Hajime Ozaki

The time-dependent process of oxidation of the surface of Bi2Te3 studied by x-ray photoelectron spectroscopy

The process of oxidation of the Bi2Te3 surface was investigated by x-ray photoelectron spectroscopy (XPS). The oxidized surface layer was found to have a definite thickness, with configurations where O is bonded with Bi and Te, and Bi and Te are bonded with three and four oxygens, respectively. The oxidation time dependence of the oxidized layer thickness d(t) estimated from the XPS behaved as (t-t0)1/2 when d(t) was smaller than the thickness of a single oxidized quintuple atomic layer in our oxide model and behaved as t-t1 when it was larger than that. Experimental data were compared to our oxidation process model for the layered structure with the van der Waals gap and very good agreement was found.

Experimental studies on the electronic structure of Pb1-xEuxTe

P- and n-type Pb1-xEuxTe (0≤x≤0.25) single-crystal films were prepared on BaF2 (111) substrates using the hot-wall epitaxy technique. The lattice constant increases nonlinearly with the composition ratio x, and the energy gap increases rapidly up to x=0.07, while its rate of increase decelerates for x>0.07. X-ray photoelectron spectroscopy measurements revealed that the localized Eu 4f level centers around 1.2-1.5 eV below the valence band maximum. It is seen from the transport measurements that all of the n-type films are metallic but for the p-type films a metal-to-insulator transition takes place at x=0.07. Using these results, the electronic structure of Pb1-xEuxTe is proposed in which the Eu 4f localized electron plays an important role in the electronic properties for intermediate composition ratio (x>0.07).

X-ray photoelectron spectroscopy study in Bi2Sr2Ca1-xNdxCu2O8+d

X-ray photoelectron spectroscopy (XPS) measurements were performed in Bi2Sr2Ca1-xNdxCu2O8+d (0≤x≤1) in order to investigate the change in the electronic states through the decrease of the holes in this system. The negative chemical shifts of the Cu 2p main peaks were observed, while their satellites did not shift but decreased in areas. These behaviors indicated that the holes at both Cu and O sites decrease with increasing Nd concentration x.

Single crystal growth of (LaO)CuS

Abstract Single crystals of (LaO)CuS with size 100×100×5 μm3 have been prepared by annealing of small crystals and polycrystalline powders of (LaO)CuS in iodine atmosphere at 1373 K. The iodine pressure at this temperature is about 5 atm. The present study indicates that there are optimum values both of iodine pressure and annealing temperature.

EFFECT OF NI SUBSTITUTION FOR CU IN 110 K PHASE OF (BI, PB)-SR-CA-CU-O SUPERCONDUCTORS

The effect of Ni substitution for Cu in the 110 K phase of (Bi, Pb)-Sr-Ca-Cu-O superconductor was investigated. Samples of Bi1.84Pb0.34Sr1.91Ca2.03(Cu1-xNix)3.06Oy were prepared. The single phase was obtained up to a Ni concentration of x=0.015. The zero-resistance temperature Tc decreased monotonically in the region of x≤0.03 and became 90 K at x=0.03. Tc remained constant at 89 K in the region of 0.03<x≤0.10. Through X-ray photoelectron spectroscopy measurements, the Cu 2p core level was found to shift toward lower binding energy with the increase of x, which shows the Ni substitution for Cu.

Superconducting Bi-Sr-Ca-Cu-O films prepared by the liquid phase epitaxial method

Abstract Bi-Sr-Ca-Cu-O superconducting films were prepared on MgO(1 0 0) single crystal substrates by the liquid phase epitaxial (LPE) method using Bi 2 Sr 2 CaCu 2 O x solution. X-ray diffraction patterns and scanning electron microscopy analysis showed the films were predominantly oriented with the c -axis perpendicular to the substrate surfaces. Two types of the growth temperature profiles, type A and B, were employed for film preparation by the LPE method. As-grown films in type A consisted of single 10 K (2201) phase. After a post-annealing, films predominantly consisted of the 80 K (2212) phase with T c ( R = 0) = 78 K. As-grown films in type B consisted of single 80 K (2212) phase. But the films did not show a superconducting transition. The films showed a superconductivity with the highest T c ( R = O ) = 82 K obtained by the post-annealing at 860°C for 5 h. We investigated the formation condition of the 80 K phase by controlling the temperature of the annealing process.

Effect of W doping in metal-insulator transition material VO2 by tunnelling spectroscopy

We used tunnelling spectroscopy to study heavily W doped VO2 crystal in the temperature region across the phase transition. A double pseudo-energy-gap structure was observed across the Fermi level at temperatures below the phase transition. When the temperature was increased across the phase transition, the outer gap structure disappeared and an inner gap structure of about 0.4 eV became sharp. The tunnelling results explained the qualitative changes in the heavily doped sample from lightly doped one in the behaviour of the temperature dependence of electrical resistivity near the metal–insulator transition.

Effect of Y substitution for the Ca site in the 110 K phase of (Bi, Pb)-Sr-Ca-Cu-O superconductors

The effect of the substitution of the trivalent Y ions for the divalent Ca ions has been studied in the 110 K phase of Bi1.84Pb0.34Sr1.91(Ca1-xYx)2.03Cu3.06O10+y (x=0~0.05). We found the solubility limit of Y to the 110 K phase around x=0.015. Within the solubility limit, the c-axis seemed to decrease with increasing x. In the region of the 110 K single phase, the critical temperature Tc gradually decreased with the increase of x, corresponding to the decrease of the carrier concentration.

Effects of Hf and W doping on the commensurate charge density waves in 1T-TaS2

Abstract The electrical resistivities of Hf-doped and W-doped 1T-TaS2 have been measured to investigate the influence of varying the Fermi level on the formation of commensurate charge density waves. It was found that W-doping is far more effective in breaking the C-CDW phase than Hf-doping. Such results are explained by the energy consideration of the system with the Hubbard gap which is produced by the Mott-localization.

Effect of Y Substitution for Ca in Bi2Sr2CaCu2O8+d Superconducting Films Prepared by Liquid Phase Epitaxial Method

The effect of Y substitution for Ca on superconducting properties has been investigated in Bi2Sr2CaCu2O8+d films prepared by liquid phase epitaxial method. The Y concentration in the film, xS, increased in proportion to that in the solution, xL, in the region of 0≤xL≤0.2, and then it was saturated. The incorporation ratio of Y in solid and in liquid, xS/xL, for 0≤xL≤0.2 was found to be about 3. The xS dependences of the zero-resistance temperature and the carrier concentration showed that about half of the Y concentration incorporated in the films contributes to the effect of the Y substitution obtained in sintered materials.