Shukichi Tanaka

Selective area growth of GaN microstructures on patterned (111) and (001) Si substrates

Selective MOVPE growth of GaN microstructure on silicon substrates has been investigated using SiO2 mask having circular or stripe window. In case of (0 0 1)substrate, grooves with (1 1 1) facets at the sides were made by using the etching anisotropy of a KOH solution. On the (1 1 1) facets of patterned silicon substrate (or on the as opened window region of (1 1 1) substrate), growth of wurtzite GaN was performed, of which the c-axis is oriented along the 〈1 1 1〉 axis of silicon. The photoluminescence and X-ray diffraction analysis were performed to characterize the single crystal to reveal the effect of the growth conditions of the intermediated layer and the microstructure.

Structural, electrical and magnetic studies of infinite-layered Sr1−xLaxCuO2 superconductor

Abstract Superconductivities were investigated in infinite-layered Sr 1− x La x CuO 2 prepared in a reductive atmosphere under high pressure. Its solid solution limit was estimated to be just above x =0.10. The T c onset values were in a range of 42–44 K independently of x and all samples attained the zero resistivities. The volume fraction of the superconductor showed a maximum at x =0.10. Their oxygen contents were analyzed to be not more than 2. The value of thermoelectric power was negative but its Hall coefficient had a positive value in a temperature range from room temperature to 77 K for the x =0.10 sample. These results indicate that electrons were doped into the conduction plane in the infinite-layered compound.

On the Anisotropy of the Superconducting Order Parameter of Cu Oxides Observed by STM/STS

The anisotropic nature of differential tunneling conductance of high- T c oxides has been studied by Scanning Tunneling Microscopy/Spectroscopy (STM/STS) with the tip approach along the [100] and [110] directions of single crystals of YBa 2 Cu 3 O 7 and La 2- x Sr x CuO 4 ( x =0.10 and 0.15). Although the observed gap is larger in the [110] direction than in the [100] direction, trial calculations of the k-dependent tunneling current which consider the shape of the electron wave functions of surface oxygens suggest that the data can be understood on the basis of the ( x 2 - y 2 )-like symmetry rather than the x y -like one. Zero-bias conductance peaks have been observed along the [100] direction.

Anomalous thermoelectric power of Bi1.6Pb0.5Sr1.9-yLayCu1.05Oz

Abstract Thermoelectric power S of Bi1.6Pb0.5Sr1.9−yLayCu1.05Oz have been studied, where as a result of the La doping, the carrier concentration decreases and the system changes from normal metallic state for y∼0 to semiconducting one for y≳0.8 through superconducting phase. S of semiconducting samples is positive and large as in the case of similar samples of other Cu-oxides. A new surprising result is that the samples with y∼-0 exhibit rather large negative values of S at high temperatures, which has not been observed in other Cu-oxides in the nonsuperconducting metallic region with an overdoped carrier concentration. For the metallic samples, S is roughly divided into two contributions: One is linear in temperature T and has a negative slope and another is nearly constant above 100∼150K and decreases with decreasing T in low temperature region. The latter term suggests an existence of unusual scattering of carriers of Cu-oxygen.

STM/STS studies on YBa2Cu3Oy single crystals

Abstract STM/STS studies have been carried out on the (110) and (100) surfaces of YBa 2 Cu 3 O y single crystals with a superconducting transition temperature T c of about 90 K. In most cases on the (110) surface, the spectra of the quasi-particle excitations in the superconducting state exhibit characteristics of the “twin gap” structure, where the “inner gap” is much smaller than the BCS value of 3.52 k B T c and seems to be important in discussing the temperature dependence of various quantities of the compound at low temperatures. Anisotropy of the spectra has also been observed, suggesting the anisotropic nature of the superconducting order parameter.

Variation of Physical Properties of Bi1.6Pb0.5Sr1.9- yLa yCu1.05O z with the La-Concentration y across the Superconducting-Nonsuperconducting Metal Boundary

Experimental results on the variation of various physical properties in Bi 1.6 Pb 0.5 Sr 1.9- y La y Cu 1.05 O z with varying y through the superconducting and nonsuperconducting metallic phase boundary are presented. The magnetic susceptibility at fixed temperatures is monotonous functions of y . The Hall coefficient does not indicate even a weak tendency of the sign change at the boundary. The thermoelectric power has a systematic change with y and exhibits quite unusual behavior even in the nonsuperconducting metallic phase.

Single crystal growth of BaCo1−xNixS2

Abstract Crystal growth of the BaCo1−xNixS2 system, which has a layered structure and exhibits a metal-insulator transition with varying x, has been carried out. The maximum volume of the ingot-like crystals obtained through peritectic reaction is as large as 1 cm3, on which preliminary neutron inelastic measurements have been carried out.

What factors determine STM/STS characteristics of high-Tc oxides?

Abstract Results of STM/STS studies on single crystals of La2−xSrxCuO4 are presented. For analyses of STM/STS data of high-Tc oxides, a phenomenological model is proposed, where the contribution of electrons with a wave vector k to the tunneling current is mainly determined by the k-dependent “form factor” of the pσ orbitals of surface oxygens in the antibonding band forme ly bythe Cu 3dx2−y2 and O 2pσ orbitals. Various characteristics of STM/STS data are shown to be consistent with the model.

On the Magnetic Properties of Systems with Low Dimensional Linkage of VO5 Pyramids.

[N(CH 3 ) 4 ]V 3 O 7 , (en) 2 ZnV 6 O 14 (en=ethylenediamine) and (C 6 H 14 N 2 )V 6 O 14 ·H 2 O with edge-sharing VO 5 chains have been prepared hydrothermally and studied by magnetic susceptibility measurements and by NMR. Observed magnetic properties, which indicate the existence of the spin gap, can be explained by a model of isolated spin dimers of V 4+ ions, where a very large exchange interaction J exists only between the electrons at V sites within two edge-sharing VO 5 pyramids whose apical oxygens are on the same side of the basal planes. It can be understood by considering the kinetic exchange mechanism acting between the electrons in the x y orbitals.

Symmetry of the Superconducting Order Parameter of High-T c Oxides and Consideration of k-Dependent Tunneling in STM/STS

By means of Scanning Tunneling Microscopy/Spectroscopy (STM/STS), the superconducting gap structure of high- T c oxides has been studied with the tip approach along the directions within CuO 2 planes. Observed gap anisotropy is, at first sight, x y -like. However, the result can be consistently understood by assuming ( x 2 - y 2 )-like symmetry and by introducing a model which calculates k-dependence of the tunneling probability of electrons in the antibonding band formed by Cu 3 d x 2 - y 2 and O2p σ orbitals. The conclusion is found to be insensitive to various parameters used in the calculations.