H. Igarashi

Variation in Tc, structure and oxygen content in Tl2Ba2CuO6 and their correlations

Abstract The preparation condition of Tl 2 Ba 2 CuO 6 (2201 compound) and its properties including superconductivities and structural variations were studied. Preparation at relatively low heat-treatment temperatures produced the 2201 compound with an orthorhombic symmetry, while heat-treatment above 840°C produced a tetragonal symmetry. It was revealed that the appearance of superconductivity was independent of the structural symmetry, and was strongly affected by oxygen content. Decrease of oxygen content by quenching or Ar-annealing increased the superconducting transition temperatures up to about 70 K. A positive correlation between T c and the c -axis length was also found for all samples including both orthorhombic and tetragonal structures.

Preferentially oriented epitaxial Y-Ba-Cu-O films prepared by the ion beam sputtering method

Preferentially oriented epitaxial Y‐Ba‐Cu‐O films were prepared on (100) SrTiO3 substrates by oxygen reactive ion beam sputtering. The epitaxial orientations were varied by controlling both substrate temperature and oxygen parital pressure. c‐axis oriented films tended to be formed at higher substrate temperatures (>620 °C) and lower oxygen pressures ( 3×10−3 Torr). The best Tc (end) of 82 K was observed in one of the c‐axis oriented film without post‐annealing. The tendency for preferential orientation can be well understood in terms of the lattice mismatch between the substrate and the film, the lattice constants of which depend on oxygen deficiency.

Rietveld analysis of Tl2Ba2Can−1CunO4+2n (n=1, 2 and 3) by powder x-ray diffraction

Abstract Crystal structures of the phases in the Tl-Ba-Ca-Cu-O system determined by powder X-ray diffraction were refined using Rietveld analysis. Three samples having the general composition Tl 2 Ba 2 Ca n −1 Cu n O 4+2 n where synthesized. These were Tl 2 Ba 2 Ca 2 Cu 3 O 10 ( n =3,2223 phase ) with a T c of 116 K, Tl 2 Ba 2 CaCu 2 O 8 ( n =2,2212 phase ) with a T c of 85 K, and Tl 2 Ba 2 CuO 6 ( n =1,2201 phase ) which did not exhibit superconductivity. The results of the Rietveld analysis revealed that as n increased the lattice parameter, a , and the interatomic distance Cu-O(2) became shorter, and the Tl-O and Ba-O layers showed a tendency to become flatter. Possible substitution of about 10% of the Ca-sites with Tl ions in the 2212 and 2223 phases was also suggested.

Effect of oxygen deficiency on the crystal structure and superconducting properties of the Ba2YCu3Oy

Effect of oxygen deficiency on crystal structure and superconductivity of Ba2YCu3O7-δ was studied by annealing and quenching experiments. With increasing oxygen deficiency superconductivity was suppressed remarkably while axial ratio, a0/b0 in the orthorhombic phase approached to unity and c0 value increased. Structural transformation from orthorhombic to tetragonal lattice took place in the temperature range, 950~980°C and maximum oxygen deficiency, δ=0.7 was observed for the tetragonal phase.

Superconductivity of TlBa1+xLa1−xCuO5 with 1201 structure

Abstract A new superconducting phase has been found in the Tl-Ba-La-Cu-O system. The powder X-ray diffraction and EPMA study revealed that this new phase has a chemical composition of TlBa 1+ x La 1− x CuO 5 ( x = 0-0.4) and a crystal structure identical with TlBa 2 CuO 5 (1201 phase). The lattice parameter c 0 became longer with increasing x , while a 0 exhibited no significant change. For x = 0, the temperature dependence of the resistivity was semiconductor-like. With increasing x value, the samples came to show superconductivity and finally changed to a metallic nonsuperconductor at x = 0.4. The maximum T c value was about 40 K at around x = 0.2.

Large Tc-variation from 0K to 80K and hole concentration in Tl2Ba2CuO6

Abstract We have found that a large variation in T c from 0K to 80K observed in Tl 2 Ba 2 CuO 6 was caused by very small oxygen deficiency of ∼0.1 per formula unit, which corresponded to the decrease in hole density of ∼0.2 per Cu atom. This was qualitatively confirmed by Hall measurement although it gave much larger decrease in carrier density (Hall number). It should be noted that all these features were very similar to those observed in La Sr Cu oxides;superconductivity disappeared at compositions with too many hole carriers, suggesting that superconductivity appeared only in a certain appropriate range of carrier density. Preliminary results for other Tl-oxide superconductors were also presented.

Determination of the Orthorhombic-Tetragonal YBa2Cu3O7-δ Phase Boundary in the δ-T Diagram

The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by a detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at δ = const., but to have a slope of dδ/dT~-4.7×10-4 K-1. The experimental results are discussed with a simple order-disorder transformation model of oxygen atoms on the Cu plane sandwiched by Ba planes.

Epitaxial film growths of artificial (Bi-O)/(Sr-Ca-Cu-O) layered structures

Epitaxial film growth of artificial (Bi‐O)/(Sr‐Ca‐Cu‐O) layered structures on MgO substrates was carried out using reactive‐oxygen‐gas dual ion beam sputtering and shuttering technique. A 12 A Sr‐Ca‐Cu‐O buffer layer seems suitable for perfect epitaxial film growth. Single‐phase films having periodicities of 12, 15, and 18 A (which correspond to bulk 24, 30, and 36 A phases in Bi oxide superconductors) were selectively grown by adjusting the thickness of a Sr‐Ca‐Cu‐O layer sandwiched by Bi‐O bi‐planes. A high Tc phase film with a total thickness of about 300 A showed an onset Tc of 110 K and a zero resistivity temperature of 45 K without post‐deposition annealing. The shuttering technique seems to enable the layer‐by‐layer film growth, and it is very effective for the formation of smooth and perfect epitaxial structures.

In situ epitaxial growth of Bi2(Sr,Ca)3Cu2Ox films by ion beam sputtering with an atomic oxygen source

In situ epitaxial growth of Bi2(Sr,Ca)3Cu2Ox films was performed by ion beam sputtering in atomic oxygen ambience at the substrate temperature of 640 °C. The films showed an epitaxial growth in which the a and b axes were parallel to 〈100〉 MgO, and the superstructure according to the incommensurate modulation along the b axis was also observed. The superconducting properties of the as‐grown films seemed to sensitively depend on the oxidation treatment during the cooling down process. The zero resistivity temperature Tc(R=0) of a 600‐A‐thick film cooled down in the same atomic oxygen density as the film growth ambience was 60 K, but it increased up to 80 K after a post‐deposition annealing at 500 °C for 1 h in air. In contrast, as‐grown films cooled down in insufficient oxidation ambience showed the Tc(R=0) of 76 K without post‐deposition annealing. The control of the oxygen concentration is critical for the superconductivity of as‐grown films.

Non-superconducting TlBa2YCu2O7 with a new crystal structure resembling superconducting YBa2Cu3O7

Abstract A new phase was found in the TlBaYCuO system. X-ray diffraction and EMPA studies revealed that this new phase had a chemical composition of TlBa 2 YCu 2 O 7 (1212 phase) and its structure was similar to both Tl 2 Ba 2 CaCu 2 O 8 and YBa 2 Cu 3 O 7 . This structure contains two Cu perovskite-like pyramids separated by monolayer TlO sheets; in contrast to that there are bilayer TlO sheets in the Tl 2 Ba 2 CaCu 2 O δ and CuO chain layer in the YBa 2 Cu 3 O 7 . The electrical properties of this phase are semiconductor-like, and non-superconducting.