Kazuhiko Yamaya

Peierls transition in TaS3

Abstract Transition-metal trichalcogenide, TaS 3 , with an orthorhombic structure was found to undergo a metal-semiconductor transition at 270 K. Electron diffraction reveals diffuse scattering above room temperature and superlattice spots at a lower temperature. These phenomena are interpreted in terms of Peierls transition.

Crystal topology: A Möbius strip of single crystals

A Möbius strip is produced by twisting a ribbon of material through 180° and joining its two ends, resulting in a distinct, one-sided topology. Here we describe a Möbius structure formed by crystals of a compound of niobium and selenium, NbSe3. It is surprising that a crystalline ribbon should adopt this exotic topology in view of its inherent rigidity, which would be expected to prevent it from either bending or twisting.

Low-temperature electronic specific heat of La2−xSrxCuO4 and La2−xSrxCu1−yZnyO4. Evidence for a d wave superconductor

Abstract The electronic specific heat C el was studied at T ⩽ 10 K both on La 2− x Sr x CuO 4 samples and on La 2− x Sr x Cu 1− y O 4 samples below and above y c at which the superconductivity disappears. In superconducting La 2− y Sr x CuO 4 samples, the C el contains only a T 2 component at T « T c in a x range between 0.14 and 0.22, suggesting a gapless superconductor with line nodes. A T linear component of C el appears in superconducting La 2− x Sr x Cu 1− y Zn y O 4 ( y y c samples, and its coefficient, γ, increases markedly with Zn content. In samples with various Zn contents, the relation between T c / T c ( y = 0) and γ/γ( y ≥ y c ) can be understood in terms of strong impurity scattering in a d wave superconductor.

Non-ohmic conductivity of TaS3 in the low-temperature semiconducting regime

Abstract Electrical conductivity of the quasi one-dimensional conductor TaS3 was measured in the low-temperature semiconducting regime. Below 100K, the conductivity along c-axis (l-d axis) in the low field limit is characterized by the activated process with the activation energy of 250K, and the current-field (I-E) characteristic is nonlinear, dI dE increasing with the field strength. These results can be explained by the excitation of phase solitons associated with pinned CDW condensates.

Crystallinity of a‐axis oriented YBa2Cu3O7−δ thin film epitaxially grown on NdGaO3 (110) by 95 MHz magnetron sputtering

An a‐axis oriented 400 A YBa2Cu3O7−δ film has been epitaxially deposited on a NdGaO3 (110) substrate by rf magnetron sputtering using a single YBa2Cu5Ox target. An excitation frequency of 94.92 MHz, seven times as high as the conventionally used 13.56 MHz, results in a lower self‐bias voltage which reduces degradation of films caused by resputtering due to negatively charged oxygen. Sharp streaks corresponding to the c‐axis lattice parameter of YBa2Cu3O7−δ have been observed by reflection high‐energy electron diffraction, showing that the c‐axis is parallel to the surface of the NdGaO3 substrate and the film surface is smooth on an atomic scale. The crystallinity has been characterized by Rutherford backscattering channeling analysis. A minimum yield, χmin of 3.2%, has confirmed excellent crystallinity of the a‐axis oriented film.

Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu3Oy

Abstract Measurements of superconducting transition temperature T c and lattice parameters of CaLaBaCu 3 O y (CLBCO) indicate that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y =6.7 to 7. The T c decreases monotonously with decreasing oxygen content and no superconducting transition is observed below y =6.7. It is concluded that disappearance of the superconductivity for y y

Superconductivity in One-Dimensional TaSe3

Monoclinic TaSe 3 was found to be superconducting below 2.1 K. The effect of magnetic field on the transition curve is very anisotropic. One-dimensional behavior is expected.

Pressure dependence of superconducting transition temperature and of hall constant in high-Tc Cu-oxide superconductors

Abstract The pressure derivative of T c in the hole-doped superconductors having relatively lower carrier concentration is shown to change the sign from positive to negative at a certain pressure with a maximum of T c , while it is negative in the highly-doped ones, which is confirmed as a general feature in these compounds. The Hall constants of the samples YBa 2 Cu 3 O 7 , CaBaLaCu 3 O 7−y , YBa 2 Cu 4 O 8 , Bi 2 Sr 2 CaCu 2 O 8 and Tl 2 Ba 2 CuO 6+y is found to increase by about 10%/GPa which is almost the same amount that estimated by Yamada et al. from the Madelung energy calculation for YBa 2 Cu 4 O 8 .

Pressure effect on the low temperature tetragonal phase and the superconductivity in (La1−xBax)2CuO4

Abstract The resistivity of (La 1−x Ba x ) 2 CuO 4 was measured under pressure up to 20 kbar. The resistive increment at low temperatures, associated with the structural phase transition to low temperature tetragonal phase, in x=0.06 was suppressed completely under pressures above 10 kbar. Dip of the superconducting transition temperature around x=0.06 is not observable under high pressures where the resistive increment at low temperatures disappears. The results give an evidence that the low temperature structural phase transition does suppress the superconductivity.

Highest crystallinity of a-axis YBCO films by DC-94.92 MHz hybrid plasma magnetron sputtering

Abstract The resputtering effects and the effect of oxygen partial pressure on the compositional deviation of films from the target in the process of a newly developed DC-94.92 MHz hybrid plasma magnetron sputtering have been investigated. The YBCO films have a -axis orientation. A full width at half maximum (FWHM) value of 0.027° in the rocking curve through the (200) peak and χ min of 2.0% and 3.8% in the Rutherford backscattering channeling measurements were obtained, respectively, for the films on SrTiO 3 and NdGaO 3 substrates. The surfaces of the films were also very smooth.