arXiv subjects

Y. Takano

Publications and source records attributed to Y. Takano.

At least 73 records · Page 4Linked to original sources

arXiv2009

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T_c) increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa. The maximum T_c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using 77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T_1T just above T_c shows a plateau as well as the pressure dependence of T_c, which gives clear evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2009

Pressure evolution of low-temperature crystal structure and bonding of 37 K $T_c$ FeSe superconductor

FeSe with the PbO structure is a key member of the family of new high-$T_c$ iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe$_4$ tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases $T_c$ which attains a broad maximum of 37 K at $\sim$7 GPa (this is one of the highest $T_c$ ever reported for a binary solid) before decreasing to 6 K upon further compression to $\sim$14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $α$-FeSe, revealing an extremely soft solid with a bulk modulus, $K_0$ = 30.7(1.1) GPa and strong bonding anisotropy between inter- and intra-layer directions that transforms to the more densely packed $β$-polymorph above $\sim$9 GPa. The non-monotonic $T_c$($P$) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties.

cond-mat.supr-con↗

arXiv2008

Superconductivity in S-substituted FeTe

We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.

cond-mat.supr-con↗

arXiv2008

Growth of superconducting single-crystalline (Lu, Ca)Ba2Cu3O7-d whiskers

Single-crystalline (Lu, Ca)Ba2Cu3O7-d (Lu(Ca)123) whiskers have been successfully grown using the Te-doping method. X-ray diffraction patterns of Lu(Ca)123 whiskers showed sharp (0 0 l) peaks corresponding to REBa2Cu3O7-d phase (RE = rare earth elements). Transport measurements showed that the superconducting transition occurred at 83 K in the obtained whiskers.

cond-mat.supr-con↗

arXiv2008

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.

cond-mat.supr-con↗

arXiv2008

Electrical properties of boron-doped MWNTs synthesized by hot-filament chemical vapor deposition

We have synthesized a large amount of boron-doped multiwalled carbon nanotubes (MWNTs) by hot-filament chemical vapor deposition. The synthesis was carried out in a flask using a methanol solution of boric acid as a source material. The scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy were performed to evaluate the structural properties of the obtained MWNTs. In order to evaluate the electrical properties, temperature dependence of resistivity was measured in an individual MWNTs with four metal electrodes. The Ramman shifts suggest carrier injection into the boron-doped MWNTs, but the resistivity of the MWNTs was high and increased strongly with decreasing temperature. Defects induced by the plasma may cause this enhanced resistivity.

cond-mat.mtrl-sci↗

arXiv2008

Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study

We report the results of $^{77}$Se--nuclear magnetic resonance (NMR) in $α$-FeSe, which exhibits a similar crystal structure to the LaFeAsO$_{1-x}$F$_x$ superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate $1/T_1$ shows $T^3$ behavior below the superconducting transition temperature $T_c$ without a coherence peak. The $T_1T=$ const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above $T_c$. The superconductivity in $α$-FeSe is also an unconventional one as well as LaFeAsO$_{1-x}$F$_x$ and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.

cond-mat.supr-con↗

arXiv2008

Nanoscale freezing of the 2D spin liquid Pr$_{3}$Ga$_{5}$SiO$_{14}$

In this letter, we report on the single crystal growth and physical characterization of the distorted kagomé system Pr$_3$Ga$_5$SiO$_{14}$. It is found that at zero magnetic field the system shows no magnetic order down to 0.035 K and exhibits a $T^{2}$ behavior for the specific heat at low temperatures, indicative of a gapless 2D spin liquid state. Application of an applied field induces nanoscale islands of ordered spins, with a concomitant reduction of the $T^{2}$ specific heat term. This state could be a possible ferro-spin nematic ordering stabilized out of an unusual spin liquid state.

cond-mat.str-el↗

arXiv2008

Crystal structure of the new FeSe1-x superconductor

The newly discovered superconductor FeSe1-x (x=0.08, Tconset=13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure - the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO1-xFx (RE = rare earth) superconductors

cond-mat.supr-con↗

arXiv2007

Thermodynamics of the Up-Up-Down Phase of the S = 1/2 Triangular-Lattice Antiferromagnet Cs$_2$CuBr$_4$

Specific heat and the magnetocaloric effect are used to probe the field-induced up-up-down phase of Cs2CuBr4, a quasi-two-dimensional spin-1/2 triangular antiferromagnet with near-maximal frustration. The shape of the magnetic phase diagram shows that the phase is stabilized by quantum fluctuations, not by thermal fluctuations as in the corresponding phase of classical spins. The magnon gaps determined from the specific heat are considerably larger than those expected for a Heisenberg antiferromagnet, probably due to the presence of a small Dzyaloshinskii-Moriya interaction.

cond-mat.str-el↗

arXiv2007

Switching Current Measurements of EuBa2Cu3Oy Intrinsic Josephson Junctions

We investigated the switching dynamics of two kinds of EuBa2Cu3Oy (Eu123) intrinsic Josephson junctions (IJJs) with different anisotropy parameters gamma (= (m*c/m*ab)1/2) tuned to 37 and 22. In contrast to weakly-coupled IJJs in Bi2212, significant deviations from the thermally activated escape model of a single-stack Josephson junction were observed in their switching current distributions, P(I), due to the strongly-coupled nature of the Eu123 IJJs, The P(I) of the two IJJs are found to be independent of temperature, below 1.4 K and 4.2 K, respectively; which indicates the observation of macroscopic quantum tunneling at high temperatures of liquid He.

cond-mat.supr-con↗

arXiv2007

Phonon Softening in Superconducting Diamond

We observe strong softening of optical phonon modes in superconducting (Tc = 4.2 K) boron doped diamond near the Brillouin zone center using inelastic x-ray scattering from a CVD-grown highly oriented sample. The magnitude of the softening, and our observation that it becomes stronger approaching zone center, supports theoretical models suggesting a phonon-mediated pairing mechanism via coupling of optical phonon modes to Fermi surfaces around the zone center. The electron-phonon coupling parameter is determined as approximately lambda= 0.33.

cond-mat.supr-con↗

arXiv2007

Spin Dynamics of the Spin-1/2 Kagome Lattice Antiferromagnet ZnCu_3(OH)_6Cl_2

We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagome lattice antiferromagnet Zn Cu_3 (OH)_6 Cl_2. The susceptibility indicates a Curie-Weiss temperature of ~ -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law with exponent less than or equal to 1. These results suggest that an unusual spin-liquid state with essentially gapless excitations is realized in this kagome lattice system.

cond-mat.str-el↗

arXiv2006

Microscopic Evidence for Evolution of Superconductivity by Effective Carrier Doping in Boron-doped Diamond:11B-NMR study

We have investigated the superconductivity discovered in boron (B)-doped diamonds by means of 11B-NMR on heteroepitaxially grown (111) and (100) films. 11B-NMR spectra for all of the films are identified to arise from the substitutional B(1) site as single occupation and lower symmetric B(2) site substituted as boron+hydrogen(B+H) complex, respectively. A clear evidence is presented that the effective carriers introduced by B(1) substitution are responsible for the superconductivity, whereas the charge neutral B(2) sites does not offer the carriers effectively. The result is also corroborated by the density of states deduced by 1/T1T measurement, indicating that the evolution of superconductivity is driven by the effective carrier introduced by substitution at B(1) site.

cond-mat.supr-con↗

arXiv2006

Low-Energy Charge-Density Excitations in MgB$_{2}$: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

A sharp feature in the charge-density excitation spectra of single-crystal MgB$_{2}$, displaying a remarkable cosine-like, periodic energy dispersion with momentum transfer ($q$) along the $c^{*}$-axis, has been observed for the first time by high-resolution non-resonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-$q$ collective mode residing in the single-particle excitation gap of the B $π$ bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB$_{2}$.

cond-mat.str-el↗

arXiv2006

Periodic Oscillations of Josephson-Vortex Flow Resistance in Oxygen-Deficient Y1Ba2Cu3Ox

We measured the Josephson vortex flow resistance as a function of magnetic field applied parallel to the ab-planes using annealed Y1Ba2Cu3Ox intrinsic Josephson junctions having high anisotropy (around 40) by oxygen content reduction. Periodic oscillations were observed in magnetic fields above 45-58 kOe, corresponding to dense-dilute boundary for Josephson vortex lattice. The observed period of oscillations, agrees well with the increase of one fluxon per two junctions ($H_{p}$\textit{=$Φ$}$_{0}$\textit{/2Ls}), may correspond to formation of a triangular lattice of Josephson vortices as has been reported by Ooi et al. for highly anisotropic (larger than 200) Bi-2212 intrinsic Josephson junctions.

cond-mat.supr-con↗

arXiv2006

Observation of superconducting gap in boron-doped diamond by laser-excited photoemission spectroscopy

We have investigated the low energy electronic state of a boron-doped diamond thin film by the ultrahigh resolution laser-excited photoemission spectroscopy. We observed a clear shift of the leading edge below 11 K indicative of a superconducting gap opening ($Δ\sim 0.78$ meV at 4.5 K). The gap feature is significantly broad and the well-defined quasiparticle peak is not recognizable even at the lowest temperature of measurement 4.5 K. We discuss our result in terms of possible disorder effect on superconductivity in this system.

cond-mat.supr-con↗

arXiv2006

Low-energy electrodynamics of superconducting diamond

Heavily-boron-doped diamond films become superconducting with critical temperatures $T_c$ well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm$^{-1}$, by also using Coherent Synchrotron Radiation. We thus determine the optical gap, the field penetration depth, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function. We conclude that diamond behaves as a dirty BCS superconductor.

cond-mat.supr-con↗