arXiv subjects

M. Ichioka

Publications and source records attributed to M. Ichioka.

At least 37 records · Page 2Linked to original sources

arXiv2007

Pauli Paramagnetic Effects on Vortices in Superconducting TmNi2B2C

The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6 Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.

cond-mat.supr-con↗

arXiv2007

Is Sr2RuO4 a triplet superconductor?

The field dependence of the specific heat γ(H) at lower temperatures in Sr2RuO4 is analyzed by solving microscopic Eilenberger equation numerically. We find that systematic γ(H) behaviors from a concaved \sqrt H to a convex H^α (α>1) under H orientation change are understood by taking account of the Pauli paramagnetic effect. The magnetizations are shown to be consistent with it. This implies either a singlet pairing or a triplet one with d-vector locked in the basal plane, which allows us to explain other mysteries of this compound in a consistent way.

cond-mat.supr-con↗

arXiv2006

Vortex Core Structure in Neutral Fermion Superfluids with Population Imbalance

Quantized vortex core structure is theoretically investigated in Fermion superfluids with population imbalance for two atom species of neutral atom clouds near a Feshbach resonance. In contrast with vortex core in balance case where the quantum depletion makes a vortex visible through the density profile measurement, the vortex core is filled in and becomes less visible because the quantized discrete bound states are occupied exclusively by the majority species. Yet it is shown that the core can be visible through the minority density profile experiment using phase contrast imaging, revealing an interesting opportunity to examine low-lying Fermionic core bound states unexplored so far.

cond-mat.supr-con↗

arXiv2006

Ubiquitous V-shape density of states in mixed state of clean limit type II superconductors; STM experiment and theory

It is demonstrated theoretically and experimentally that the low energy density of states $N(E)$ is described by a singular V-shape form $N(E)=N_0(H)+α|E|+O(E^2)$ for all clean superconductors in a vortex state, irrespective of the underlying gap structure. The linear term $α|E|$ which has been not recognized so far is obtained by exactly evaluating the vortex contribution. Based on microscopic Eilenberger theory $N(E)$ is evaluated for isotropic gap, line and point node gaps to yield a V-shape $N(E)$. STS-STM experiments on NbSe$_2$ and YNi$_2$B$_2$C give direct evidence for this. We provide arguments on significance of this finding and on relevance to other experiments.

cond-mat.supr-con↗

arXiv2006

Generic Phase Diagram of Fermion Superfluids with Population Imbalance

It is shown by microscopic calculations for trapped imbalanced Fermi superfluids that the gap function has always sign changes, i.e., the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state like, up to a critical imbalance $P_c$, beyond which normal state becomes stable, at temperature T=0. A phase diagram is constructed in $T$ vs $P$, where the BCS state without sign change is stable only at $T\neq 0$. We reproduce the observed bimodality in the density profile to identify its origin and evaluate $P_c$ as functions of $T$ and the coupling strength. These dependencies match with the recent experiments.

cond-mat.supr-con↗

arXiv2006

Basal-Plane Magnetic Anisotropies of High-kappa d-Wave Superconductors in a Mixed State: A Quasiclassical Approach

We study the basal-plane anisotropies of reversible magnetization and torque in a mixed state of layered d-wave superconductors based on the quasiclassical version of the BCS-Gor'kov theory. Both the longitudinal magnetization ($M_L$) and torque ($τ$) show fourfold oscillations as a function of the field angle $χ$. The relationship between the node position and the oscillatory patterns shown by $M_L$ and $τ$ is clarified. It is also shown that the sign of the $τ(χ)$-oscillation does not change between $H_{c1}$ and $H_{c2}$, while the sign of the $M_L (χ)$-oscillation changes. The newly obtained result for $τ$ indicates that the torque experiment can allow us to detect the in-plane anisotropies of $H_{c2}$ even in a material with strong fluctuations such as cuprate or organic superconductors, where the $H_{c2}$ itself cannot be determined experimentally.

cond-mat.supr-con↗

arXiv2006

Theory of tunneling spectroscopy in the Larkin-Ovchinnikov state

A theory of the tunneling spectroscopy of normal metal / Larkin-Ovchinnikov (LO) state is presented by fully taking account of the periodic modulation of pair potentials in real space. The resulting tunneling spectra have characteristic line shapes with several maxima and minima reflecting minigap structures due to the periodic pair potentials. These features are qualitatively different from tunneling spectra of Fulde-Ferrell state and those of uniform $s$- and d-wave superconductors. On the basis of calculated results, we propose a method to identify the LO state in actual experiments.

cond-mat.supr-con↗

arXiv2006

Specific heat and low-lying excitations in the mixed state for a type II superconductor

Low temperature behavior of the electronic specific heat $C(T)$ in the mixed state is by the self-consistent calculation of the Eilenberger theory. In addition to $γT$-term ($γ$ is a Sommerfeld coefficient), $C(T)$ has significant contribution of $T^2$-term intrinsic in the vortex state. We identify the origin of the $T^2$-term as (i) V-shape density of states in the vortex state and (ii) Kramer-Pesch effect of vortex core shrinking upon lowering $T$. These results both for full-gap and line node cases reveal that the vortex core is a richer electronic structure beyond the normal core picture.

cond-mat.supr-con↗

arXiv2005

Mixed-State Thermodynamics of Superconductors with Moderately Large Paramagnetic Effects

Effects of Pauli paramagnetism on thermodynamic quantities in a vortex state, such as the specific heat $C$ and magnetization $M$, are studied using the quasiclassical Eilenberger formalism. We demonstrate that with an increase of paramagnetic depairing effect, the sigh of the curvature of the field dependence of $C$ changes from negative to positive, and that the Maki parameter $κ_2$ becomes an increasing function of temperature. Our results provide a natural explanation for the unusual field dependence of $C$ seen in CeCoIn$_5$ in terms of the paramagnetic effect.

cond-mat.supr-con↗

arXiv2005

Topological Structure of a Vortex in Fulde-Ferrell-Larkin-Ovchinnikov State

We find theoretically that the vortex core in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is quite different from the ordinary core by a simple topological reason. The intersection point of a vortex and nodal plane of the FFLO state empties the excess spins. This leads to observable consequences in the spatial structure of the spontaneous magnetization. We analyze this topological structure based on the low lying excitation spectrum by solving microscopic Bogoliubov-de Gennes equation to clarify its physical origin.

cond-mat.supr-con↗

arXiv2005

Anisotropic Diamagnetic Response in Type-II Superconductors with Gap and Fermi-Surface Anisotropies

Effects of anisotropic gap structures on a diamagnetic response are investigated in order to demonstrate that the field-angle-resolved magnetization ($M_L(χ)$) measurement can be used as a spectroscopic method to detect gap structures. Our microscopic calculation based on the quasiclassical Eilenberger formalism reveals that $M_L(χ)$ in a superconductor with four-fold gap displays a four-fold oscillation reflecting the gap and Fermi surface anisotropies, and the sign of this oscillation changes at a field between $H_{c1}$ and $H_{c2}$. As a prototype of unconventional superconductors, magnetization data for borocarbides are also discussed.

cond-mat.supr-con↗

arXiv2005

Direct Imaging of Spatially Modulated Superfluid Phases in Atomic Fermion Systems

It is proposed that the spatially modulated superfluid phase, or the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state could be observed in resonant Fermion atomic condensates which are realized recently. We examine optimal experimental setups to achieve it by solving Bogoliubov-de Gennes equation both for idealized one-dimensional and realistic three-dimensional cases. The spontaneous modulation of this superfluid is shown to be directly imaged as the density profiles either by optical absorption or by Stern-Gerlach experiments.

cond-mat.supr-con↗

arXiv2004

How to detect gap nodes of a superconductor by angle-resolved specific heat experiment?

The specific heat oscillation in the mixed state of type II superconductors is studied theoretically when rotating field within a plane containing gap minimum and maximum. The calculations are performed microscopically by solving quasi-classical Eilenberger equation for vortex lattices. The field dependence of the oscillation amplitude can discriminate between the nodal and anisotropic gap with a finite minimum and the oscillation phase gives the gap minimum position on the Fermi surface. These also provide a way to separate out the anisotropic behavior due to the Fermi velocity.

cond-mat.supr-con↗

arXiv2004

Field dependence of the zero energy density of states around vortices in an anisotropic gap superconductor

It is shown theoretically that the Sommerfeld coefficient $γ(B)$ in the mixed state contains useful information on the gap topology in a superconductor. We accurately evaluate $γ(B)$ against magnetic induction $B$ by using microscopic quasi-classical theory. The linear portion in {$γ(\propto B)$} relative to $B_{\rm c2}$ and its slope in low field faithfully corresponds to the gap anisotropy, so that we can distinguish the nodal gap and the anisotropic full gap, and estimate the magnitude of the gap anisotropy.

cond-mat.supr-con↗

arXiv2004

Expansion of Vortex Cores by Strong Electronic Correlation in La$_{2-x}$Sr$_x$CuO$_4$ at Low Magnetic Induction

The vortex core radius \rv, defined as the peak position of the supercurrent around the vortex, has been determined by muon spin rotation measurements in the mixed state of \lscox for $x=0.13$, 0.15, and 0.19. At lower doping (x=0.13 and 0.15), \rv(T) increases with decreasing temperature T, which is opposite to the behavior predicted by the conventional theory. Moreover, \rv(T\to0) is significantly larger than the Ginsburg-Landau coherence length determined by the upper critical field, and shows a clear tendency to decrease with increasing the doping x. These features can be qualitatively reproduced in a microscopic model involving antiferromagnetic electronic correlations.

cond-mat.supr-con↗

arXiv2003

The effect of nonmagnetic impurities on the local density of states in s-wave superconductors

We study the effect of nonmagnetic impurities on the local density of states (LDOS) in s-wave superconductors. The quasiclassical equations of superconductivity are solved selfconsistently to show how LDOS evolves with impurity concentration. The spatially averaged zero-energy LDOS is a linear function of magnetic induction in low fields, N(E=0)=cB/H_{c2}, for all impurity concentration. The constant of proportionality "c" depends weakly on the electron mean free path. We present numerical data for differential conductance and spatial profile of zero-energy LDOS which can help in estimating the mean free path through the LDOS measurement.

cond-mat.supr-con↗

arXiv2003

Orientational field-dependence of low-lying excitations in mixed state of unconventional superconductors

Orientational field-dependence of the zero energy density of states (ZEDOS) is calculated for superconductors with the polar state (line node), axial state (point node) and 3D d-wave state. Depending on the gap topology and relative field direction the field dependencies of ZEDOS sensitively differ, providing us a useful and practical method to identify the gap topology. It is also demonstrated that for d-wave state the field rotation in the basal plane shows a sizable oscillation ~3% of ZEDOS. This is directly measurable in low-T specific heat experiment in the mixed state.

cond-mat.supr-con↗

arXiv2002

Beliaev Damping and Kelvin Mode Spectroscopy of a Condensate in the Presence of a Vortex Line

It is demonstrated theoretically that the counter-rotating quadrupole mode in a vortex of Bose-Einstein condensates can decay into a pair of Kelvin modes via Beliaev process. We calculate the spectral weight of density-response function within Bogoliubov framework, taking account of both Beliaev and Landau processes. Good agreement with experiment on $^{87}$Rb by Bretin et al. [cond-mat/0211101] allows us to unambigiously identify the decayed mode as the Kelvin wave propagating along a vortex line.

cond-mat.soft↗