A.M. Sun

HgBa2CaCu2Oy superconducting thin films prepared by laser ablation

Abstract HgBa 2 CaCu 2 O y superconducting thin films were prepared on SrTiO 3 (100) and LaAlO 3 (100) substrates using laser ablation and post-Hg-vapor annealing. X-ray diffraction and electromagnetic measurements show that these films are dominated by c -axis-oriented Hg-1212 phase. For the Hg-1212/SrTiO 3 sample, a zero-resitance superconducting transition temperature ( T c0 ) of 110 K and critical current density ( J c ) of up to 10 6 A/cm 2 at 77 K and zero field have been obtained. The Hg-1212/LaAlO 3 sample exhibits slightly lower T c0 ) ( = 100 K) and J c (= 2 × 10 5 A/cm 2 ). Our investigations suggest that by using superconducting HgBaCaCuO target mercury can be incorporated into the films easily during post annealing.

Current and magnetic field distribution of disk-shaped superconducting film

Abstract When the applied field B → a =μ 0 H → a parallel to the rotation axis of a disk-shaped superconducting thin film with radius R and thickness d in a cylindrical coordinate system, and the self-field effects of the screening current B → ′=B z ′ z +B ρ ′ ρ ( z ‖ B → a ⊥ ρ ) are considered, an effective method using finite-element analysis and matrix operation, to obtain the field dependent critical current function J c ( B ) from magnetic moment hysteresis loop m ( B a ) in a self-consistent way, is presented. Then also in a self-consistent way, the current density J ( ρ ) and field B z ( ρ )= B a + B z ′( ρ ) distributions can be obtained, regardless whether the film being fully penetrated by the applied field or not. The effects of the radial stray field B ρ ( ρ , z ) on the current cannot be neglected when B a ≈0. Due to the weak self-field effects, or intense demagnetic effects of the film compared with the long cylinder-shaped superconductor, between the average values of field 〈 B z 〉 and magnetization 〈 M 〉 there is a quantitative relation 〈 B z 〉= μ 0 [ H a +(0.8 d / R )〈 M 〉] for a disk-shaped film and a qualitative relation 〈B z 〉=μ 0 [H a +〈M〉/ 1+(2R/d) 2 ] for an arbitrary cylinder-shaped superconductor under the Bean model approximation.

Magnetic response of disk shaped YBa2Cu3O7−δ thin film to a perpendicular magnetic field

Abstract Magnetic measurements have been performed on a disk shaped YBa2Cu3O7−δ thin film in a perpendicular field. The critical state model developed by Mikheenko and Kuzovlev and Zhu et al. is extended to analyze the demagnetizing effects of the film. Using the field-dependent critical current density through the two-current model, the virgin magnetic moment as a function of the applied field and the hysteresis loops at several temperatures can be well fit by the extended model. The current density profiles and the flux density profiles of the film have been obtained and discussed.

Temperature, current and field dependencies of activation energy in Tl2Ba2CaCu2O8 superconducting thin film

Abstract The dynamic magnetic hysteresis loops m(T, B a , B a ) of an epitaxial Tl2Ba2CaCu2O8 superconducting thin film are measured at various field sweep rates B a and at temperatures ranging from (1/2)Tc to (4/5)Tc. The magnetization current densities J m (T, B a , B a ) are obtained using the Bean model approximation. Followed by the determination of the maximum creep velocity for fluxons or flux bundles, the optimum temperature, current and field dependent functions of the effective activation energy, g(T), F(Jm) and Bγa are uniquely determined for a range of temperatures and applied fields. The inverse function of F(Jm), i.e. J c m (T, B a , B a ) , is in excellent agreement with the experimental results of J m (T, B a , B a ) . Hence in a range of temperatures and applied fields, the optimum temperature, current and field dependencies of activation energy can be ascertained without a priori assumption of their explicit forms.

Dependence of the Glassy Exponent on Field and Temperature in Textured YBCO Specimen

The glassy exponent Μ of a melt-textured YBa2Cu3O7-x sample is derived from measurements of the magnetic hysteresis loops with various applied field sweep rates and at different temperatures and magnetic fields. Thus, the dependences of the exponent Μ on temperature and field are obtained. Furthermore, we report for the first time the crossover-line of the vortex creep regime from a single vortex to a small bundle.

Glassy exponent of melt textured YBCO extracted from magnetic hysteresis loops with various field sweep rates

Abstract The glassy exponent μ of the melt-textured YBCO sample is extracted from the magnetic hysteresis loops with the various applied field sweep rates at the different temperatures. Our results show that the field and temperature dependence of μ is consistent with the predictions of the collective pinning theory.

Structural and physical properties of Al substituted Y1−xAlxBa2Cu3Oy

Abstract The Y 1− x Al x Ba 2 Cu 3 O y superconductors have studied. An anomalous behavior of T c , oxygen content and structure at doping concentration x=0.5 was found. Our investigations reveal that the abnormal of these properties is due to Al substituted at different positions.

Linear AC response in the high-Tc superconductor of cylinder sample

Abstract In this paper, we discussed the ac linear response in the high-Tc superconductor for the cylinder sample and derived the expression of the ac susceptibility χ theoretically. The equation of the criterion for the peak of the ac susceptibility in the ac linear-response case was also given.

Frequency dependence of ac magnetic response in type-II superconductors

Abstract The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A modified model was proposed to describe the variation of current density in a sample immersed in an alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa 2 Cu 3 O 7 bulk sample was measured and explained in this model. Also discussed was the relation between our model and other models that appeared in literature.

Surface pinning and influence of field reduction on magnetic relaxation in YBa2Cu3O7 bulk sample

Abstract Differences between the magnetic relaxation due to surface barriers and bulk pinning is studied. It is shown that a small field reduction can affect the two relaxation processes quite differently. Experimental results on a melt-gown YBa 2 Cu 3 O 7 bulk sample and theoretical calculations based on surface pinning support this conclusion. We suggest that field-reduction relaxation experiments can be used to probe surface pinning.