Magnetic penetration depth measurements of superconducting thin films by a microstrip resonator technique

The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth, n(T), in superconducting thin films. Because the method relies on measuring the resonant frequency of a microwave transmission line resonator, one can very precisely measure small changes in A( T). This technique can resolve changes in ,4 on the order of several angstroms, allowing a direct measurement of the low-temperature behavior of A( T), which is a measure of the low-lying pair breaking excitations of the superconductor. Absolute penetration depth values can also be obtained from a self-consistent fit to the data to an assumed temperature dependence. Measurements of the penetration depth of Nb and NbCN film give results that are consistent with the predictions of BCS theory in which 2A/kT, is treated as an adjustable parameter, while YBa2Cu307--6 films give results that are not completely understood at this time. We also compare this technique with other methods of measuring the penetration depth of superconducting thin films, and discuss the systematic errors present in the measurement.