EVIDENCE OF 2-GAP S-WAVE SUPERCONDUCTIVITY IN YBA2CU3O7-X FROM MICROWAVE SURFACE IMPEDANCE MEASUREMENTS

Epitaxial thin films of YBa2Cu3O7-x with low cation and oxygen disorder exhibit two features in the temperature dependence of the microwave surface impedance which are consistent with a weak s-wave superconducting state associated with the CuO chains. First, both the microwave surface resistance and the magnetic field penetration depth exhibit a weak exponential temperature dependence below 30 K corresponding to an energy gap of about 6 meV. Second, the penetration depth exhibits a small bump at about 60 K, which is consistent with a two-band BCS calculation using 6 meV for the smaller gap.