arXiv subjects

Hirono Kaneyasu

Publications and source records attributed to Hirono Kaneyasu.

17 recordsLinked to original sources

arXiv2025

Theoretical Study of Impurity Effects on Superconductivity in UTe2

This study investigates the impurity effects on UTe2 within the self-consistent Born approximation using the six-orbital f-d-p model which contains two uranium and tellurium atoms in the minimum unit cell. We analyze the dependence of superconducting transition temperature (Tc) on impurity concentration for various pairing symmetries proposed by experiments and theories. It clarifies that the decrease of Tc significantly depends on which atom sites the impurities reside. Particulalry, the analysis shows that the impurity at U-site has dominant effect on the change of Tc. Then, either the singlet state in the case of magnetic impurities or the triplet states in both non-magnetic and magnetic impurities are consistent with experiments. Thus, this indicates that elucidating the magnetic properties of impurities (i.e. magnetic or non-magnetic) is crucial for identifying the pairing symmetry of UTe2.

cond-mat.supr-con↗

arXiv2024

Spin-lattice relaxation for point-node-like s-wave superconductivity in f-electron systems

In this study, we examined the temperature dependence of the spin-lattice relaxation using an f-d-p model, which is an effective model of UTe2. Solving the linearized Eliashberg equation in the f-d-p model based on third-order perturbation theory, we obtain a point-node-like s-wave pairing state. Our result shows that the Hebel-Slichter peak in the point-node-like s-wave pairing state is smaller than that in the isotropic s-wave pairing state. However, the Hebel-Slichter peak remains robust even in the point-node-like s-wave pairing state, and the point-node-like s-wave state is inconsistent with the results of nuclear magnetic resonance measurements.

cond-mat.supr-con↗

arXiv2023

Unconventional $s$-Wave Pairing with Point-Node-Like Gap Structure in UTe$_2$

We explore the pairing state and gap structure of UTe$_2$ using a six-orbital model which we call the $f$-$d$-$p$ model. Our model accurately reproduces the quasi-two-dimensional Fermi surfaces consistent with recent de Haas-van Alphen oscillation measurements and the $(0, \pm \pi, 0)$ antiferromagnetic spin fluctuations observed by neutron scattering. We incorporate on-site Coulomb repulsion for $f$ electrons and solve the linearized Eliashberg equation within the third-order perturbation theory to investigate the superconducting symmetry in UTe$_2$. The most likely state is found to be an $s$-wave state with a highly anisotropic superconducting gap structure that exhibits a point-node-like behavior of the specific heat at low temperatures.

cond-mat.supr-con↗

arXiv2022

Field-Induced Spin Nematic Liquid of the $S=1/2$ Bond-Alternating Chain with the Anisotropy

The $S=1/2$ ferromagnetic-antiferromagnetic bond-alternating spin chain with the anisotropy on the ferromagnetic exchange interaction in magnetic field is investigated using the numerical diagonalization and the density matrix renormalization group analyses. It is found that the nematic-spin-dominant Tomonaga-Luttinger liquid phase is induced by the external magnetic field for sufficiently large anisotropy. The phase diagram with respect to the anisotropy and the magnetization is presented.

cond-mat.str-el↗

arXiv2022

Field-induced spin nematic Tomonaga-Luttinger liquid of the $S=1/2$ spin ladder system with the anisotropic ferromagnetic rung interaction

The $S=1/2$ quantum spin ladder system with the anisotropic ferromagnetic exchange interaction on the rung under magnetic field is investigated using the numerical diagonalization and the density matrix renormalization group (DMRG) analyses. It is found that the nematic-spin-correlation-dominant Tomonaga-Luttinger liquid (TLL) appears in some high magnetic field. It is included in the TLL phase where the two-magnon bound state is realized. For some suitable parameters, after the field-induced phase transition from this two-magnon-bound TLL phase to the single-magnon TLL one, the re-entrant transition to the two-magnon-bound TLL phase occurs, which is confirmed by the magnetization curves by the DMRG. Several phase diagrams on the plane of the coupling anisotropy versus the magnetization and the magnetic field are presented. The present result is a proposal of the candidate system which exhibits the spin nematic phase without the biquadratic interaction or the frustration.

cond-mat.str-el↗

arXiv2022

Translational Symmetry Broken Magnetization Plateau of the $S=2$ Antiferromagnetic Chain with Anisotropies

The magnetization plateau of the $S=2$ antiferromagnetic chain with interaction and single-ion anisotropies is investigated using the numerical diagonalization of finite-size clusters and some size scaling analyses. The previous level spectroscopy analysis indicated that two different magnetization plateau phases appear at half of the saturation magnetization. One is due to the large-$D$ mechanism and the other is due to the Haldane one. In the present study the phase diagram is extended to wider region of the anisotropies. As a result we find another half magnetization plateau phase, where the translational symmetry is spontaneously broken .

cond-mat.str-el↗

arXiv2022

S=2 Quantum Spin Chain with the Biquadratic Exchange Interaction

The $S=2$ quantum spin chain with the single-ion anisotropy $D$ and the biquadratic exchange interaction $J_{\rm BQ}$ is investigated using the numerical diagonalization of finite-size clusters and the level spectroscopy analysis. It is found that the intermediate-$D$ phase corresponding to the symmetry protected topological (SPT) phase appears in a wide region of the ground state phase diagram. We also obtain the phase diagram at the half of the saturation magnetization which includes the SPT plateau phase.

cond-mat.str-el↗

arXiv2017

Evolution of the filamentary 3-Kelvin phase in Pb-Ru-Sr2RuO4 Josephson junctions

The evolution of the filamentary 3-Kelvin (3K) superconducting phase at the interface between Sr2RuO4 and Ru-metal inclusions is discussed for Pb-Ru-Sr2RuO4 contacts. Using the Ginzburg-Landau model, the influence of proximity-induced superconductivity in Ru on the topology of the 3K phase is analyzed. Because the s-wave order parameter in Ru favors a 3K state of trivial topology, the onset temperature of the phase with a non-trivial topology, which is compatible with the bulk phase of Sr2RuO4, is essentially reduced to the bulk transition temperature. Because the topology of the superconducting state in Sr2RuO4 is crucial for the Josephson effect through Pb-Ru-Sr2RuO4 contacts, this model qualitatively reproduces the experimental observation of the anomalous temperature dependence on the critical current.

cond-mat.supr-con↗

arXiv2014

Limiting mechanism for critical current in topologically frustrated Josephson junctions

Eutectic Sr$_2$RuO$_4$-Ru samples with $μ$m-sized Ru-metal inclusions support inhomogeneous superconductivity above the bulk transition of Sr$_2$RuO$_4$ in the so-called 3-Kelvin phase. In Pb/Ru/Sr$_2$RuO$_4$ Josephson junctions as realized by Maeno et al., a Pb film is indirectly coupled to the superconductor Sr$_2$RuO$_4$ mediated by the proximity-induced superconducting Ru-inclusions, yielding an extended Josephson contact through the interface between Ru and Sr$_2$RuO$_4$. Motivated by this experimental setup, we formulate a sine-Gordon model for the Josephson phase of the interface, assuming a simple cylindrical shape for the Ru-inclusion hosting the proximity-induced $s$-wave superconducting phase. Considering the Sr$_2$RuO$_4$ as a chiral $p$-wave superconductor, we discuss two types of Josephson junctions, a frustrated one due to the nature of the order parameter in Sr$_2$RuO$_4$, and an unfrustrated one for the topologically trivial 3-Kelvin phase. While the latter situation displays standard junction behavior, the former yields an unusual limiting mechanism for the critical current, based on a pinning-depinning transition of a spontaneously induced magnetic flux driven by an externally applied current. We analyze different coupling limits and show that different critical currents can arise for the two topologies. This concept fits well to recent experimental data obtained for the above setup showing an anomalous temperature dependence of the critical current at the transition temperature $T_c $ of bulk Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2010

Phase transition in the 3 Kelvin phase in the eutectic Sr2RuO4-Ru

The inhomogeneous 3-Kelvin (3K) phase of the eutectic Sr2RuO4 with Ru inclusions nucleates superconductivity at the interface between Ru and Sr2RuO4. The structure of the interface state and its physical properties are examined here. Two superconducting phases are identified between the transitions to the bulk phase at 1.5K and to the 3K phase. The nucleation of the 3K phase results in a state conserving time reversal symmetry, which generates an intrinsically frustrated superconducting network in samples with many Ru inclusions. At a lower temperature (>1.5K), a discontinuous (first order) transition to an interface state breaking time reversal symmetry is found leading to an unfrustrated network phase. It is shown that this phase transition located at a temperature between 1.5 and 3K would yield the anomalous property that the critical current in such a network depends on the sign of the current, reproducing recent experimental observations.

cond-mat.supr-con↗

arXiv2010

Nucleation of Vortex State in Ru-inclusion in Eutectic Ruthenium Oxide Sr2RuO4-Ru

Eutectic samples of Sr2RuO4-Ru possess many micrometer-sized Ru-metal inclusions. Based on a Ginzburg-Landau formulation we analyze the interplay of the chiral p-wave state of Sr2RuO4 (Tc = 1.5 K) and the s-wave state of Ru metal (Tc,Ru = 0.5 K) for an inclusion of cylindrical geometry. As a consequence of the mismatch of the order parameter phase, the occurrence of a spontaneous flux distribution appears for T < Tc,Ru which evolves into a "Josephson vortex" on the Sr2RuO4-Ru interface. At a sufficiently low temperature a depinning transition can occur whereby the vortex moves to the center of the cylinder.

cond-mat.supr-con↗

arXiv2010

Increase in d-Wave Superconducting Transition Temperature near Imperfect Layer in Correlated Electron System

The effect of the site potential in an imperfect layer is studied in a d-wave layer superconductor on the basis of the electron correlation. The site potential binds electrons to the imperfect layer, and then, the superconductivity of the imperfect layer is independent of that of the bulk. We found that the superconducting transition temperature of the imperfect layer becomes higher than that of the bulk owing to the effect of the site potential. In this situation, the Fermi surface of the imperfect layer has a strong nesting feature leading to increases in both the antiferromagnetic spin fluctuation and the density of states near the Fermi level, which are in favor of d-wave pairing.

cond-mat.supr-con↗

arXiv2006

Nonuniform Spin Triplet Superconductivity due to Antisymmetric Spin-Orbit Coupling in Noncentrosymmetric Superconductor CePt$_3$Si

We show that the nonuniform state (Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state) of the spin triplet superconductivity in noncentrosymmetric systems is stabilized by antisymmetric spin-orbit coupling even if the magnetic field is absent. The transition temperature of the spin triplet superconductivity is reduced by the antisymmetric spin-orbit coupling in general. This pair breaking effect is shown to be similar to the Pauli pair breaking effect due to magnetic field for the spin singlet superconductivity, in which FFLO state is stabilized near the Pauli limit (or Chandrasekhar-Clogston limit) of external magnetic field. Since there are gapless excitations in nonuniform superconducting state, some physical quantities such as specific heat and penetration depth should obey the power low temperature-dependences. We discuss the possibility of the realization of nonuniform state in CePt$_3$Si.

cond-mat.supr-con↗

arXiv2006

Fulde-Ferrel-Larkin-Ovchinnikov State due to Antisymmetric Spin-Orbit-Coupling in Noncentrosymmetric Superconductivity CePt$_3$Si

When the inversion symmetry is broken, the spin-orbit coupling reduces the transition temperature of some types of spin triplet superconductivity, which is similar to the case that magnetic field reduces the spin singlet superconductivity due to Zeeman splitting. It is well known that Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state of spin singlet superconductivity is realized near the Pauli limit (or Chandrasekhar-Clogston limit) of external magnetic field. In FFLO state the amplitude of the order parameter is not uniform in space. In this paper we study the FFLO state in the spin triplet superconductivity in the absence of magnetic field due to the spin-orbit coupling. Although the FFLO state is not realized in the simple model with spherical Fermi surface, it will be stabilized if some condition is favorable for it. We discuss the possibility of FFLO state in CePt$_3$Si in the absence of external magnetic field.

cond-mat.supr-con↗

arXiv2006

Theoretical Study on Coexistence of Ferromagnetism and Superconductivity

On the basis of a two-dimensional t-t' Hubbard model in ferromagnetic and paramagnetic states, the triplet superconducting mechanism is investigated by the third-order perturbation theory with respect to the on-site Coulomb interaction U. In general, the superconducting state is more stable in the paramagnetic state than in the ferromagnetic state. As a special case, the dominant ferromagnetic superconductivity is obtained by the electron-electron correlation between the electronlike majority and holelike minority bands. Furthermore, it is pointed out that in some cases the two bands play an essential role for the coexistence of superconductivity and ferromagnetism.

cond-mat.str-el↗

arXiv2006

Effects of Electron Correlation near Spin-Density-Wave on Angle Dependence of Magnetoconductivity

The effect of electron correlation on the angle dependence of magnetoconductivity is studied in quasi-one-dimensional organic conductors. We investigated the effect on the basis of the momentum dependence of both quasi-particle's lifetime and velocity near the spin-density-wave (SDW). We found that the momentum dependence of quasi-particle's lifetime mainly governs the angle dependence of one-dimensional axis magnetoconductivity. On the other hand, the change in velocity originating from the electron correlation gives the dominant effects on the angle dependence of the interchain axis magnetoconductivity. The effect of electron correlation is clarified from the association of the momentum dependences of lifetime and velocity with the momentum dependences of commensurate orbitals on a Fermi surface in magnetic fields.

cond-mat.str-el↗

arXiv2005

Magic angle effects of the one-dimensional axis conductivity in quasi-one dimensional conductors

In quasi-one-dimensional conductors, the conductivity in both one-dimensional axis and interchain direction shows peaks when magnetic field is tilted at the magic angles in the plane perpendicular to the conducting chain. Although there are several theoretical studies to explain the magic angle effect, no satisfactory explanation, especially for the one-dimensional conductivity, has been obtained. We present a new theory of the magic angle effect in the one-dimensional conductivity by taking account of the momentum-dependence of the Fermi velocity, which should be large in the systems close to a spin density wave instability. The magic angle effect is explained in the semiclassical equations of motion, but neither the large corrugation of the Fermi surface due to long-range hoppings nor hot spots, where the relaxation time is small, on the Fermi surface are required.

cond-mat.str-el↗