Microwave surface impedance of high-quality YBa 2Cu 3O 7− x thin films

Abstract

The parallel plate resonator method has been used for measuring high quality YBa 2Cu 3O 7− x (YBCO) thin films. The surface resistance and the real part of 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 linearly at low temperatures. The lowest penetration depth linear fitting has a slope value of 2.2 to 2.5 Å/K up to 20 K which is lower than previous measurements on YBCO single crystals. An interpretation of this smaller slope in terms of the generally accepted d-wave order parameter symmetry presents difficulties.