G. Ravi Kumar

Texture formation and enhanced critical currents in YBa2Cu3O7

We report an increase of a factor up to 5 in JC on mixing varying amounts of Ag2O with YBa2Cu3O7. The increase correlates with an increase in texture as seen by x‐ray diffraction intensities. Microstructure indicates the formation of platelets with c axis normal to the platelets.

Critical state model and the magnetic behaviour of high Tc superconductors

Intragrain critical current densities (Jc) are usually inferred from magnetization measurements using Beans critical state model. Using this model results are presented for the case of Jc decaying exponentially with field and explanations are given for the observed changes in the shape of the magnetization hysteresis curve as (1) the sample temperature is changed; (2) the sample dimension is changed; (3) the maximum field to which the sample is cycled is varied; and (4) the field direction is changed for a single crystal sample. Some of the observed thermomagnetic history effects in the magnetization of the high temperature superconductors are also explained. Finally, d.c. and low-frequency a.c. magnetization data on various Rga2Cu3O7 superconductors are presented where features originating from intergranular and intragrain regions are identified and explained.

Dynamic contribution to the fishtail effect in a twin-free DyBa2Cu3O7−δ single crystal

Abstract Superconducting current densities j s and dynamic relaxation rates Q  d ln j s /d In(d B e /d t ), where d B e /d t is the sweep rate of the external magnetic field B e , were measured as a function of temperature (5 K T 2 Cu 3 O 7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j s ( B e ) curve at fields around B e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j s ( B e ) curve. Both the j s versus B e and Q versus B e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j c which is by definition independent of relaxation effects. Interestingly, the j c ( B e , T ) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j c ( B e , T ) decreases weakly with increasing B e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.

Low field anomaly in magnetization curves

We present a calculation of the isothermal magnetization hysteresis curves appropriate to highTcsuperconductors. We discuss the nature of the low field anomaly as one goes from this strong pinning case to the weak pinning case. We show that the shape of the equilibrium (thermodynamic) magnetization curve is recovered in the limit ofJcapproaching zero.

Do we really need a superconducting glass model

Muller and coworkers have introduced the concept of a superconducting glass to explain some magnetization data in highTcsuperconductors. We explain this data using conventional ideas of type-II superconductivity.

Angular scaling of critical current measurements on laser-ablated YBa2Cu3O7−δ thin films

Abstract Orientation dependent measurements of torque hysteresis loops on high quality laser-ablated YBa2Cu3O7−δ thin films were performed at a temperature of 30 K in magnetic fields up to 7 T. From these measurement we extract the field and orientation dependence of the in-plane critical current density Js(H,θ), where θ is the angle between the applied external magnetic field H and the plane of the film. Following the scaling arguments of Blatter, Geshkenbein and Larkin based on the anisotropic Ginzburg-Landau theory, we have been able to scale the measured Js(H, θ) into a form ∫(ϵ(θ)H), where ϵ (θ) = (sin 2 θ + cos 2 θ/gg 2 ) 1 2 with γ = 3.4±0.3. The obtained value of the anisotropy parameter γ is much smaller than the values reported earlier for single crystals close to Tc.

Magnetization curves and Bean's model in high-T c superconductors

High-Tcsuperconductors have critical currents that decay sharply with increasing magnetic field. We solve Beans model forJc decaying exponentially withH and obtain qualitative agreement with existing magnetization data. We show thatHc1cannot be obtained from the linear part of the magnetization curve; it can only be inferred from a low-field anomaly in the hysteresis curve. Presently quoted values ofHc1,based on the linear part of the magnetization curve, are gross overestimates.

Critical current measurements on extruded YBa2Cu3O y wires

The successful extrusion of short lengths of brittle wires of YBa2Cu3Oy that are superconducting above 77 K is reported. Detailed critical current measurements are presented on wires having undergone different physical treatments. The highest critical current obtained from V-I data is ⋍ 90 A/cm2, but this particular wire shows clear evidence of flux-pinning forces corresponding to ⋍ 130 A/cm2.

Critical current measurement on Y-Ba-Cu-O

We report critical current measurements on sintered Y0.35Ba0.65CuOy. The sample, in the perovskite phase, shows zero resistance at 87 K. The critical current transition is seen, in zero field and at 77 K, at a current densityJc of 50 A/cm2.

Microstructural and magnetic properties of textured GdBa2Cu3Oy/Gd2BaCuO5 composites fabricated from Gd2BaCuO5 preforms

Abstract Bulk GdBa2Cu3Oy superconductors with uniformly distributed inclusions of micron-sized Gd2BaCuO5 (211) have been prepared by the infiltration and texturing of pre-shaped porous 211 preforms with liquid phases, in a high purity Ar atmosphere. The microstructural features at various stages of processing with particular emphasis on the nature of the 211 inclusions are studied. The superconducting properties like the transition temperatures and critical current densities of the resulting composites are discussed.