Zhao Zhongxian

Microstructure and Superconductivity of Ultrathin YBa2Cu3O7-δ Films

Two unit cells to 200 A thick ultrathin YBa2Cu3O7-δ films have been synthesized and their growth mechanism and physical properties have been studied. Atomic force microscopy showed that the films grow with both spiral and layer by layer modes and the screw dislocations appeared even in the 6 unit cells thick films. In the initial growth stages of the films, the strong dependence of Tc on the film thickness is discussed.

Pinning Potential and Critical Current Densities in Powder Melting Processed Y(Ho)BCO Samples

Magnetic relaxation and magnetization of the powder melting Y1-xHoxBa2Cu3O7 (x = 0.2, 0.4 and 0.6) samples have been measured at the temperature of 70 K. The pinning potential U and magnetization critical current density Jc have been obtained according to the Anderson-Kim model and Bean model. It is found that both U and Jc were influenced by doping Ho. In the sample with x = 0.4 the pinning potential U has a value of 3.2 eV when the field is equal to 1 kG and parallel to c-axis, which is the highest one we have known in the high Tc superconductors.

Relationship between Tc and σJ in Doped Superconducting Copper Oxides

A relation of Tc similar σJn is proposed after analysing the phase diagram and magnetic susceptibility experiments for La2-xSrxCuO4 and Bi2+xSr2Ca1-xCu2Oy. This empirical rule agrees with the phase diagram for La2-xSrxCuO4. We suggest that it should also hold for other superconducting cuprites.

Structural Anomalies of Bi-2223 Phase in the Temperature Range of 8-305 K

A nearly single-phased Bi-2223 sample with Tc 108 K was studied by low-temperature powder x-ray diffraction. Several discontinuous changes of lattice parameters are observed around 245, 160, 125, 100 and 20 K. The most pronounced anomalies occur close to the superconducting transition temperature. Our observation indicates that electron-lattice interaction is quite strong near Tc.

Doping Effect on Superconducting Transition of Ba-K-Bi-O System

Chemical doping or substitutions were carried out in both the Ba site and Bi site in Ba-K-Bi-O superconductor. The large single crystals of Ba-K-M-Bi-O (M=Ca, Eu, In and Tl) were prepared by using the electrocrystallization method. The resulting crystalline materials were characterized by x-ray photoelectron spectroscopy and magnetic method. The results show that partial substitutions of Ca or Eu in the Ba site suppress Tc more drastically than that of Tl in the Bi site. All the present chemical substitutions result in a decrease in Tc.

Phase Transformation in the Infinite Layer Structure Compound (Sr, Nd) CuO2 and its Effect on Superconductivity

Superconducting compound (Sr0.86Nd0.14) CuO2 with the infinite layer structure was synthesized under high pressure. This tetragonal structure a × a transforms into a superstructure 2a × 2a when the sample was in situ heated in transmission electron microscope to about 600 K. An anormalous resistance change of the sample during heating in vacuum to 400°C confirmed the phase transformation. After the superlattice formation, the sample lost its superconductivity and became semiconducting.

Insulator to superconductor transition of Pb doped 2201 single crystal in Bi-system by oxygen deficiency

Flux method was used to grow large 2201 single crystal in Bi-Sr-Cu oxide. The as-grown crystal was nonsuperconductor and showed semiconductivity in resistivity curve. After it was reduced in flowing N2 at 570°C and appropriate oxygen vacancies were induced, superconductivity with Tcon = 6K was obtained. The great influence of oxygen deficience on superconductivity is evident. Also, the incorporation of Pb adjusted the modulation pattern.

Preparation of a-axis oriented YBa2Cu3Ox thin films

The a-axis oriented YBa2Cu3Ox (YBCO) thin films could be grown on (100) SrTiO3 (STO) substrates with STO buffer layers by dc and rf magnetron sputtering either by lowering the deposition temperature, or by using a self-template technique. For the latter, the resistivity of the thin film at 290 K along the substrate [001] direction is about four times larger than that in the [010] direction. The zero resistance temperatures Tc0 are 89 K in both directions. So high quality a-axis oriented YBCO thin films can be prepared by the self-template technique. Also the Tc0 increase monotonously with the reduction of the thickness of the YBCO seed layer.

Superconductivity in a new (Sr, Y)CuO2 System Synthesized Under High Pressure

Superconductivity has been observed in a new series (Sr1-xYx)CuO2 (x = 0.00 - 0.30) synthesized under high pressure of 3.5 GPa and 950°C. Compared with the previous (Sr1-xLnx)CuO2 (Ln = Nd, Pr and La) series, this new series shows a broader superconducting composition range of x = 0.05 - 0.30. The onset transition temperature of this series is 36-38 K.

Irreversibility line and thermal depinning in YBa2Cu3O7-δ

By measuring magnetic torque moment in a field-sweeping process, the temperature and field dependence of the critical current density j (with a criterion of electric field) and the normalized relaxation rate Q = d ln j/d ln E of a YBa2Cu3O7-δ thin film were obtained. With a minimum current density (jmin = 10 Acm-2) the irreversibility lines at different sweeping rates were determined. It was found that these irreversibility lines cannot be fitted to either the melting line or the vortex-glass transition. All the data can be interpreted by the thermally-assisted-flux-flow model. Further investigation shows that, at irreversible temperature and field, Uc is much smaller than kBT, which indicates that the thermal depinning is the real origin of the irreversibility line.