Penetration depths of high T c films measured by two-coil mutual inductances

A fundamental parameter of sample quality in epitaxial films of high Tc oxides is the effective penetration depth λ∥ of the superconducting sheet. A contactless audio-frequency method is described, in which an epitaxial YBa2Cu3O7 film is sandwiched between sets of stacked coils, and the procedure is given for computing the complex sheet impedance and hence λ∥ from the change in mutual inductance produced by screening currents in the film. Temperature dependence of the complex impedance is presented.