X.Z. Wang

Crystal structure and superconductivity in REBaSrCu3Ox

Abstract The preparation, crystal structure and physical properties of REBaSrCu 3 O x compounds have been studied (Re≡La, Pr, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm, or Lu). These compounds are found to be superconducting with a transition temperature within the range 54 K ≤ T c ≤86 K, except for RE = Pr, which is semiconducting. The maximum transition temperature is observed with RE=Gd and Dy where the lattice structure changes from tetragonal to orthorhombic.

High-Tc superconductivity in GdBaSrCu3O7

Abstract The new compound GdBaSrCu 3 O 7 shows high- T c superconductivity with zero-resistance temperature of about 86 K. The material seems to have a tetragonal structure.

Fabrication of ceramic Y1−xCaxBa2Cu4O8+δ and YBa2Cu3O7 superconductors from oxalate precursors at ambient pressure

Abstract Ceramic Y 1− x Ca x Ba 2 Cu 4 O 8+δ and YBa 2 Cu 3 O 7 superconductors were prepared from coprecipitated oxalate precursor material. Calcination and firing at 820°C under atmospheric oxygen pressure yielded pure phase Ca 2+ doped and undoped YBa 2 Cu 4 O 8+δ superconducting material. The T co values increased from 80 to 85 K with an increase in the Ca 2+ content from x =0 to 0.1. YBa 2 Cu 3 O 7 was obtained after calcination of the oxalates at 860°C and sintering at 920°C under ambient oxygen pressure. This superconductor had a T co of 92 K with a transition width of 1 K.

Oxygen ordering and superconductivity in GdBaSrCu3O7−x

Abstract A new superconducting compound, DyBaSrCu 3 O y , has been synthesized. By different thermal treatments, this compound was crystalized in either orthorhombic or quasitetragonal structure, with oxygen content of y ⋍6.93 to 6.94. Superconducting properties are similar for both phases. The magnetic moment of the Dy atoms is not influenced by the difference in structures and is close to the value for Dy 3+ . At low temperatures a tendency towards magnetic ordering was observed.

Synthesis of YBa2-xSrxCu3O7-δ by pulsed-laser deposition

Abstract This superconducting films of single-phase YBa2-xSrxCu3O7-δ with values of x up to 1.8 have been synthesized by means of pulsed-laser deposition (PLD). When increasing x from zero to 1.8, the zero resistance temperature decreases from 91 K to 57 K; simultaneously the length of the c-axis decreases from 11.68 A to 11.45 A. The temperature dependence of the critical current density has been measured for materials with various values of x.

Rare-earth magnetism and superconductivity in RBaSrCu3O7-x (R=Gd, Pr)

Abstract We present specific-heat, susceptibility, X-ray absorption (EXAFS and XANES) and 155 Gd Mossbauer experiments of RBaSrCu 3 O 7−δ ( R = Gd , Pr ) in order to examine the effect of isoelectronic substitution of Ba by Sr upon superconductivity and magnetism with respect to the parent reference compounds RBaSrCu 3 O 7−δ . The RBaSrCu 3 O 7−δ compounds offer the possibility to vary the oxygen content without changing the long-range tetragonal structure. From 155 Gd Mossbauer spectroscopy a small local orthorhombic distortion is observed for tetragonal GdBaSrCu 3 O 7 with a superconducting transition at 69 K. The XANES spectra of PrBaSrCu 3 O 7−δ indicate that Pr is mainly trivalent with a considerable extent of hybridization, in agreement with a recent description of the specific-heat and susceptibility data in terms of Pr 3+ crystal-field effects. From the comparison of the isoelectronic Ba/Sr substitution with hole doping (oxygen removal or huydrogen absorption) we deduce that the hybridization of the Pr 4f states with the Cu-O valence bands appears to be important for the suppression of superconductivity and the high antiferromagnetic ordering temperature in these Pr compounds.

Fabrication of YBa2Cu4O8 from tartrate gel precursors

Abstract Ceramic YBa 2 Cu 4 O 8 (Y-124) has been prepared by thermal decomposition of tartaric acid gels. The synthesis was performed in an oxygen atmosphere of 1 bar at a temperature, T s , of only 710°C. The onset temperature of the material to the superconducting phase was around T on ≈83 K. Zero resistance was measured at T c0 ≈47 K. When increasing T s to 814°C an increase in T c0 to 72 K was observed. At temperatures T s =890° C the Y-124 phase starts to decompose.

Formation of superconducting Bi2−yPbySr2Ca2Cu3Ox from coprecipitated oxalates

The conditions for the coprecipitation of Bi{sup 3+}, Pb{sup 2+}, Ca{sup 2+} as oxalates are reported. These oxalates were used as precursors for the formation of Bi{sub 2-y}Pb{sub y}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} superconducting ceramics. The behavior of the oxalates upon heat treatment was studied by thermogravimetry. Both the oxalates and the superconducting oxides were analyzed and characterized by atomic absorption spectroscopy and by X-ray diffraction.Abstract The conditions for the coprecipitation of Bi 3+ , Pb 2+ , Sr 2+ , Ca 2+ , and Cu 2+ as oxalates are reported. These oxalates were used as precursors for the formation of Bi 2− y Pb y Sr 2 Ca 2 Cu 3 O x superconducting ceramics. The behavior of the oxalates upon heat treatment was studied by thermogravimetry. Both the oxalates and the superconducting oxides were analyzed and characterized by atomic absorption spectroscopy and by X-ray diffraction. For 0.2 ≤ y ≤ 0.4 and proper heat treatment of the oxalates, followed by compression of the oxides and by sintering, single-phase ceramic samples with a zero resistance temperature of T c (0) ∼ 106 K can be produced in a convenient way.

The preparation and electrical properties of Bi2Sr2CaCu2Oy thick films with high Tc on (100) MgO substrate

Thick c-oriented films, of single-phase Bi2Sr2CaCu2Oy (Bi-2212) were prepared on (100) MgO substrates by employing a partial melting method. The effect of quenching on Tc0 was studied. The highest Tc0 obtained by resistance measurement was 92 K, whereas 93.2 K was achieved by AC susceptibility measurements. The best results for the critical current density Jc, measured in zero magnetic field, were 1.3*103 A cm-2 at 77 K and 7*103 A cm-2 at 53 K. Near Tc the temperature dependence of Jc can be described by Jc approximately (1-T/Tc)2. At temperatures Tc<<Tc the dependence of Jc on temperature can be described by a homogeneous superconductor flux creep model.

Current-density dependence of the vortex pinning energy in YBaCuO

Abstract Measurements of the electrical resistivity, using pulsed currents of various magnitudes, in the mixed state of YBaCuO microbridges are reported. The onset of resistance follows a thermally activated behaviour of flux line motion. The dependence of the pinning potential barrier U on the current density j was investigated, revealing two distinctly different regions. At moderate current densities j 4 A/cm 2 , no change of U upon current density was observed. We associate this behaviour with thermally assisted flux flow. At high current densities, U declines logarithmically, in accordance with the flux creep picture and a particular pinning potential well proposed by Zeldov et al.