B. Yin

Evidence for flux pinning induced by spatial fluctuation of transition temperatures in single domain (Y1−xPrx)Ba2Cu3O7−δ samples

Abstract The current density j s has been measured versus temperature at B = 1 T for single domain (Y 1− x Pr x )Ba 2 Cu 3 O 7−δ ( x = 0.00, 0.03, 0.05, 0.08) samples. It is found that the measured current density j s is enhanced by the substitution of Y with Pr, which manifests that extra pinning centers have been introduced into the samples. Detailed analysis show that the temperature dependence of the measured current density j s ( T ) as well as the normalized relaxation rate S ( T ) for all the four samples can be well explained basing on the model for flux pinning induced by the spatial fluctuation of the transition temperatures, while the theoretical results obtained basing on the model for flux pinning induced by the spatial fluctuation of the charge carrier mean free path are completely inconsistent with the experiment data. Finally we conclude that the dominant pinning in our samples are resulted from the spatial fluctuation of the transition temperatures rather than the mean free path.

Evidence for the lattice-mismatch-stress-field induced flux pinning in (Gd1 − xYx)BA2Cu3O7 − σ thin films

Abstract By partially substituting Gd with Y in GdBa 2 Cu 3 O 7 − σ thin films, the measured critical current density ( j s ) was increased, while the relaxation rate ( S ) was clearly lowered, which was attributed to the presence of extra pinning centers. The temperature dependence of the true critical current density j c ( T ) and the pinning potential U c ( T ) were determined from the experimental data both for the pure and the substituted thin films by means of a generalized inversion scheme (GIS). It is found that the experimental results for the pure film are close to the predictions of the σl pinning model, while those for the substituted thin film follow the theoretical expressions derived for the single-vortex collectively pinned by the small size stress-field pinning centers, which may suggest that the enhanced pinning in the substituted sample is probably due to the presence of a local stress field induced by the lattice mismatch.

The fish-tail effect and the flux pinning induced by the spatial fluctuation of critical temperatures in melt-textured YBa2Cu3O7−δ bulk samples

Abstract The irreversible magnetization loops and the relaxation of the remnant magnetization were measured for a melt-textured YBa2Cu3O7−δ bulk with two different oxygen states. It is found that introducing the extra oxygen deficiencies into the sample will enhance the critical current density in the low-temperature region and the fish-tail effect in the intermediate-temperature region, but lower the critical current density in the high-temperature region. Further investigation shows that the fish-tail is not due to the crossover between different collective pinning and creep regions, but rather to the presence of the flux pinning induced by the critical temperature fluctuation on the background pinning induced probably, by the mean free path fluctuation.

Structural and superconducting properties of the infinite-layer (Sr, Ca)1−yCuO2 prepared under high pressure

The Rutherford backscattering/channelling technique of 2.1 MeV He ions has been used to study the damage distribution in Si irradiated with 1.0 MeV Ti ions to a dose of 5 x 10(14) ions cn-2 under 7-degrees and 60-degrees incidence. A dechannelling analysis of multiple scattering has been made. The longitudinal damage straggling and lateral damage spread are estimated. The values obtained are compared with transport of ions in matter. The results show that the differences between experimental and calculated values for longitudinal damage straggling and the lateral damage spread are 25% and 28%, respectively. Also, it is observed that the damage distribution shape under 60-degrees irradiation is found in good qualitative agreement with an earlier simulation.

STRUCTURE AND SUPERCONDUCTIVITY IN THE INFINITE-LAYER SR1-XCUO2 SYSTEM PREPARED UNDER HIGH-PRESSURE

Abstract Infinite-layer Sr 1− x CuO 2 ( x = 0.0−0.3) samples have been prepared under high pressure. The average structure of the infinite- layer SrCuO 2 was refined by the Rietveld technique to be tetragonal with a = 3.9269 (2) A and c = 3.4346 (2) A . The infinite- layer Sr 1− x CuO 2 samples could be formed under high pressure for x = 0.0−0.3 in which the Sr deficiency was partially released by forming Cu rich impurity phase (s) such as CuO or Sr 14 Cu 24 O 41 . The microstructure of the Sr 1− x CuO 2 samples was characterized by high-resolution electron microscopy and Sr vacancies were not found to cluster into defect layers along the c -axis. The resistivity-temperature dependence of the Sr 1 − x CuO 2 ( x = 0.0−0.3) samples suggested that the Sr deficient infinite-layer phase might be electron-doped. An indication of superconductivity with T c onset = 60 K was observed in the Sr 1− x CuO 2 samples. When the samples were annealed in an oxidizing atmosphere, the indication of superconductivity remained at 60 K but the superconducting property was not improved remarkably. The origin of the superconductivity was discussed.

Susceptibility behaviour and specific heat anomaly in single crystals of alanine and valine

Magnetization of single crystals ofD-,L-alanine andD-valine were measured as a function of temperature using the SQUID magnetometer. An obvious lambda transition at 270 ± 1 K was shown in the specific heat measurement of alanine and valine enantiomers by differential scanning calorimetry (DSC). Magnetic transition temperature seems coincident with that of lambda transition. Temperature dependence of the X-ray powder diffraction forD-valine showed no crystal lattice change under the temperature cooling down from 293 K to 123 K. We propose that the difference of theX ∼T curve between theD-alanine andL-alanine is attributable to the variation of intramolecular geometry of chirality density in the presence of an external magnetic field. The chirality characteristics of alanine and valine enantiomers by the specific heat and susceptibility behaviour are discussed.

Critical current density and flux pinning in Gd1−xYxBa2Cu3O7−y epitaxial thin films

Low-electric-field E=j(s) curves, normalized dynamical relaxation rate Q(T)=d ln(j(s))/d In(E), and experimental critical current density j(s)(E = E(c)) were measured from 10 to 85 K with the magnetic sweeping method. By taking a general assumption for the activation energy U(j(s), T) = U-c( T)f [j(s)(T)/j(c)(T)], with t = T/T-c, we obtained the U(j, T=O K) relation by scaling all the E-j(s) curves of different temperatures to one smooth curve with U-c(T) and j(c)(T) determined by the so-called generalized inversion scheme. It is shown that the important parameter c = In(upsilon(o)B), which was actually taken as a fitting parameter in, the scaling process in previous work, can be predetermined. It is also found that the resulting U(j, T=O K) relation based on the E=j(s) curves for T less than or equal to 79 K can be well interpreted with the collective-pinning model. However, the data for T>79 K start to deviate from this interpretation, which may be attributed to the reverse hopping of vortices.

THE VORTEX-GLASS TRANSITION IN TL2BA2CACU2O8 SUPERCONDUCTING THIN-FILMS

Abstract We have measured the I–V curves of Tl2Ba2CaCu2O8 thin films at different temperatures in fixed magnetic field. The curves collapsed nicely onto two branches by plotting V/I/|T − Tg|v(z−1) vs. I/T|Tg − T|2v, with the temperature Tg, critical exponents v and z determined from experiments. When T

Epitaxial growth of Bi2Sr2-xLaxCuO6+δ thin films by RF-magnetron sputtering

Superconducting Bi2Sr2-xLaxCuO6+delta thin films were successfully deposited on (100) LaAlO3 and (100) SrTiO3 substrates by in situ RF-magnetron sputtering. The highest zero resistance temperature T-c0, reached 29 K. The film morphology is shown to be related with the sputtering gas pressure. The microstructures. of the films show ab-twinning. Atomic force microscopy image showed the films grow as terrace islands with no screw dislocation. For obtaining high quality La-2201 superconducting thin films, the sputtering gas pressure should be higher, and the compositions of the targets have to be modified by trial and error

Critical current density and flux pinning in Ho‐doped melt‐processed YBa2Cu3Oy

We fabricated highly textured Y1−xHoxBa2Cu3Oy (x=0, 0.2, 0.4, and 0.6) bulk samples by a ‘‘powder melting process’’ method under identical conditions and investigated their critical current densities and flux pinning behavior with a superconducting quantum interference device magnetometer. The dependence of Jc on the magnetic field, estimated by the Bean model, is studied. The results indicate that Jc drops with the magnetic field according to a power law Jc∝H−n (n<0.5), which implies that the Jc value of these samples is controlled by the flux pinning rather weak links. It is found that Jc and flux pinning can be significantly improved by Ho substitution for Y in Y‐based superconductors. The reduction of the size of Y2BaCuO5 particles, stress‐field pinning, and magnetic pinning created by the Ho addition are considered to be responsible for the increase in Jc and flux pinning.