Low frequency (5 MHz) impedance measurements of thick YBCO films

Abstract

Two-point low frequency impedance measurements of microstrip transmission lines, fabricated from thick YBCO ( + 10 wt, 9% Ag) films on YSZ and sapphire substrates, have provided intrinsic properties for these granular thick films. The variation of the resistance with temperature for all the films studied, with and without an external DC magnetic field, followed that found by the conventional four-point DC technique. However, the temperature variation of the reactance for these granular thick films showed four important features: (1) for temperatures below T c ( R=0) the reactance was purely inductive even when superconductivity had been destroyed by the application of an external magnetic field, but the tail of the resistive transition was still present; (2) high quality, melt textured, thick films had a small double peak (or broad single peak) structure in the inductance for temperatures above T c which corresponded to changes in the tail of the resistivity transition and these peaks are interpreted in terms of magnetic field penetration into the inter- and intra- granular regions; (3) poor quality films and those intentionally doped with Y 2BaCuO 5 had a constant inductive value extending from low temperatures to a temperature corresponding to the end of the resistive tail; (4) as the temperature is increased beyond the onset of superconductivity the inductive reactance decreased rapidly, for all films studied, to a negative value (i.e. a capacitive reactance) and then decreased approximately linearly for further increases in temperature. The value of the capacitive reactance appeared to depend on the amount of (211) YBCO present in the film. Analysis of the film conducting path in terms of parallel conductance and susceptance components gives rise to a temperature dependence very similar to the susceptibility curves obtained from bulk YBCO material and their observed structure may be interpreted in terms of a granular model.