Evidence of two-gap s-wave superconductivity in YBa2Cu3O7-x from microwave surface impedance measurements.

Abstract

Epitaxial thin films of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ with low cation and oxygen disorder exhibit two features in the temperature dependence of the microwave surface impedence 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.