Microwave surface impedance of high-quality YBa2Cu3O7-x thin films

The parallel plate resonator method has been used for measuring high qualit y YBa2Cu3O7-x (YBCO) thin films. The surface resistance and the real part o f the conductivity show a non-monotonic behaviour with a broad peak around 45 K. The penetration depth and the real part of the conductivity vary line arly at low temperatures. The lowest penetration depth linear fitting has a slope value of 2.2 to 2.5 Angstrom/K up to 20 K which is lower than previo us measurements on YBCO single crystals. An interpretation of this smaller slope in terms of the generally accepted d-wave order parameter symmetry pr esents difficulties. (C) 1999 Published by Elsevier Science B.V. All rights reserved.