arXiv subjects

Noriyuki Nakai

Publications and source records attributed to Noriyuki Nakai.

12 recordsLinked to original sources

arXiv2015

Interplay between effective mass anisotropy and Pauli paramagnetic effects in a multiband superconductor--Application to Sr2RuO4--

We investigate the mixed state properties in a type II multiband superconductor with uniaxial anisotropy under the Pauli paramagnetic effects. Eilenberger theory extended to a multiband superconductor is utilized to describe the detailed vortex lattice properties, such as the flux line form factors, the vortex lattice anisotropy and magnetic torques. We apply this theory to Sr$_2$RuO$_4$ to analyze those physical quantities obtained experimentally, focusing on the interplay between the strong two-dimensional anisotropy and the Pauli paramagnetic effects. This study allows us to understand the origin of the disparity between the vortex lattice anisotropy ($\sim$60) and the $H_{\rm c2}$ anisotropy ($\sim$20). Among the three bands; $γ$ with the effective mass anisotropy $\sim$180, $α$ with $\sim$120, and $β$ with $\sim$60, the last one is found to be the major band, responsible for various magnetic responses while the minor $γ$ band plays an important role in the vortex formation. Namely, in a field orientation slightly tilted away from the two dimensional basal plane those two bands cooperatively form the optimal vortex anisotropy which exceeds that given by the effective mass formula with infinite anisotropy. This is observed by small angle neutron scattering experiments on Sr$_2$RuO$_4$. The pairing symmetry of Sr$_2$RuO$_4$ realized is either spin singlet or spin triplet with the d-vector strongly locked in the basal plane. The gap structure is that the major $β$ band has a full gap and the minor $γ$ band has a $d_{x^2-y^2}$ like gap.

cond-mat.supr-con↗

arXiv2015

Pauli-paramagnetic effects on mixed state properties in a strongly anisotropic superconductor ---Application to Sr_2_RuO_4_---

We study theoretically the mixed state properties of a strong uniaxially-anisotropic type II superconductor with the Pauli paramagnetic effect, focusing on their behaviors when the magnetic field orientation is tilted from the conduction layer ab plane. On the basis of Eilenberger theory, we quantitatively estimate significant contributions of the Pauli paramagnetic effects on a variety of physical observables, including transverse and longitudinal components of the flux line lattice form factors, magnetization curves, Sommerfeld coefficient, field distributions and magnetic torques. We apply these studies to Sr_2_RuO_4_ and quantitatively explain several seemingly curious behaviors, including the H_c2_ suppression for the ab plane direction, the larger anisotropy ratio and intensity found by the spin-flip small angle neutron scattering, and the first order transition observed recently in magneto-caloric, specific heat and magnetization measurements in a coherent and consistent manner. Those lead us to conclude that Sr_2_RuO_4_ is either a spin-singlet or a spin-triplet pairing with the d-vector components in the ab plane.

cond-mat.supr-con↗

arXiv2014

Eilenberger theory and London theory for transverse components of flux line lattice form factors in uniaxial superconductors

We theoretically study the magnetic field orientation dependence of longitudinal and transverse flux line lattice form factors in uniaxial superconductors with anisotropy ratio corresponding to YBa_2_Cu_3_O_{7-delta}. We discuss influences of the anisotropy ratio of coherence length, and differences between the s-wave and the d_{x^2-y^2}-wave pairings. The calculations are performed by two methods, the Eilenberger theory and the London theory comparatively, and we study the cutoff function of the extended London theory, which will be helpful in the analysis of the small angle neutron scattering in the vortex states.

cond-mat.supr-con↗

arXiv2014

Knight shift spectrum in vortex states in s-wave and d-wave superconductors on the basis of Eilenberger theory

From the spatial structure of vortex lattice state calculated by Eilenberger theory, we study the resonance line shape of Knight shift of the paramagnetic moments in the s-wave and the d-wave superconductors, comparing with the Redfield pattern of the internal field distribution. We discuss the deviation from the temperature dependence of the Yosida function, and the magnetic field dependence of the paramagnetic susceptibility. In addition to the calculation in the clean limit, influences of the impurity scattering are estimated in the Born limit and in the unitary limit. These results are helpful for the analysis of NMR experiments to know properties of the superconductors.

cond-mat.supr-con↗

arXiv2012

Direct Numerical Demonstration of Sign-preserving Quasiparticle Interference via Impurity inside Vortex Core in Unconventional Superconductors

We perform large-scale numerical calculations self-consistently solving the Bogoliubov-de Gennes (BdG) equations in the magnetic field together with random impurities to directly demonstrate the typical quasi-particle interference (QPI) in the presence of vortices as observed by scanning tunneling microscopy/spectroscopy experiments in unconventional superconductors. The calculations reveal that vortex itself never works as a scatter causing the QPI pattern but vortex core containing impurity brings about the enhancement of the sign-preserving QPI peaks. Its origin is Andreev bound-states distorted by impurity, and all the measurement findings are consistently explained by the scenario based on the numerical results.

cond-mat.supr-con↗

arXiv2011

Field Evolution of the Fulde-Ferrell-Larkin-Ovchinnikov State in a Superconductor with Strong Pauli Effects

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the vortex lattice state is quantitatively studied using the selfconsistent Eilenberger theory in three-dimensional (3D) space. We estimate free energy to determine the FFLO phase diagram in the H-T plane and stable FFLO wave number in the isotropic system with the 3D Fermi sphere and s-wave pairing. To facilitate the experimental identification of the FFLO state, we investigate the field evolution of NMR spectra and flux line lattice form factors obtained in neutron scattering in the FFLO vortex states. Possible applications of our results to experimental data on CeCoIn5 are mentioned.

cond-mat.supr-con↗

arXiv2010

Microwave Surface Impedance Measurements of LiFeAs Single Crystals

We report results of microwave surface impedance measurements of LiFeAs single crystals. The in-plane penetration depth depends on temperature exponentially at low temperatures, which strongly suggests that this material has the nodeless superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs is a multi-gap superconductor with at least two isotropic gaps. In addtion, the real part of the microwave conductivity exhibits a large enhancement below $T_\mathrm{c}$, indicating that the quasi-particle relaxation time, $τ$, increases rapidly below $T_\mathrm{c}$. We believe that this enhancement is rather common to all superconductors where an inelastic scattering is dominant above $T_\mathrm{c}$, irrespective of the strength of the electron correlation.

cond-mat.supr-con↗

arXiv2010

Ambegaokar-Baratoff relations of Josephson critical current in heterojunctions with multi-gap superconductors

An extension of the Ambegaokar-Baratoff relation to a superconductor-insulator-superconductor (SIS) Josephson junction with multiple tunneling channels is derived. Appling the resultant relation to a SIS Josephson junction formed by an iron-based (five-band) and a single-band Bardeen-Cooper-Schrieffer (BCS) type superconductors, a theoretical bound of the Josephson critical current ($I_{\rm c}$) multiplied by the resistance of the junction ($R_{\rm n}$) is given. We reveal that such a bound is useful for identifying the pairing symmetry of iron-pnictide superconductors. One finds that if a measured value of $I_{\rm c}R_{\rm n}$ is smaller than the bound then the symmetry is $\pm s$-wave, and otherwise $s$-wave without any sign changes. In addition, we stress that temperature dependence of $I_{\rm c}R_{\rm n}$ is sensitive to the difference of the gap functions from the BCS type gap formula in the above heterojunction.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

arXiv2004

Electronic state around vortex in a two-band superconductor

Based on the quasiclassical theory, we investigate the vortex state in a two-band superconductor with a small gap on a three dimensional Fermi surface and a large gap on a quasi-two dimensional one, as in MgB_2. The field dependence of zero-energy density of states is compared for fields parallel and perpendicular to the ab plane, and the anisotropy of the vortex core shape is discussed for a parallel field. The Fermi surface geometry of two-bands, combining the effect of the normal-like electronic state on the small gap band at high fields, produces characteristic behavior in the anisotropy of c- and ab-directions.

cond-mat.supr-con↗

arXiv2003

On Superconducting Double Transition in PrOs_4_Sb_12

Superconducting double transition in PrOs_4_Sb_12 is investigated by analyzing the anisotropy of the upper critical field H_c2 in the ab-plane, and the possible pairing state is discussed. When mixing due to gradient coupling is active, the twofold-symmetric component is necessarily induced in the fourfold symmetric phase, leading to the twofold oscillation of H_c2, contrary to the experimental result. To avoid the mixing effect, the weak spin-orbit coupling triplet pairing state is considered as a likely pairing function, where time-reversal symmetry is broken.

cond-mat.supr-con↗

arXiv2001

Field Dependence of Electronic Specific Heat in Two-Band Superconductors

The vortex structure is studied in light of MgB$_2$ theoretically based on a two-band superconducting model by means of Bogoliubov-de Gennes framework. The field dependence of the electronic specific heat coefficient $γ(H)$ is focused. The exponent $α$ in $γ(H)\propto H^α$ is shown to become smaller by adjusting the gap ratio of the two gaps on the major and minor bands. The observed extremely small value $α\sim 0.23$ could be explained reasonably well in this two-band model with the gap ratio $\sim 0.3$.

cond-mat.supr-con↗