Ryoji Sugise

Sputter Synthesis of Ba2YCu3Oy As-Deposited Superconducting Thin Films from Stoichiometric Target–A Mechanism of Compositional Deviation and Its Control

A mechanism of compositional deviation of Ba-Y-Cu-O films during sputtering has been investigated, and a process for obtaining as-deposited superconducting films from a stoichiometric target has been developed. The main factor of the deviation is the radiation of high-energy electrons emitted from the oxide target with a low work function onto the growing surface. By eliminating the electron bombardment, films with the same composition were obtained reproducibly. As-grown films 4000 A thick deposited at 580°C showed Tc ( ρ=0) of 82.5 K and an excellent surface smoothness.

Possibility of Superconductivity at 65°C in Sr-Ba-Y-Cu-O System

Transition to a zero-resistance state at 65°C (338 K) was observd in a Sr-Ba-Y-Cu-O system for the first time. A nonlinear I-V characteristic curve was observed at that temperature. The resistivity changed from 5×10-4 Ωcm to below a measurable limit of 1×10-8 Ωcm between 71 and 65°C. The zero-resistance state reminiscent of superconductivity was continued for ten days at room temperature in air. A magnetization measurement indicated a very small volume fraction of the room temperature superconducting state, if any. X-ray diffractogram showed almost a single phase of oxygen-deficient perovskite with a trace of unidentified phase. Photoemission measurement showed the coexistence of multivalent Cu ions and a relatively low density of state at Fermi level. The superconductivity can be interpreted by the previously proposed d-hole mechanism.

The formation process of a new high-Tc superconductor, TlBa2Ca4Cu5Oy, with five CuO layers

Abstract The formation process of a new high-Tc superconductor with five CuO layers and a single TlO layer, TlBa2Ca4Cu5Oy (1245), was analysed by the powder X-ray diffraction method. The phase transformation from TlBa2Ca3Cu4Oy (1234) phase to the 1245 phase was observed with an evaporation of the Tl component. The two kinds of steps for the phase transformations were observed. The control of thallium content was essential to prepare the 1245 phase.

Preparation and Formation Process of TlBa2Can-1CunOy

The Tl–Ba–Ca–Cu–O compounds, TlBa2Can-1CunOy (n = 1–5), with a single Tl–O layer and multiple Cu–O layers per unit cell were prepared. The formation process of these phases was studied. The numbers of Cu–O and Tl–O layers were influenced by the thallium content. Control of the thallium content was essential to prepare the desired phases in the Tl–Ba–Ca–Cu–O system. The value of Tc increased with increasing number of Cu–O layers (1≤n ≤4). At n=5 Tc was lower than that at n =4. Therefore, the highest Tc value was obtained at n=4.

The Reversible Reactions in Tl-Ba-Ca-Cu-O System

The reversible reactions in the Tl-Ba-Ca-Cu-O system were found. The phase transformations from the Tl2Ba2CaCu2Oy (2212) phase to the Tl2Ba2Ca2Cu3Oy (2223) phase and to the Ba-Ca-Cu-O mixed phases by heating in an oxygen atmosphere were observed. Moreover, the phase transformations from the Ba-Ca-Cu-O mixed phases to the 2223 and to the 2212 phases by heating in the presence of thallium oxide vapor were observed. Therefore, these phase transformations are reversible. These reversible reactions are very useful in stabilizing the structure of each superconductor.

Orientation of CaCuO2 Thin Films in RF Sputtering

The orientation of CaCuO2 films was examined under various RF sputtering conditions. Two types of crystal growth, (001) and (110) orientation, were controlled by varying the partial pressure of O2 and Ar gas. (001) orientation was observed at lower gas pressure and (110) orientation was observed at higher gas pressure. This suggests that the orientation of CaCuO2 film is related to the total pressure of the sputtering gas.

Preparation of Tl2Ba2Ca2Cu3Oy thick films from Ba−Ca−Cu−O films

The preparation of Tl2Ba2Ca2Cu3Oy thick films from spin-coated Ba-Ca-Cu-O films in the presence of thallium oxide vapor was studied. The starting Ba-Ca-Cu-O films were prepared by pyrolysis of organic acid salts. The preparation of the Tl2Ba2Ca2Cu3Oy thick films was carried out for cases of the presence of Tl2O3 powder and of mixed powders of Tl2O3, BaO2, CaO2 and CuO. A Tl2Ba2Ca2Cu3Oy film with Tc onset=126 K, Tc mid=115 K and Tc zero=108 K was obtained. Control of the thallium content of the films was more easily accomplished when using the mixed powders of Tl2O3, BaO2, CaO2 and CuO than when using Tl2O3 powder alone.

Strong correlation between Tc and lattice parameters in Ba2YCu3Oy

Superconducting properties and lattice parameters were systematically studied for Ba 2 YCu 3 O y ( 6 T c value and lattice parameter have a strong correlation in a stepwise change with the oxygen content. The lattice parameters of c and (b-a) change stepwisely at y=6.7 and 6.35 like Cu valency. The bond length of Cu-0 along the pyramidal height strongly correlates to the Tc value. The distortion of the pyramidal basal plane also has a relation to the Tc value.

The formation process of new high-Tc superconductors with single-layer thallium-oxide, TlBa2Ca2Cu3Oy and TlBa2Ca3Cu4Oy

The formation process of new high-Tc superconductors with a single Tl-O layer, TlBa2Ca2Cu3Oy (1223 phase) and TlBa2Ca3Cu4Oy (1234 phase), was studied. The thallium content decreased continuously with increased sintering time. At the first sintering stage, the Tl2Ba2Ca2Cu3Oy phase (2223 phase) was formed. Next, the 1223 phase was formed with the demise of the 2223 phase. Then, after further sintering, the 1234 phase was formed with the disappearance of the 1223 phase. These results indicate that one Tl-O layer of the double Tl-O layers of the 2223 phase evaporated owing to the volatility of the thallium component, and the 1223 phase was formed. Then the 1223 phase with three Cu-O layers changed to the stable 1234 phase with four Cu-O layers.

Photoemission study of the chemical bond nature of Ba2YCu3OY

Abstract The Chemical bond nature of Ba 2 YCu 3 O y (6 6.4). The peak intensity ratio of the satellite peak to the main peak ofCu2p 3/2 state was always smaller in Ba 2 YCu 3 O y than in CuO. The peak width of the Cu 2p main peak changes from a wider value to a narrower value than that of CuO. This means that the Ba 2 YCu 3 O y compound always contain trivalent or monovalent Cu ions as well as divalent ions. A weak-bonding oxygen peak reduces with y, but still remains even below y=6.2 It suggests the existence of hole in the O2p state of O atoms at the top of pyramid. The two valence states of Ba ion indicates the valence charge fluctuation of Ba atom. These results indicate the possibility of the charge-fluctuation superconducting mechanism.