximation, considering the onsite Coulomb interaction. We demonstrate that spin susceptibility can be decomposed into a trivial part, dependent solely on the band dispersion, and a geometric part, where the quantum distance plays a crucial role. Focusing on a model of a quadratic band-touching semimetal, we show that a magnetic phase transition between ferromagnetic and antiferromagnetic order can be induced solely by tuning the wavefunction geometry, even while the energy spectrum is held constant. This highlights the versatility of quantum geometry as a mechanism for tuning magnetic properties independent of the energy spectrum. Applying our framework to the Fe-pnictide and kagome lattice models, we further show that the geometric contribution is decisive in stabilizing their known antiferromagnetic and ferromagnetic states, respectively. Our work sheds light on the hidden quantum geometric aspects necessary for understanding and engineering magnetic order in quantum materials.

cond-mat.str-el↗