arXiv subjects

T. Masui

Publications and source records attributed to T. Masui.

28 records · Page 2Linked to original sources

arXiv2003

Enhancement of the irreversibility field by carbon substitution in single crystal MgB$_2$

We report the detailed study of the irreversibility field {\hirr} of single crystals of Mg(B$_{1-x}$C$_{x}$)$_2$ ($x$=0, 2, 3.5, 5 and 7%) in strong magnetic fields of up to 40 T. High sensitive torque measurements revealed that the {\hirr} is greatly enhanced by a factor of two ($μ_{0}${\hirr}(0)$\simeq$33 T in $x$=5%) by carbon substitution compared to the pristine {\mgb} owing to a reduction in the inplane coherence length. We also observed the temperature-dependent {\hirr} anisotropy in all the carbon contents. This fact strongly suggests that the two-gap superconductivity of {\mgb} is less influenced by a small amount of carbon substitution. We discuss these results focusing on the impact of carrier doping and impurity scattering on two-gap superconductor.

cond-mat.supr-con↗

arXiv2003

Oxygen Phonon Branches in YBa2Cu3O7

We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa2Cu3O6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying the in-plane modes involving out-of-phase motion for the two Cu-O layers within a unit cell as well as those with in-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axis-polarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to T = 10 K at the wave vector q = 0.27 r.l.u..

cond-mat.str-el↗

arXiv2003

Evidence for Dynamic Charge Stripes in the Phonons of Optimally Doped YBCO

Inelastic neutron scattering investigations on optimally doped YBCO revealed a very pronounced temperature dependence in the Cu-O in-plane bond-stretching vibrations along the (010) direction: a shift of spectral weight by at least 10 meV has been observed in a narrow range of wave vectors halfway to the zone boundary. The temperature evolution starts at about 200 K, well above the superconducting transition temperature. The displacement pattern of the anomalous phonons indicates a dynamic one-dimensional charge-density modulation within the planes, but excludes a direct contribution from the Cu-O chains.

cond-mat.str-el↗

arXiv2003

Raman study of carrier-overdoping effects on the gap in high-Tc superconducting cuprates

Raman scattering in the heavily overdoped (Y,Ca)Ba_2Cu_3O_{7-d} (T_c = 65 K) and Bi_2Sr_2CaCu_2O_{8+d} (T_c = 55 K) crystals has been investigated. For the both crystals, the electronic pair-breaking peaks in the A_{1g} and B_{1g} polarizations were largely shifted to the low energies close to a half of 2Delta_0, Delta_0 being the maximum gap. It strongly suggests s-wave mixing into the d-wave superconducting order parameter and the consequent manifestation of the Coulomb screening effect in the B_{1g}-channel. Gradual mixing of s-wave component with overdoping is not due to the change of crystal structure symmetry but a generic feature in all high-T_c superconducting cuprates.

cond-mat.supr-con↗

arXiv2002

Origin of superconductivity transition broadening in MgB2

We report resistivity and magnetization of single crystal MgB2, focusing on the broadening of superconducting (SC) transition in magnetic fields. In-plane and out-of-plane resistivity indicate that the broadening of superconducting transition is independent of Lorentz force and that it is merely dependent on the magnetic field direction. In magnetization, diamagnetic signal begins to appear at almost the same temperature as the onset temperature of resistivity transition. These results suggest that the broadening is attributed not to the surface superconductivity but to the superconducting fluctuation or the vortex-liquid picture, owing to the short coherence length and the high transition temperature of MgB2.

cond-mat.supr-con↗

arXiv2002

Oxygen Phonon Branches in Detwinned YBa2Cu3O7

We report results of inelastic neutron scattering measurements of phonon dispersions on a detwinned sample of YBaCu3O7 and compare them with model calculations. Plane oxygen bond stretching phonon branches disperse steeply downwards from the zone center in both the a and the b direction indicating a strong electron-phonon coupling. Half way to the zone boundary, the phonon peaks become ill-defined but we see no need to invoke unit cell doubling or charge stripe formation: lattice dynamical shell model calculations predict such behavior as a result of branch anticrossings. There were no observable superconductivity-related temperature effects on selected plane oxygen bond stretching modes measured on a twinned sample.

cond-mat.supr-con↗

arXiv2002

High-pressure synthesis of superconducting Nb{1-x}B2 (x = 0-0.48) with the maximum Tc = 9.2 K

Superconductivity with Tc above 9 K was found in metal-deficient NbB2 prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB2. The superconductivity was observed above x = 0.04 in Nb1-xB2. and the lattice parameters also changed abruptly at x = 0.04. As x increased, the transition temperature Tc slightly rose and fell with the maximum value of 9.2 K at x = 0.24 for the samples sintered at 5 GPa and 1200 C. The Tc-value changed in the range from 7 K to 9 K, depending on the sintering pressure. A series of Ta1-xB2 (0 =< x =< 0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.

cond-mat.supr-con↗

arXiv2002

Crystal growth and characterization of MgB2: Relation between structure and superconducting properties

We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in Mg-B-N system, including the optimisation of P-T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single crystals morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defects concentration, which revealed and possible effects of Mg-deficiency and lattice strain on the superconducting properties of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).

cond-mat.supr-con↗

arXiv2002

Anisotropic resistivity and Hall effect in MgB2 single crystals

We report resistivity and the Hall effect measurements in the normal and superconducting states of MgB2 single crystal. The resistivity has been found to be anisotropic with slightly temperature dependent resistivity ratio of about 3.5. The Hall constant, with a magnetic field parallel to the Mg and B sheets is negative in contrast to the hole-like Hall response with a field directed along the c-axis indicating presence of both types of charge carriers and, thus, multi-band electronic structure of MgB2. The Hall effect in the mixed state shows no sign change anomaly reproducing the Hall effect behavior in clean limit type-II superconductors.

cond-mat.supr-con↗

arXiv2001

Growth, structure analysis and anisotropic superconducting properties of MgB2 single crystals

Here we report the growth of sub-millimeter MgB2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a single-crystal X-ray diffraction analysis gives lattice parameters a=3.0851(5) A and c=3.5201(5) A with small reliability factors (Rw =0.025, R=0.018), which enables us to analyze the Fourier and Fourier difference maps. The maps clearly show the B sp2 orbitals and covalency of the B-B bonds. The sharp superconducting transitions at Tc =38.1-38.3K were obtained in both magnetization (DTc =0.6K) and resistivity (DTc <0.3K) measurements. Resistivity measurements with magnetic fields applied parallel and perpendicular to the Mg and B sheets reveal the anisotropic nature of this compound, with upper critical field anisotropy ratio of about 2.7.

cond-mat.supr-con↗