arXiv subjects

Takeshi Mizushima

Publications and source records attributed to Takeshi Mizushima.

At least 19 recordsLinked to original sources

arXiv2026

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↗

arXiv2026

Magnetic penetration depth in topological superconductors: Effect of Majorana surface states and application for UTe$_2$

In this study, we examine how orbital degrees of freedom and Majorana surface states influence the magnetic penetration depth in the superconductor UTe$_2$. Using a two-orbital model, we analyze pairing states belonging to the irreducible representations of the $D_{2h}$ crystal symmetry: $A_u$, $B_{1u}$, $B_{2u}$, and $B_{3u}$. For bulk nodal states such as $B_{2u}$, we find that the penetration depth for screening currents along the antinodal direction and the cylindrical axis scales as $T^2$, in strong contrast to the conventional $T^4$ law. This behavior originates from quasiparticles near the point nodes contributing to the interorbital paramagnetic current. We further show that Majorana surface states can dominate the low-temperature response. The fully gapped $A_u$ state hosts Majorana cones, which produce a $T^3$ dependence of the penetration depth when the ratio of penetration depth to coherence length ($κ$) is small. In contrast, the other pairing states exhibit Majorana Fermi arcs: the exponent is $n=2$ along the dispersive direction, while along the dispersionless direction it depends on whether the arcs terminate at endpoints. The exponent $n=2$ in the dispersive direction is robust, while it in the dispersionless direction relies on the presence or absence of the endpoints of the arcs and deviates from $n=2$ when endpoints are absent. Our results demonstrate that penetration-depth measurements provide a direct probe of Majorana surface states in low-$κ$ superconductors. For larger $κ$, the surface contribution becomes negligible and the temperature dependence is governed by bulk quasiparticles.

cond-mat.supr-con↗

arXiv2026

Theory of Andreev and shot noise spectroscopy for topological superconductors probed by $s$-wave superconducting tips

Scanning tunneling microscopy (STM) and spectroscopy (STS) with $s$-wave superconducting tips has been widely applied to probe exotic superconductors, but its potential for investigating topological superconductors remains unclear. In junctions between an $s$-wave superconductor and a topological superconductor, the dominant tunneling process is Andreev reflection, in which Cooper pairs from the $s$-wave superconductor tunnel as particle--hole excitations into the surface state of the topological superconductor. In this work, we theoretically investigate the fundamental properties of Andreev and shot noise spectroscopy on topological superconductors, focusing on the $dI/dV$ characteristics and current noise. We develop a real-time description of an effective tunneling action incorporating Andreev reflection processes in the Keldysh formalism and derive analytical expressions for the Andreev reflection current and the associated current noise. Furthermore, we perform numerical simulations for representative topological superconductors and provide a catalog of $dI/dV$ spectra and the Fano factor. Our results establish guidelines for probing topological superconductivity using STM with $s$-wave superconducting tips, and provide theoretical benchmarks for future STS experiments.

cond-mat.supr-con↗

arXiv2026

Two-dimensional flat band on the (011) surface of UTe$_2$: Implication for STM measurements with a superconducting tip

Scanning tunneling microscopy (STM) measurements have been extensively performed on the easily cleavable (011) surface of UTe$_2$, using both normal-metal and superconducting tips. Motivated by these experiments, we theoretically investigate the topological surface states on the (011) surface of UTe$_2$. We find that a two-dimensional nearly flat band emerges in the $B_{3u}$ state, giving rise to a pronounced zero-energy peak in the surface density of states. This flat band is supported by two key mechanisms: (i)~nontrivial Berry phases defined at multiple momenta give rise to low-energy in-gap states, and (ii)~weak spin conservation allows the gap function to acquire phase winding. Furthermore, to investigate the relation between the zero-bias peak observed in recent STM experiments with a superconducting tip and the topological surface states, we calculate the nonequilibrium dc tunneling current in a junction between an $s$-wave superconductor and the (011) surface of UTe$_2$. Our results provide crucial insights into the superconducting pairing symmetry realized in UTe$_2$.

cond-mat.supr-con↗

arXiv2025

Optical-vortex-pulse induced nonequilibrium spin textures in spin-orbit coupled electrons

Optical vortex beams are a type of topological light characterized by their inherent orbital angular momentum, leading to the propagation of a spiral-shaped wavefront. In this study, we focus on two-dimensional electrons with Rashba and Dresselhaus spin-orbit interactions and examine how they respond to pulsed vortex beams in the terahertz frequency band. Spin-orbital interactions play a vital role in transferring the orbital angular momentum of light to electron systems and generating spatiotemporal spin textures. We show that the spatiotemporal spin polarization of electrons reflects orbital angular momentum carried by optical vortex pulses. These findings demonstrate how optical vortices facilitate ultrafast spin manipulation in spin-orbit-coupled electrons. Our results can be straightforwardly extended to the case of higher-frequency vortex beams for other two-dimensional metals with a larger Fermi energy.

cond-mat.mes-hall↗

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

Collective modes in Fulde-Ferrell-Larkin-Ovchinnikov superconductors: The role of long-range Coulomb interaction and signatures in density response

We theoretically investigate collective excitations in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states of Pauli-limited superconducting films. When the long-range Coulomb interaction is absent, excitation spectra consist of two gapless and three gapped modes. The gapless modes are the Nambu-Goldstone modes associated with the spontaneous breaking of the ${\rm U}(1)$ symmetry and the translational symmetry. The gapped modes include the Higgs mode and the twofold degenerate modes that cause the oscillation of the domain width and grayness of FFLO nodal planes. We find that the long-range Coulomb interaction only gaps out the gapless phase mode through the Anderson-Higgs mechanism, while the other modes remain unaffected. Furthermore, the field evolution of the dispersion of the gapless elastic mode, the Nambu-Goldstone mode associated with the translational symmetry breaking, is associated with that of the bandwidth of the mid-gap Andreev bound states. We demonstrate that the signature of the elastic mode can be detected by measuring the density-density response function.

cond-mat.supr-con↗

arXiv2025

Spin caloritronics as a probe of nonunitary superconductors

Superconducting spintronics explores the interplay between superconductivity and magnetism, sparking significant interest in nonunitary superconductors as a platform for novel magneto-superconducting phenomena. However, identifying nonunitary superconductors remains challenging. We demonstrate that spin current driven by thermal gradients sensitively probes the nature of the condensate in nonunitary superconductors. Spin polarization of the condensate in momentum space induces the superconducting spin Seebeck effect, where a spin current is generated along thermal gradients without a thermoelectric charge current. Notably, the nonvanishing superconducting spin Seebeck effect provides a smoking gun evidence of nonunitary superconductivity because it reflects the spin polarization of the condensate in momentum space, irrespective of whether the net pair spin magnetization vanishes. At the same time, the spin-chirality of the condensate induces the spin-Nernst effect, where a spin current is generated perpendicular to thermal gradients in nonunitary superconductors. These spin caloritronics phenomena offer a definitive probe of nonunitary superconductors.

cond-mat.supr-con↗

arXiv2024

Nonreciprocal heat transport in the Kitaev chiral spin liquid

Nonreciprocal transport, characterized by its direction-selective nature, holds significant potential for applications in various devices. In this study, we investigate nonreciprocal heat transport in Majorana systems, specifically focusing on the Kitaev chiral spin liquid under external magnetic fields. Our theoretical examination focuses on effects of open boundaries in which the Majorana edge modes exist, and the inversion symmetry is broken, which leads to the Dzyaloshinskii-Moriya interaction (DMI). Through perturbation theory, we demonstrate that DMI induces asymmetric hopping, resulting in the asymmetry of the Majorana band. The results of nonreciprocal heat currents are presented for various directions of external magnetic fields, and we discuss the relation between the current and the field-directions. The potential exists to manipulate both of the directions and magnitude of the nonreciprocal current by varying external magnetic fields and apply to heat transfer devices.

cond-mat.str-el↗

arXiv2024

Impurity bands, line-nodes, and anomalous thermal Hall effect in Weyl superconductors

We investigate the anomalous thermal Hall effect (ATHE) in Weyl superconductors realized by the $E_{1u}$ ($p$-wave and $f$-wave) chiral superconducting order for the point group $D_{6h}$. Using the quasiclassical transport theory, we analyze the influence of the impurity scatterings and the line-nodal excitations on the ATHE. We compare the extrinsic (impurity-induced) ATHE with the intrinsic (topological) ATHE to identify the dominant contribution. Because the transverse response is sensitive to the slope in the density of states (DOS) at the Fermi energy, the extrinsic ATHE vanishes in both the Born (weak impurity potential) and unitarity (strong impurity potential) limits. The amplitude of the impurity contribution to the thermal Hall conductivity (THC) reaches maximum between these limits when the slope of the DOS becomes large due to impurity bands near the Fermi energy. In such situations, the extrinsic ATHE dominates the intrinsic ATHE even at low temperatures. The extrinsic ATHE is sensitive to line-nodal excitations, whereas the intrinsic ATHE is insensitive to bulk excitations. When line nodes involve the sign change of the order parameter, the impurity contribution to the THC is suppressed even though the phase space for low-energy excitation is large. In contrast, if line nodes are not accompanied by such sign changes, the extrinsic ATHE is significantly enhanced. Our results form a basis for the comprehensive analysis of anomalous thermal transport in Weyl superconductors.

cond-mat.supr-con↗

arXiv2024

Anisotropic paramagnetic response of topological Majorana surface states in the superconductor $\text{UTe}_2$

Identifying the superconducting gap symmetry and topological signatures in the putative spin-triplet superconductor $\text{UTe}_2$ is an important issue. Especially, a smoking-gun detection scheme for Majorana surface states hallmarking topological superconductivity in $\text{UTe}_2$ is still lacking. In this study, we examine the surface spin susceptibility of $\text{UTe}_2$ with a particular focus on the contribution of the surface states. We find that Majorana surface states contribute significantly to the surface spin susceptibility, and give rise to an Ising-like anisotropy and anomalous enhancement in the surface spin susceptibility. We calculate the surface spin susceptibility as well as the local density of states using the recursive Green's function method and examine the anisotropy of the surface spin susceptibility in terms of the topological surface states and symmetry for all irreducible representations of odd-parity pairing states. Our results indicate that the Ising anisotropy and the anomalous enhancement are attributed to the Majorana surface state protected by the crystalline symmetry. These findings suggest the possibility of detecting the Majorana surface state via magnetic measurements.

cond-mat.supr-con↗

arXiv2024

Majorana modes in striped two-dimensional inhomogeneous topological superconductors

Majorana zero modes have gained significant interest due to their potential applications in topological quantum computing and in the realization of exotic quantum phases. These zero-energy quasiparticle excitations localize at the vortex cores of two-dimensional topological superconductors or at the ends of one-dimensional topological superconductors. Here we describe an alternative platform: a two-dimensional topological superconductor with inhomogeneous superconductivity, where Majorana modes localize at the ends of topologically nontrivial one-dimensional stripes induced by the spatial variations of the order parameter phase. In certain regimes, these Majorana modes hybridize into a single highly nonlocal state delocalized over spatially separated points, with exactly zero energy at finite system sizes and with emergent quantum-mechanical supersymmetry. We then present detailed descriptions of braiding and fusion protocols and showcase the versatility of our proposal by suggesting possible setups that can potentially lead to the realization of Yang-Lee anyons and the Sachdev-Ye-Kitaev model.

cond-mat.mes-hall↗

arXiv2024

Noise-Tolerance of Majorana Teleportation in Mesoscopic Topological Superconductors

We investigate teleportation interference associated with the non-local character of Majorana zero modes (MZMs) as a probe of MZMs focusing on the tolerance of teleportation against disturbances, such as inhomogeneous potentials at junctions and disorder. We develop a method for calculating non-local conductance in mesoscopic topological superconductors with fixed parity. In the trivial phase, the non-local conductance exhibits the $h/2e$-periodicity, while in the topological phase with fixed parity, it exhibits the $h/e$-periodicity, indicative of Majorana teleportation. We find that the $h/e$-periodicity is stable against changes in inhomogeneous potential structures and disorder. These results imply that MZMs can cause teleportation interference even in the presence of disturbances, leading to a clear distinction between the trivial and topological phases.

cond-mat.mes-hall↗

arXiv2024

Pairing symmetries of multiple superconducting phases in UTe2: Competition between ferromagnetic and antiferromagnetic fluctuations

The putative spin-triplet superconductor UTe2 exhibits multiple superconducting phases under applied pressure [D. Braithwaite et al., Commun. Phys. 2, 147 (2019)]. The clarification of pairing mechanisms and symmetries of gap functions are essentially important for understanding the multiple-phase diagram. Since the coexistence of ferromagnetic and antiferromagnetic spin fluctuations with Ising-like anisotropy is suggested from measurements of magnetic susceptibilities and neutron scattering measurements, it is expected that the interplay between these spin fluctuations plays a crucial role in the emergence of the multiple superconducting phases. Motivated by these observations, we examine the spin-fluctuation-mediated pairing mechanism, analyzing the linearized Eliashberg equations for an effective model of f-electron bands. It is found that the Ising-like ferromagnetic fluctuations stabilize spin-triplet pairings in either the Au or B3u states, whereas Ising-like antiferromagnetic fluctuations stabilize spin-triplet pairings in the B1u state. These results provide a plausible scenario elucidating the multiple superconducting phases under pressure.

cond-mat.supr-con↗

arXiv2023

Non-Abelian Anyons and Non-Abelian Vortices in Topological Superconductors

Anyons are particles obeying statistics of neither bosons nor fermions. Non-Abelian anyons, whose exchanges are described by a non-Abelian group acting on a set of wave functions, are attracting a great attention because of possible applications to topological quantum computations. Braiding of non-Abelian anyons corresponds to quantum computations. The simplest non-Abelian anyons are Ising anyons which can be realized by Majorana fermions hosted by vortices or edges of topological superconductors, $ν=5/2$ quantum Hall states, spin liquids, and dense quark matter. While Ising anyons are insufficient for universal quantum computations, Fibonacci anyons present in $ν=12/5$ quantum Hall states can be used for universal quantum computations. Yang-Lee anyons are non-unitary counterparts of Fibonacci anyons. Another possibility of non-Abelian anyons (of bosonic origin) is given by vortex anyons, which are constructed from non-Abelian vortices supported by a non-Abelian first homotopy group, relevant for certain nematic liquid crystals, superfluid $^3$He, spinor Bose-Einstein condensates, and high density quark matter. Finally, there is a unique system admitting two types of non-Abelian anyons, Majorana fermions (Ising anyons) and non-Abelian vortex anyons. That is $^3P_2$ superfluids (spin-triplet, $p$-wave paring of neutrons), expected to exist in neutron star interiors as the largest topological quantum matter in our universe.

cond-mat.supr-con↗

arXiv2023

Imprinting spiral Higgs waves onto superconductors with vortex beams

A vortex beam, akin to a quantized vortex in superfluids, possesses inherent orbital angular momentum (OAM), resulting in the propagation of a spiral-shaped wavefront. Here we demonstrate that a pulsed vortex beam with OAM in the terahertz frequency band can induce a spiral Higgs wave, which is a spiral-shaped oscillation mode of the superconducting order parameter. By utilizing the gauge-invariant theory for the superconducting order, we demonstrate that the phase mode is driven to screen the longitudinal magnetic field of the vortex beam, which facilitates the imprinting of the spiral-shaped wavefront and the transfer of OAM to the condensate. Furthermore, we find that increasing the OAM of light amplifies the intensity of the third harmonic generation. These findings highlight the potential of terahertz vortex beams as a spectroscopic probe of collective modes.

cond-mat.supr-con↗

arXiv2023

Possible Realization of Topological Crystalline Superconductivity with Time-Reversal Symmetry in UTe2

The recent measurement of the de Haas-van Alphen effect in the spin-triplet superconductor UTe2 [D. Aoki et al., J. Phys. Soc. Jpn. 91, 083704 (2022)] supports cylindrical electron and hole Fermi surfaces, which implies that UTe2 is trivial as a 3D time-reversal-invariant topological superconductor. Inspired by this observation, we investigate the possible realization of a topological crystalline superconductor protected by the crystalline symmetry of UTe2. We examine Majorana surface states protected by mirror and two-fold rotational symmetries for all symmetry-allowed odd-parity pairing states with time-reversal symmetry and clarify the corresponding topological invariants. It is found that topological crystalline superconductivity can be realized for all irreducible representations of odd-parity pairing states of UTe2 even for cylindrical Fermi surfaces.

cond-mat.supr-con↗

arXiv2023

Enhanced $2π$-periodic Aharonov-Bohm Effect as a Signature of Majorana Zero Modes Probed by Nonlocal Measurements

We propose the $2π$-periodic Aharonov-Bohm (AB) effect as a nonlocal probe of Majorana zero modes (MZMs) without the restriction of fermion parity. We demonstrate the enhancement of the AB effect, where the topological protection of MZMs yields amplified and robust Andreev reflection mediated by MZMs at multiple superconductor-normal metal junctions. We investigate the influence of trivial bound states and show that a nonlocal index enables a more explicit distinction between the trivial and topological bound states than local probes.

cond-mat.mes-hall↗