Toshihiro Kotani

Crystal Growth and Superconductivity of Tl-Ca-Ba-Cu-O System

Single crystals of oxides in the Tl-Ca-Ba-Cu-O system with the general formula Tl2Can-1Ba2CunOx (n=1, 2, 3, 4) have been successfully grown by the flux method. All crystals were of the platelet form ranging from 0.6 to 5 mm2. The transition temperatures of as-grown crystals, determined by DC SQUID magnetometer, were 84 K, 110 K, and 118 K for n=1, 2, and 3, respectively, but slightly lower for 115 K, where n=4. The results of the differential thermal analysis (DTA), X-ray diffraction measurement and energy dispersive X-ray microanalysis (EDX) will be also reported.

Superconductivity in the Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O Systems

The influence of the substitution of Bi for Tl in forming the superconducting phases in Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O systems was investigated. In the Tl-Bi-Ca-Ba-Cu-O system, the Tl Ca2Ba2Cu3O9 phase forms from a 10% to 40% substitution of Bi for Tl in the nominal composition. With a further increaseing Bi, the superconducting phase disappears because of the formation of BaBiO3. On the other hand, the superconducting phase of (Tl, Bi)CaSr2Cu2O7 forms from a 10% to 90% substitution of Bi for Tl in the Tl-Bi-Ca-Sr-Cu-O system. It is recognized that the substitution of Bi for Tl stabilizes and accelerates the formation of the (Tl, Bi)CaSr2Cu2Ox phase.

Superconducting Phases with Single Tl-O Layer Structure Preparation: (Tl, Pb)Can-1Ba2CunO2n+3 (n=2, 3, 4, 5, 6)

Single Tl-O layer structures with two to six Cu-O layers, (Tl, Pb)Can-1Ba2CunO2n+3 (n=2, 3, 4, 5, 6), are prepared, and the superconducting properties of their phases are investigated. The c-lattice constants of the observed phases obey the c-axis rule of c=6.38+3.19n for single Tl-O layer structures. With an increase in Cu-O layers, the superconducting transition temperature, determined from electrical resistivity-temperature dependency and DC susceptibility, rises, reaching a maximum value of 121 K at n=4, and then falls with a further increase of the number of Cu-O layers from four to six.

Superconductivity on Single Crystals of Tl-Ca-Ba-Cu-O System

The investigation on the growth of single crystals of the Tl-Ca-Ba-Cu-0 superconducting system was carried out using the flux growth method. The crystal structure and superconducting properties of mm-sized single crystals were measured using X-ray diffraction and the SQUID magnetometer. The grown crystals are typically in platelet form ; 0.5 to 2.5 mm wide and about 0.2 mm thick. The crystals exhibit a mirror-like surface and some growth steps can be observed through a scanning electron microscope. The superconducting onset transition temperature of 108K to 118K was determined by DC magnetic susceptibility obtained for as-grown single crystals. The transition temperature has a tendency to increase as the amount of CaO-CuO in the starting nominal composition rises. According to the X-ray diffraction measurement, the c-axis of the grown crystals is 35.8A for those with Tc of 118K, and 29.3A for those with Tc of 108K.

Structural Study of Tl2Ca3Ba2Cu4Ox Single Crystal by Transmission Electron Microscopy

The microstructure of Tl2Ca3Ba2Cu4Ox (2324-phase) single crystals grown by the slow-cooling method was investigated using transmission microscopy. These single crystals showed superconducting onset transition temperatures of 113 to 115 K, and lattice constants were a=b=3.85 A and c=42.0 A by X-ray diffraction measurement. The high-resolution transmission electron microscopy (TEM) observations revealed intergrowths of Tl-O single-layered structures, 1324 and 1425, with stacking spacings of 19 A and 22 A, in addition to a 2223 phase of 36 A.

Growth and superconductivity of Bi2Sr2CaCu2Ox single crystal by slow-cooling method

Abstract We have succesfully grown a Bi 2 Sr 2 CaCu 2 O x single crystal with an area of up to 4 × 12mm 2 by a simple slow-cooling method. The growth experiments revealed that a large-area crystal was typically grown from a starting composition of Bi:Sr:Ca:Cu=2:2:1:2 and at a low cooling rate. Based on an investigation of the temperature dependence of formed phases in the Bi system, the growth mechanism is discussed. The evaluations of transition temperature, crystal composition and pinning potential of a grown crystal are also reported.

Growth and Structure of (Tl,Bi,Pb)Sr2CaCu2OX Single Crystal

Crystal growth and structural analysis of (Tl,Bi,Pb)Sr2CaCu2Ox single crystals were performed. A single crystal with superconducting onset transition temperature of 85K was grown in a sealed gold tube by the slow-cooling method. The crystal structure was determined to be tetragonai(P4/mmm) with lattice constants of a=3.80A and c=12.09A. The refinement of the crystal structure was carried out based on the single-crystal X-ray diffraction data.

Structure and Superconductivity of Tl2Ca3Ba2Cu4Ox Single Crystals

The crystal structure and superconducting properties of Tl2Ca3Ba2Cu4Ox single crystals were investigated. Based on the preliminary study of phase relations in the Tl2Can-1Ba2CunOx system, 2324-phase single crystals with dimensions of from 0.5 to 1mm2 were grown by the flux method. The superconducting onset transition temperatures of grown crystals, determined by DC SQUID magnetometer, were 113 to 115K. According to anQX-ray diffraction measurement, the lattice constant of the c-axis was 42.0A, and the crystal was confirmed to be single phase of 2324. The results of a high-resolution transmission electron microscopic study are also reported.

Magnetic Properties of Bi- and Tl-Based Single Crystals

Crystal growth and magnetic relaxation of Tl2Ba2CaCu2Ox, Tl2Ba2Ca2Cu3Ox, (Tl,Bi,Pb)Sr2CaCu2Ox and Bi2Sr2CaCu2Ox single crystals were investigated. According to a study on the phase relations of these systems, four kinds of mm-sized single crystals with different transition temperatures were grown by the self-flux method. The single crystals showed a large magnetic relaxation, which was approximately linear with the logarithm of the time. Based on the thermally activated flux creep model, the effective flux-pinning potential energy for each crystal was estimated from these results.