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 disorde r exhibit two features in the temperature dependence of the microwave surface impedance which are consistent with a weak s-wave superconduct ing state associated with the CuO chains. First, both the microwave su rface resistance and the magnetic field penetration depth exhibit a we ak exponential temperature dependence below 30 K corresponding to an e nergy gap of about 6 meV. Second, the penetration depth exhibits a sma ll bump at about 60 K, which is consistent with a two-band BCS calcula tion using 6 meV for the smaller gap.