Magnetic Penetration Depth Measurements with the Microstrip Resonator Technique

The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth A (T) in superconducting thin films. Because the method relies on measuring the resonant frequency of a high-Q transmission line resonator at microwave frequencies, one can very precisely measure small changes in it(T). This technique is applied to studying the low-temperature dependence of X (T), since that is in principle a measure of the low-lying pair-breaking excitations of the superconductor. We find that the penetration depth in niobium films is consistent with the predictions of weak coupled BCS theory. The low-temperature dependence of A (T) in c-axis YBa2CU3OT-8 films can be interpreted as either a weak exponential or as a power law. In addition, the measured value of A (0) is found to be strongly dependent on the form of the temperature dependence for A (T) used in fitting the data. Best fits over the entire temperature range are obtained with a BCS temperature dependence having values for 2A(O)/kBTc strictly less than 3.5, consistent with our measurements of the temperature dependence of A(T) at low temperatures in YBa2Cu307_8