Searcharxiv⌕ Search

arXiv subjects

K. Miyake

Publications and source records attributed to K. Miyake.

At least 19 recordsLinked to original sources
arXiv2018

Anomalous ^{125}Te Nuclear Spin Relaxation Coincident with Charge Kondo Behavior in Superconducting Pb_{1-x}Tl_{x}Te

We report a ^{125}Te-NMR study of single-crystalline Pb_{1-x}Tl_{x}Te for x= 0 (undoped non-superconducting "parent"), 0.35 at% (doped but on the borderline of superconductivity), and 1.0 at% (superconducting, with a critical temperature Tc ~ 1.0 K). The Knight shift in the normal state is enhanced as x increases, corresponding to an increase in the average density of states (DOS). The NMR line-width also increases significantly with increasing x, indicative of a strong spatial variation in the local DOS surrounding each Tl dopant. Remarkably, for the superconducting composition, the ^{125}Te nuclear spin relaxation rate (1/T1T) for Te ions that are close to the Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~10 K. This temperature coincides with the temperature below which the normal state resistivity experiences an upturn. Such a simultaneous upturn in both the resistivity and (1/T1T) is highly anomalous, and puts Pb_{1-x}Tl_{x}Te in a distinct new class of doped semiconductors. These observations provide microscopic evidence for dynamical charge fluctuations in the absence of paramagnetism, and are consistent with expectations for charge Kondo behavior associated with the Tl dopant ions. In contrast, such anomalies were not detected in the non-superconducting samples (x=0 and 0.35 at%), suggesting a connection between dynamical valence fluctuations and the occurrence of superconductivity in Pb_{1-x}Tl_{x}Te.

cond-mat.supr-con↗
arXiv2017

Construction of KAGRA: an Underground Gravitational Wave Observatory

Major construction and initial-phase operation of a second-generation gravitational-wave detector KAGRA has been completed. The entire 3-km detector is installed underground in a mine in order to be isolated from background seismic vibrations on the surface. This allows us to achieve a good sensitivity at low frequencies and high stability of the detector. Bare-bones equipment for the interferometer operation has been installed and the first test run was accomplished in March and April of 2016 with a rather simple configuration. The initial configuration of KAGRA is named {\it iKAGRA}. In this paper, we summarize the construction of KAGRA, including the study of the advantages and challenges of building an underground detector and the operation of the iKAGRA interferometer together with the geophysics interferometer that has been constructed in the same tunnel.

gr-qc↗
arXiv2017

Coincidence of magnetic and valence quantum critical points in CeRhIn5 under pressure

We present accurate electrical resistivity measurements along the two principle crystallographic axes of the pressure-induced heavy-fermion superconductor CeRhIn5 up to 5.63 GPa. For both directions, a valence crossover line is identified in the p-T plane and the extrapolation of this line to zero temperature coincides with the collapse of the magnetic ordering temperature. Furthermore, it is found that the p-T phase diagram of CeRhIn5 in the valence crossover region is very similar to that of CeCu2Si2. These results point to the essential role of Ce-4f electron delocalization in both destroying magnetic order and realizing superconductivity in CeRhIn5 under pressure.

cond-mat.supr-con↗
arXiv2017

Ubiquity of Unconventional Phenomena Associated with Critical Valence Fluctuations in Heavy Fermion Metals

Ubiquity of unconventional phenomena observed in a series of heavy fermion metals is discussed on the basis of an idea of critical valence fluctuations. After surveying experimental aspects of these unconventional behaviors in prototypical compounds, CeCu_2(Si,Ge)_2, under pressure, we propose that sharp valence crossover phenomena are realized in CeCu_6, CeRhIn_5, and Ce(Ir,Rh)Si_3 by tuning the pressure and the magnetic field simultaneously, on the basis of previous results for an extended Anderson lattice model with the Coulomb repulsion U_fc between localized f-electron and itinerant conduction electrons.

cond-mat.str-el↗
arXiv2017

Thermal Expansion of the Heavy-fermion Superconductor PuCoGa$_{5}$

We have performed high-resolution powder x-ray diffraction measurements on a sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the highest critical temperature $T_{c}$ = 18.7 K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the $a$ and $c$ lattice parameters. The observed thermal expansion is isotropic down to 150 K, and becomes anisotropic for lower temperatures. This gives a $c/a$ ratio that decreases with increasing temperature to become almost constant above $\sim$150 K. The volume thermal expansion coefficient $α_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change predicted by the Ehrenfest relation for a second order phase transition. The volume expansion deviates from the curve expected for the conventional anharmonic behaviour described by a simple Grüneisen-Einstein model. The observed differences are about ten times larger than the statistical error bars but are too small to be taken as an indication for the proximity of the system to a valence instability that is avoided by the superconducting state.

cond-mat.supr-con↗
arXiv2015

Spin Polarization Enhanced by Spin-Triplet Pairing in Sr$_2$RuO$_4$ Probed by NMR

We report a novel phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr$_2$RuO$_4$ with nuclear magnetic resonance (NMR) that the spin susceptibility originating from the Ru-4$d$ electron slightly $increases$ by $\sim 2 $\% of total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted for superfluid $^3$He forty years ago, but had never been demonstrated in any superconductor.

cond-mat.supr-con↗
arXiv2013

Superconducting Transition Temperatures for Spin-Fluctuation Promoted Superconductivity in Heavy Fermion Compounds

The quantum critical Antiferromagnetic (AFM) fluctuation spectra measured by inelastic neutron scattering recently in two heavy fermion superconductors are used together with their other measured properties to calculate their D-wave superconducting transition temperatures $T_{\rm c}$. To this end, the linearized Eliashberg equations for D-wave superconductivity induced by AFM fluctuations are solved in models of fermions with various levels of nesting. The results for the ratio of $T_{\rm c}$ to the characteristic spin-fluctuation energy are well parametrized by a dimensionless coupling constant and the AFM correlation length. Comparing the results with experiments suggests that one may reasonably conclude that superconductivity in these compounds is indeed caused by AFM fluctuations. This conclusion is strengthened by a calculation with the same parameters of the measured coefficient of the normal state quantum-critical resistivity $\propto T^{3/2}$ characteristic of {\it gaussian} AFM quantum-critical fluctuations. The calculations give details of the superconducting coupling as a function of the correlation length and the integrated fluctuation spectra useful in other compounds.

cond-mat.supr-con↗
arXiv2012

Disorder Effects on Competition between Antiferromagnetism and Superconductivity in Cuprate Superconductors through the Enhancement in Charge Susceptibility

The coexistence state of antiferromagnetism (AF) and superconductivity (SC) has been observed in five-layered cuprates. However, this coexistence state disappears, and the AF phase and SC phase lose contact in the doping phase diagram toward double- and single-layered cuprates. We investigate the mechanism of the disappearance of the coexistence of AF and SC in disordered cuprate superconductors in order to understand these doping phase diagrams. In single- and double-layered cuprates, electrons on the CuO$_2$ plane experience the disorder effect through inhomogeneity in the charge reservoir layer. These impurity potentials can be effectively enhanced toward the underdoped region by the effect of many-body corrections that involve an increase in charge susceptibility. As a result, strong disorder effects are expected particularly in the competing regions of AF and SC, where the coexistence phase of AF and SC is extremely suppressed. We show the validity of this suppression mechanism by considering the Aslamazov-Larkin-type vertex correction to the effective impurity potential in the effective mean-field phase diagram.

cond-mat.str-el↗
arXiv2012

Heavy Fermion superconductor CeCu$_2$Si$_2$ under high pressure: multiprobing the valence crossover

The first heavy fermion superconductor CeCu$_2$Si$_2$ has not revealed all its striking mysteries yet. At high pressures, superconductivity is supposed to be mediated by valence fluctuations, in contrast to ambient pressure, where spin fluctuations most likely act as pairing glue. We have carried out a multiprobe (electric transport, thermopower, ac specific heat, Hall and Nernst effects) experiment up to $7 \text{GPa}$ on a high quality CeCu$_2$Si$_2$ single crystal. Reliable resistivity data reveal for the first time a scaling behavior close to the supposed valence transition, and allow to locate the critical end point at $4.5\pm0.2 \text{GPa}$ and a slightly negative temperature. In the same pressure region, remarkable features have also been detected in the other physical properties, acting as further signatures of the Ce valence crossover and the associated critical fluctuations.

cond-mat.str-el↗
arXiv2012

Spin wave modes in magnetic nanodisks under in-plane magnetic field

The size dependence of spin wave modes in a circular Permalloy (Py) nanodisk under an in-plane magnetic field is systematically studied by using micromagnetics simulations. We show that as the disk diameter is increased, the resonance frequency of the backward mode deceases while that of the uniform mode increases. The avoided crossing of resonance frequencies of the uniform mode and the backward mode appears in the plot of the size dependence of resonance frequencies and the backward mode turns into the so-called "edge mode" for large nanodisks.

cond-mat.mes-hall↗
arXiv2010

Effect of Spin-Orbit Interaction in Spin-Triplet Superconductor: Structure of ${\bf d}$-vector and Anomalous $^{17}$O-NQR Relaxation in Sr$_2$RuO$_4$

Supposing the spin-triplet superconducting state of Sr$_2$RuO$_4$, the spin-orbit (SO) coupling associated with relative motion in Cooper pairs is calculated by extending the method for the dipole-dipole coupling given by Leggett in the superfluid $^{3}$He. It is shown that the SO coupling works only in the equal-spin pairing (ESP) state to make the pair angular momentum $\hbar{\vec L}$ and the pair spin angular momentum ${\rm i}{\vec d}\times{\vec d}^{*}$ parallel with each other. The SO coupling gives rise to the internal Josephson effect in a chiral ESP state as in superfluid A-phase of $^3$He with a help of an additional anisotropy arising from SO coupling of atomic origin which works to direct the {\bf d}-vector into $ab$-plane. This resolves the problem of the anomalous relaxation of $^{17}$O-NQR and the structure of {\bf d}-vector in Sr$_2$RuO$_4$.

cond-mat.supr-con↗
arXiv2009

Pressure-induced change of the pairing symmetry in superconducting CeCu2Si2

Low-temperature (T) heat-capacity measurements under hydrostatic pressure of up to p=2.1 GPa have been performed on single-crystalline CeCu2Si2. A broad superconducting (SC) region exists in the T-p phase diagram. In the low-pressure region antiferromagnetic spin fluctuations and in the high-pressure region valence fluctuations had previously been proposed to mediate Cooper pairing. We could identify these two distinct SC regions. We found different thermodynamic properties of the SC phase in both regions, supporting the proposal that different mechanisms might be implied in the formation of superconductivity.

cond-mat.str-el↗
arXiv2009

d-Wave Spin Density Wave phase in the Attractive Hubbard Model with Spin Polarization

We investigate the possibility of unconventional spin density wave (SDW) in the attractive Hubbard model with finite spin polarization. We show that pairing and density fluctuations induce the transverse d-wave SDW near the half-filling. This novel SDW is related to the d-wave superfluidity induced by antiferromagnetic spin fluctuations, in the sense that they are connected with each other through Shiba's attraction-repulsion transformation. Our results predict the d-wave SDW in real systems, such as cold Fermi atom gases with population imbalance and compounds involving valence skipper elements.

cond-mat.mtrl-sci↗
arXiv2008

Coupled SDW and Superconducting Order in FFLO State of CeCoIn$_5$

The mechanism of incommensurate (IC) spin-density-wave (SDW) order observed in the Flude-Ferrell-Larkin-Ovchinnikov (FFLO) phase of CeCoIn$_5$ is discussed on the basis of new mode-coupling scheme among IC-SDW order, two superconducting orders of FFLO with B$_{1{\rm g}}$ ($d_{x^{2}-y^{2}}$) symmetry and $π$-pairing of odd-parity. Unlike the mode-coupling schemes proposed by Kenzelmann et al, Sciencexpress, 21 August (2008), that proposed in the present Letter can offer a simple explanation for why the IC-SDW order is observed only in FFLO phase and the IC wave vector is rather robust against the magnetic field.

cond-mat.str-el↗
arXiv2008

BCS-BEC crossover and effects of density fluctuations in a two-component Fermi gas loaded on an optical lattice

We investigate the superfluid phase transition in a gas of Fermi atoms loaded on a three-dimensional optical lattice. When the lattice potential is strong, this system can be well described by an attractive Hubbard model. In this model, we calculate the superfluid phase transition temperature Tc, including both superfluid and (spin and charge) density fluctuations within the self-consistent t-matrix theory and fluctuation exchange approximation, respectively. Since we treat these fluctuations in a consistent manner, our theory satisfies the required particle-hole symmetry over the entire BCS-BEC crossover region. We show that charge density fluctuations compete against superfluid fluctuations near the half-filling, leading to the suppression of Tc. As a result, the maximum Tc is obtained away the half-filling. Since the strong density fluctuations originate from the nesting property of the Fermi surface at the half filling (which is absent in a uniform gas with no lattice potential), our results would be useful in considering lattice effects on strong-coupling superfluidity.

cond-mat.mtrl-sci↗
arXiv2007

Theory of Ultrasonic Dispersion in Local Phonon Systems Coupled with Conduction Electrons

The physical origin of frequency dependence in elastic constants, which are often found in an ultrasound propagation in filled skutterudites and clathrate compounds, is investigated theoretically. This dependence arises from a coupling between the acoustic phonon and some optical phonons, which strongly interact with electrons. Using a self-consistent ladder approximation together with a pseudofermion mapping of the phonon to the single site Holstein Anderson model, a soft mode of the optical phonon at zero frequency is shown to emerge. The temperature dependence of the spectral weight of this soft mode shows an activation-type behavior, which is characterized by the optical phonon frequency. These features can generate the frequency dependence and the shoulder in the elastic constants observed in some filled skutterudites and clathrate compounds.

cond-mat.str-el↗
arXiv2005

Triplet Pairing Superconductivity Induced by Short-Range Ferromagnetic Correlations in Sr$_2$RuO$_4$

The microscopic origin of triplet superconductivity in Sr$_2$RuO$_4$ is discussed, paying attention to the role of Coulomb interaction, $U_{pp}$, at the O site. It is shown on the $d$-$p$ model that $U_{pp}$ induces a ferromagnetic exchange interaction between "d-electrons" (molecular orbital with $d_{xy}$-symmetry) at adjacent Ru sites, leading to short-range ferromagnetic correlations and promoting Cooper pairing with ($\sin p_x\pm\ii\sin p_y$)-symmetry on the $γ$-band. The reason why such ferromagnetic correlations work effectively may be traced back to the fact that the level of 4d-electrons at Ru sites is relatively low and located near that of $2p$-electrons at O sites.

cond-mat.str-el↗