Microwave surface resistance of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ films: studies on oxygen deficiency and disordering

Abstract

The authors have studied the microwave surface resistance R/sub S/ at 19 GHz of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ films prepared by high oxygen pressure DC-sputtering on NdGaO/sub 3/ and LaAlO/sub 3/ substrates. Oxygen deficiency x, O<or=x<or=0.3, and disordering were varied by subsequent annealing in molecular as well as in activated oxygen. Fully oxygenated films exhibit an exp (- alpha T/sub c//T) dependence of R/sub S/ below T/sub c//2, with alpha =0.4. Between 60 and 80 K a much stronger exponential behavior with ( alpha and 4) could be clearly separated from the steep falloff of R/sub S/(T) between 80 and 92 K. For an oxygen-deficient film with T/sub c/=83 K, R/sub S/(T) was changed significantly by postannealing between 100 degrees C and 250 degrees C. It is suggested that the weak exponential behavior below T/sub c//2 and the strong dependence of R/sub S/(T) on oxygen disordering is associated with induced superconductivity of charge carriers in the CuO chains. The strong exponential behavior at higher temperatures for fully oxygenated film is attributed to the intrinsic energy gap of the CuO/sub 2/ planes.<<ETX>>