arXiv subjects

Kazuyoshi Yoshimura

Publications and source records attributed to Kazuyoshi Yoshimura.

26 records · Page 2Linked to original sources

arXiv2011

Enhancement of the Superconducting Transition Temperature with Hydrostatic Pressure in Ca_{3}Ir_{4}Sn_{13} Single Crystals

We report high pressure magnetic susceptibility and electrical resistivity measurements on Ca_{3}Ir_{4}Sn_{13} single crystals up to 60 kbar. These measurements allow us to follow the evolution of the superconducting critical temperature T_c, the resistivity anomaly temperature T*, the superconducting coherence length and the Fermi velocity under pressure. The pressure-temperature phase diagram constructed for Ca_{3}Ir_{4}Sn_{13} shows a dome-shaped pressure dependence of T_c. The initial rise in T_c, which is accompanied by a decrease in T*, is consistent with a reduction in the partial gapping of the Fermi surface under pressure.

cond-mat.supr-con↗

arXiv2010

Universal Phase Diagram and Reentrant Superconductivity of $Bi$layer Hydrated Na$_x$CoO$_2\cdot y$H$_2$O

We report duration dependence of superconductivity in a 43 \% humidity atmosphere and $^{59}$Co nuclear quadrupole resonance (NQR) of polycrystalline samples of $bi$layer hydrated Na$_x$CoO$_2 \cdot y$H$_2$O. We found a reentrant behavior of superconductivity with respect to the time duration. We obtained the universal phase diagram where two superconducting phases and an in-between magnetic phase are classified by $^{59}$Co NQR frequency. Zero field $^{59}$Co nuclear spin-lattice relaxation rates $1/T_1$ indicated a magnetic critical slowing down at 5 K in the magnetic phase, excluding a charge density wave ordering, and an enhancement at and just above $T_\mathrm{c}$ in a new superconducting phase, suggesting a coexistence of magnetic ordering.

cond-mat.supr-con↗

arXiv2010

Spectroscopic Study of $^{75}$As and $^{139}$La NMR on Layered Structure Ferromagnet LaCoAsO

$^{75}$As and $^{139}$La field-swept NMR spectra were obtained for the novel weakly itinerant ferromagnet LaCoAsO with 2D layered structure above the Curie temperature of 55 K. By analyzing NMR spectra, temperature dependences of Knight shift $K$ and nuclear quadrupole resonance frequency $ν_Q$ were obtained successfully for each nucleus. We confirmed from the so-called $K$-$χ$ plots that the macroscopic magnetization of our {LaCoAsO} powder sample is intrinsic and does not contain the contribution from impurity phases. We estimated hyperfine coupling constants from the slope of $K$-$χ$ plots and compared to that of iron-arsenide superconductor.

cond-mat.str-el↗

arXiv2009

Magnetic and Superconducting Properties of Single Crystals of Fe$_{1+δ}$Te$_{1-x}$Se$_x$ System

The spin-fluctuation effect in the Se-substituted system Fe$_{1+δ}$Te$_{1-x}$Se$_x$ ($x$ = 0, 0.05, 0.12, 0.20, 0.28, 0.33, 0.45, 0.48 and 1.00; $0<δ< 0.12$) has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with $x$ = 0.05, 0.12, 0.20, 0.28, 0.33, 0.45 and 0.48 show superconducting transition temperatures in the ranger of 10 K$\sim$14 K. We obtained their intrinsic susceptibilities by the Honda-Owen method. A nearly linear-in-$T$ behavior in magnetic susceptibility of superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field $μ_0H_{c2}^{orb}$ for $T\to$ 0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki-Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the superconducting coherence length and the electron mean free path were also discussed. These superconducting parameters indicate that the superconductivity in the Fe$_{1+δ}$Te$_{1-x}$Se$_x$ system is unconventional.

cond-mat.supr-con↗

arXiv2009

Unconventional Superconductivity in the Novel Layered Superconductor Fe(Te-Se) Investigated by $^{125}$Te NMR on the Single Crystal

We have performed $^{125}$Te NMR on a single crystal of the novel layered superconductor Fe$_{1.04}$Te$_{0.67}$Se$_{0.33}$ for the first time. The spin parts of the Knight shifts for both $H // a$, $H // c$ are suppressed in the superconducting state, indicating the spin singlet superconductivity. At superconducting state, $1/T_{1}$ shows the power-law behavior ($\sim T^{3}$) without any coherent peaks. Observations of the residual density of state and the $T^{3}$-law indicate the presence of the line node in the superconducting gap. In the normal state, $1/T_{1}T$ which probes the \textbf{q}-summation of spin fluctuations enhances at low temperatures. Our results suggest that the superconductivity in Fe$_{1+δ}$Te$_{1-x}$Se$_{x}$ is stabilized by the growth of the antiferromagnetic spin fluctuations as well as in FeSe.

cond-mat.supr-con↗

arXiv2009

Synthesis, Structural and magnetic properties of the solid solution (CuCl1-xBrx)LaNb2O7 (0<x<1)

The two-dimensional (2D) quantum spin system (CuCl)LaNb2O7 has a spin-singlet ground state with a gap of 2.3 meV, while the isostructural material (CuBr)LaNb2O7 displays collinear antiferromagnetic order at TN = 32 K. Here we report on the synthesis of solid solution (CuCl1-xBrx)LaNb2O7 (0 < x < 1), and its structural and magnetic properties by means of magnetic susceptibility, high-field magnetization, and neutron diffraction studies. The x-dependence of cell parameters follows Vegards law, verifying the uniform distribution of Cl and Br atoms at the halide site, though a more complex structural evolution is inferred from an opposing correlation between the intra- and interlayer cell distances (vs. x). 5%-Br substitution is found to induce antiferromagnetic order with TN = 7 K, consistent with a recent mSR study, and the magnetic structure is collinear, having a significantly reduced moment. Further Br substitution leads to a linear increase in TN up to x = 1. These results indicate that (CuCl)LaNb2O7 is located in the vicinity of the quantum phase boundary.

cond-mat.mtrl-sci↗

arXiv2005

Novel phase diagram of superconductor NaxCoO2-yH2O in a 75 % relative humidity

We succeeded in synthesizing the powder samples of bilayer-hydrate sodium cobalt oxide superconductors NaxCoO2-yH2O with Tc = 0 ~ 4.6 K by systematically changing the keeping duration in a 75 % relative humidity atmosphere after intercalation of water molecules. From the magnetic measurements, we found that the one-day duration sample does not show any superconductivity down to 1.8 K, and that the samples kept for 2 ~ 7 days show superconductivity, in which Tc increases up to 4.6 K with increasing the duration. Tc and the superconducting volume fraction are almost invariant between 7 days and 1month duration. The 59Co NQR spectra indicate a systematic change in the local charge distribution on the CoO2 plane with change in duration.

cond-mat.supr-con↗

arXiv2005

$^{17}$O NMR Measurements on Superconducting Na$_{0.35}$CoO$_{2} \cdot y$H$_{2}$O

An $^{17}$O NMR measurement was performed on nonoriented polycrystalline Na$_{0.35}$CoO$_2 \cdot y$H$_2$O with superconducting (SC) transition temperature $T_\text{c} = 4.6$ K. A weak temperature dependence was observed in the Knight shift at the O site ($^{17}K$). The spin part of $^{17}K$ ($^{17}K_{\rm spin}$) is estimated from the plot of $^{17}K$ against bulk susceptibility $χ$. The $^{17}K_{\rm spin}$ decreases in the SC state, indicative of the decrease in the in-plane component of the spin susceptibility. The nuclear spin-lattice relaxation rate $1/T_1$ at the O site $^{17}(1/T_1)$ shows a good scaling with $1/T_1$ at the Co site $^{59}(1/T_1)$. This indicates that the spin fluctuations at the O site originate from the Co spin dynamics. The relationships between $^{17}(1/T_1T)$ and $^{17}K_{\rm spin}$ and between $^{17}(1/T_1T)$ and $^{59}(1/T_1T)$ show the development of incommensurate fluctuations at {\bf q $\sim$ 0} other than {\bf q $=$ 0} below 30 K. A clear indication of ferromagnetic correlations at {\bf q $=$ 0} was not observed from the present $^{17}$O-NMR studies.

cond-mat.supr-con↗