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Nobuhiko Hayashi

Publications and source records attributed to Nobuhiko Hayashi.

At least 19 recordsLinked to original sources
arXiv2015

Effects of Weak and Strong Scatterers on the Spectra of Vortex Andreev Bound States in Two-Dimensional Chiral p-wave Superconductors

The vortices of two-dimensional chiral $p$-wave superconductors are predicted to exhibit some exotic behaviors; one of their curious features is the existence of two types of vortices (each vortex has vorticity either parallel or antiparallel to the Cooper pair's chirality) and the robustness of the antiparallel vortices against nonmagnetic Born-like impurities. In this work, we study the impurity effect on the vortex of the chiral $p$-wave superconductors through the quasiclassical Green's function formalism. We take account of impurities via the self-consistent $t$-matrix approximation so that we can deal with strong as well as Born-like (i.e., weak) scatterers. We found that the spectrum is heavily broadened when the phase shift $δ_0$ of each impurity exceeds a critical value $δ_{\text{c}}$ above which the impurity band emerges at the Fermi level. We also found a quantitative difference in the impurity effects on the two types of vortex for $δ_0<δ_{\text{c}}$. Part of the numerical results for $δ_0<δ_{\text{c}}$ can be understood by a variant of the analytical theory of Kramer and Pesch for bound states localized within vortex cores.

cond-mat.supr-con↗
arXiv2014

Field-angle Resolved Flux-flow Resistivity as a Phase-sensitive Probe of Unconventional Cooper Pairing

We theoretically investigate the applied magnetic field-angle dependence of the flux-flow resistivity $ρ_{\rm f}(α_{\rm M})$ for an uniaxially anisotropic Fermi surface. $ρ_{\rm f}$ is related to the quasiparticle scattering rate $\varGamma$ inside a vortex core, which reflects the sign change in the superconducting pair potential. We find that $ρ_{\rm f}(α_{\rm M})$ is sensitive to the sign-change in the pair potential and has its maximum when the magnetic field is parallel to the gap-node direction. We propose the measurement of the field-angle dependent oscillation of $ρ_{\rm f}(α_{\rm M})$ as a phase-sensitive field-angle resolved experiment.

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arXiv2014

Analysis of Magnetic Field-Angle Dependent Electronic Raman Scattering to Probe the Superconducting Gap

We study the field-angle resolved electronic Raman scattering in 2-dimensional d-wave superconducting vortex states theoretically by quasi-classical approximation, the so-called Doppler-shift method. An analytic expression is obtained for the field angle dependence of the Raman scattering amplitude at zero temperature. After numerical integration, we obtain the scattering intensity for various field angles by changing the Raman shift energy. Field-angle resolved electronic Raman scattering turns out to be an effective method for probing unconventional superconducting gap structures. It shows a novel phenomenon: reversal of extrema as a function of frequency without changing temperature or field magnitude.

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arXiv2013

Impurity effects in a vortex core in a chiral p-wave superconductor within the t-matrix approximation

We study the effects of non-magnetic impurity scattering on the Andreev bound states (ABS) in an isolated vortex in a two-dimensional chiral p-wave superconductor numerically. We incorporate the impurity scattering effects into the quasiclassical Eilenberger formulation through the self-consistent $t$-matrix approximation. Within this scheme, we calculate the local density of states (LDOS) around two types of vortices: "parallel" ("anti-parallel") vortex where the phase winding of the pair-potential coming from vorticity and that coming from chirality have the same (opposite) sign. When the scattering phase-shift $δ_0$ of each impurity is small, we find that impurities affect differently low energy quasiparticle spectrum around the two types of vortex in a way similar to that in the Born limit ($δ_0\rightarrow 0$). For a larger $δ_0(\leq π/2)$ however we find that ABS in the vortex is strongly suppressed by impurities for both types of vortex. We found that there are some correlations between the suppression of ABS near vortex cores and the low energy density of states due to impurity bands in the bulk.

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arXiv2013

Effect of anisotropic Fermi surface on the flux-flow resistivity under rotating magnetic field

We numerically investigate the effect of in-plane anisotropic Fermi surface (FS) on the flux-flow resistivity $ρ_{\rm f}$ under rotating magnetic field on the basis of the quasiclassical Green's function method. We demonstrate that one can detect the phase in pairing potential of Cooper pair through the field-angular dependence of $ρ_{\rm f}$ even if the FS has in-plane anisotropy. In addition, we point out one can detect the gap-node directions irrespective of the FS anisotropy by measuring $ρ_{\rm f}$ under rotating field.

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arXiv2013

Field-angle dependence of the quasiparticle scattering inside a vortex core in unconventional superconductors

We theoretically investigate the quasiparticle scattering rate $\varGamma$ inside a vortex core in the existence of non-magnetic impurities distributed randomly in a superconductor. We show that the dependence of $\varGamma$ on the magnetic field direction is sensitive to the sign of the pair potential. The behavior of $\varGamma$ is quite different between an s-wave and a d-wave pair potential, where these are assumed to have the same amplitude anisotropy, but a sign change only for the d-wave one. It is suggested that measurements of the microwave surface impedance with changing applied-field directions would be used for the phase-sensitive identification of pairing symmetry.

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arXiv2013

Effect of uniaxially anisotropic Fermi surface on the quasiparticle scattering inside a vortex core in unconventional superconductors

We theoretically study the dependence of the quasiparticle (QP) scattering rate $\varGamma$ on the uniaxial anisotropy of a Fermi surface with changing the magnetic field angle $α_{\rm M}$. We consider the QP scattering due to the non-magnetic impurities inside a single vortex core. The field-angle dependence of the quasiparticle scattering rate $\varGamma(α_{\rm M})$ is sensitive to the sign-change of the pair potential. We show that with increasing the two dimensionality of the system, $\varGamma(α_{\rm M})$ reflects more clearly whether there is the sign-change in the pair potential.

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arXiv2013

Phase-Sensitive Flux-Flow resistivity in Unconventional Superconductors

We theoretically investigate the magnetic-field-angle dependence of the flux-flow resistivity $ρ_{\rm f}$ in unconventional superconductors. Two contributions to $ρ_{\rm f}$ are considered: one is the quasiparticle (QP) relaxation time $τ(\bm{k}_{\rm F})$ and the other is $ω_0(\bm{k}_{\rm F})$, which is a counterpart to the interlevel spacing of the QP bound states in the quasiclassical approach. Here, $\bm{k}_{\rm F}$ denotes the position on a Fermi surface. Numerical calculations are conducted for a line-node s-wave and a d-wave pair potential with the same anisotropy of their amplitudes, but with a sign change only for a d-wave one. We show that the field-angle dependence of $ρ_{\rm f}$ differs prominently between s-wave and d-wave pairs, reflecting the phase of the pair potentials. We also discuss the case where $τ$ is constant and compare it with the more general case where $τ$ depends on $\bm{k}_{\rm F}$.

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arXiv2013

Impurity scattering effect on the zero-energy peak of the local density of states in a multi-quantum vortex core

We theoretically study a non-magnetic impurity effect on the vortex bound states of a multi-quantum vortex. The zero-energy peak of the local density of states is investigated for vortex cores with the winding numbers 2 and 4 within the framework of the quasiclassical theory of superconductivity. We find that the zero-energy peaks, which appear away from the vortex center in the clean limit, move towards the vortex center with increasing the impurity scattering rate, resolving a contradiction between an experimental result and previous theoretical predictions.

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arXiv2012

Quasiclassical numerical method for mesoscopic superconductors: bound states in a circular d-wave island with a single vortex

We demonstrate an efficient numerical method for obtaining unique solutions to the Eilenberger equation for a mesoscopic or nanoscale superconductor. In particular, we calculate the local density of states of a circular d-wave island containing a single vortex. The "vortex shadow" effect is found to strongly depend on the quasiparticle energy in such small systems. We show how to construct by geometry quasiparticle trajectories confined in a finite-size system with specular reflections at the boundary, and discuss the stability of the numerical solutions even in the case of vanishing order parameter as for nodal quasiparticles in a d-wave superconductor, or for quasiparticles passing through the vortex center with zero energy.

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arXiv2011

Torque Theory of Anisotropic Superconductors with No Phenomenological Parameter in determining Vortex Core Size

The contribution of vortex core has been taken into account properly in constructing a torque theory for multiband superconductors. We employ the prescription of describing internal magnetic field in the vortex lattice by Hao {\it et al.} and by Yaouanc {\it et al.} to derive a torque formula as a natural extension of a preceding London theory. In marked contrast with the preceding model, our novel formula does not contain a phenomenological parameter $η$, which prevents us from obtaining a {\it true} upper critical field $H_{\rm c2}$ by analyzing an experimental torque curve. The parameter $η$ was originally introduced to take care of the uncertainty in determining the vortex core size $ξ_v$. Furthermore, we reveal that the $η$ value is universally scaled by anisotropy $γ$, magnetic field $B$, and $H_{\rm c2}$ due to field dependence of $ξ_v$. This may revitalize the single-band Kogan model in combination with a universal function $η(γ, B, H_{\rm c2})$ instead of a constant $η$.

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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.

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arXiv2009

Surface-angle dependence of the tunneling spectroscopy in iron-based superconductors: sign-reversing s-wave scenarios

We discuss the surface Andreev bound states in Fe-based superconductors with the use of an effective five-band model and investigate the surface-angle dependence of the tunneling spectroscopy by a quasiclassical approach for an isotropic and an anisotropic /pm s-wave gap superconductivity. We show that information on the normal state is important for the Andreev bound state and its peak positions do not depend on the gap amplitude anisotropy.

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arXiv2009

Analysis of field-angle dependent specific heat in unconventional superconductors: a comparison between Doppler-shift method and Kramer-Pesch approximation

We theoretically discuss the magnetic-field-angle dependence of the zero-energy density of states (ZEDOS) in superconductors. Point-node and line-node superconducting gaps on spherical and cylindrical Fermi surfaces are considered. The Doppler-shift (DS) method and the Kramer-Pesch approximation (KPA) are used to calculate the ZEDOS. Numerical results show that consequences of the DS method are corrected by the KPA.

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arXiv2009

Surface Bound States in n-band Systems with Quasiclassical Approach

We discuss the tunneling spectroscopy at a surface in multi-band systems such as Fe-based superconductors with the use of the quasiclassical approach. We extend the single-band method by Matsumoto and Shiba [J. Phys. Soc. Jpn. 64, 1703 (1995)] into $n$-band systems ($n \geq 2$). We show that the appearance condition of the zero-bias conductance peak does not depend on details of the pair-potential anisotropy, but it depends on details of the normal state properties in the case of fully-gapped superconductors. The surface density of states in a two-band superconductor is presented as a simplest application. The quasiclassical approach enables us to calculate readily the surface-angular dependence of the tunneling spectroscopy.

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arXiv2009

Field angle dependence of the zero-energy density of states in unconventional superconductors: analysis of the borocarbide superconductor YNi2B2C

We investigate the field-angle-dependent zero-energy density of states for YNi2B2C with using realistic Fermi surfaces obtained by band calculations. Both the 17th and 18th bands are taken into account. For calculating the oscillating density of states, we adopt the Kramer-Pesch approximation, which is found to improve accuracy in the oscillation amplitude. We show that superconducting gap structure determined by analyzing STM experiments is consistent with thermal transport and heat capacity measurements.

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arXiv2008

Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic \pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like βFermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.

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