Yasukuni Matsumoto

119Sn-Mössbauer Spectroscopic Study of High-Tc Superconducting YBa2(Cu0.995Sn0.005)3O7-y and the Gamma-Ray Irradiation Effect

119Sn-Mossbauer measurement of the superconducting Y-Ba-Cu(Sn)-O system has been made at temperatures between 18 and 298 K. All the spectra show a superposition of an intensive singlet and a weak doublet peaks due to Sn4+ species located at different Cu(1) sites; the singlet is assigned to a square planar Cu(Sn)O4 and the doublet to a triangular Cu(Sn)O3. 60Co-γ ray (~108 R) irradiation results in an increased fraction of the Cu(Sn)O3 species. Isomer shift (δ) and absorption area (A) increase linearly with decreasing temperature and become constant at temperatures lower than 95-100 K, which is comparable to the onset temperature of superconducting transition (94.8 K). This finding indicates that softening of lattice vibration at the Cu(1) sites, followed by a strong electron-phonon coupling, is closely concerned with the superconductivity in Cu(2)O4 layer.

Softening of Cu(1)-site vibration matched with the onset temperature of superconducting YBa2(Cu0.995Sn0.005)3O6.60

The 119Sn-Mossbauer spectrum of superconducting YBa2(Cu0.995Sn0.005)3O6.60 ceramic, prepared by quenching at 815°C, consists of an intense singlet peak due to square planar Cu(Sn)O4 and a weak doublet due to triangular Cu(Sn)O3. The absorption area increases linearly with decreasing temperature and becomes constant below the onset temperature of superconducting transition (ca. 60 K). This indicates that the mean square displacement of the Cu(1)-site vibration becomes constant, as was observed below 95 K in the YBa2(Cu0.995Sn0.005)3O7-y system (y\fallingdotseq0.15). The decrease of Tc is ascribed to the displacement of O(4) atoms from the c-axis.

Softening of Cu(1)-site vibration at the onset temperature of high-Tc superconducting transition

119 Sn-Mossbauer spectroscopy is applied to the lattice dynamic study of superconducting YBa 2 (Cu 0·995 Sn 0·005 ) 3 O 7−y. . The absorption area vs temperature plot shows a softening of Cu(1)-site vibration at temperatures matched with the individual onset temperatures; the higher is the fraction of oxygen (O(1)) vacancies, the lower become the softening and onset temperatures.

Superconductivity and Crystal Structure of the Y1Ba2Cu3O7-y-WO3 System

The relationship between the superconductivity and crystal structure of the Y1Ba2Cu3O7-y-WO3 system has been studied. Tungsten oxide has various oxidized states, and mixed WO3 in YBCO easily donates its oxygen atoms to YBCO. It is found that the orthorhombic structure of YBCO is modified by mixing of WO3. The lattice constant a increases with increasing concentration of WO3, while the others, b and c, do not significantly change. Although the atomic arrangement in the Cu(2)-O plane of YBCO is consequently modified, superconducting transition temperature Tc is kept nearly constant.

Superconducting Character of Nb3Ge Single Crystals

The object of our study was the investigation of the superconducting transition curve of individual crystals in a Nb3Ge film. The growth direction of the A-15 crystals was (100). By using a local four-terminal measuring technique, we successfully recorded the superconducting transition curve of one individual crystal. Increasing the potential probe distance, we obtained a stepped transition curve. These experimental results show that our sample is not homogeneous and each individual crystal has its own superconducting transition temperature. An analysis of the chemical composition was done, too. Finally, the relation between Tc and the stoichiometry is discussed.

119Sn-Mössbauer spectroscopic study of oxygen deficient YBCO

Temperature dependence of 119 Sn-Wossbauer spectra has been measured in superconducting Y 1 Ba 2 (Cu 0.995 Sn 0.005 ) 3 O 7-y , y=0.51. The absorptiom area A(T) of the spectrum which is related to the Debye-Waller factor shows a tendency of quenching at temperatures below T c . There is an apparent correlation between the superconducting transition and the lattice vibration

Effects of WO3 mixing on the crystal structure and the superconductivity in YBCO

Effects of WO3 mixing on the crystal structure and the superconductivity in Y1Ba2Cu3O7-y have been studied. X-ray diffraction and the electrical resistance have been measured for the samples with several kinds of WO3 concentration. Experimental results of the X-ray diffraction patterns are refined precisely by the Rietvelt analysis. It has been elucidated that mixed WO3 supplies the oxygen atoms to the oxygen vacant sites along the a-axis of Y1Ba2Cu3O7-y.

Superconducting Transition Temperature of Single Crystal Nb3Ge

The electrical resistance of single crystal Nb3Ge was measured by using precise micromanipulating equipment. Samples were prepared on the sapphire substrate by the CVD method. The largest grain size of the sample was more than 10 µm and the growth direction of these crystals (100). The superconducting transitions were measured when the distance between two potential probes was changed from less than 10 µm to more than 2 mm. If this distance was 2.3 mm, the transition curve consisted of several structures. When we measured the superconducting transition of one crystal, its shape was very sharp and the transition width was less than 0.2 K. The shorter the distance between two potential microprobes, the narrower the superconducting transition width.