P.K. Srivastava

Vortex dynamics in Y Ba2Cu3O7 films

The low frequency complex ac impedance Z of c-axis oriented Y Ba 2 Cu 3 O 7 thin films was measured as a function of the applied magnetic field B . At temperatures not too close to the transition temperature the imaginary part of Z is proportional to B 2 , while the real part consists of a term proportional to B 3 and an additional term proportional to exp (-1/ B ) which appears at higher fields. This behavior is in agreement with a description of vortex dynamics with a Langevin equation of motion.

Inductive conductance measurements of YBaCuO films

Abstract Measurements of the complex ac conductance of sputtered YBaCuo films reveal, in the critical transition region, features strikingly similar to those observed at the phase transition of artificially prepared two-dimensional Josephson junction arrays.

Penetration depth in YBaCuO films

Abstract With a two coil mutual inductance technique the in plane penetration depth in compact YBaCuO films was measured to be λ (0) ≅ 0.23 μm. In granular YBaCuO films the penetration depth is larger, its value is being determined by the strength of the intergranular Josephson coupling energy.

Magnetoconductance oscillations in high temperature superconducting networks

The complex AC impedance Z of superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ triangular networks exposed to a perpendicular magnetic field was measured with a two-coil mutual inductance technique. At temperatures close to the transition temperature, both the imaginary and the real parts of Z as a function of the magnetic field show oscillations resulting from flux quantization in the unit cell of the network. The limited number of oscillations observed is probably an indication of the reduction of the superconducting phase coherence by disorder and inhomogeneities in the network.

Complex AC conductance of YBaCuO films

The AC linear response of YBaCuO films prepared by RF magnetron sputtering to a small oscillating field was studied as a function of temperature and transverse magnetic field. The structures observed are quite similar to those found in artificially prepared two-dimensional periodic junction arrays where the transition from a phase-ordered to a phase-disordered state at T/sub c/ is attributed to the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory for phase transitions in two dimensions. The magnetic field data suggest that the typical cell size of the YBCO array is a few mu m, on the order of the grain size. This suggests that the ceramic films can be modeled as two-dimensional networks of Josephson tunnel junctions, the junctions being located at the grain boundaries. >

Thermally activated vortex motion in YBa2Cu3O7 wire networks

With a mutual inductance method we have measured the temperature dependence of the complex AC impedance Z = R + iωL of high quality YBa2Cu3O7 wire networks. In a perpendicular magnetic field the real and the imaginary part of Z exhibit oscillations resulting from flux quantization in the loops of the network. The relative amplitude of the oscillations of the inverse effective sheet inductance 1/L is consistent with a model where 1/L is renormalized by thermally activated vortex motion.