Magnetic penetration depth measurements with the microstrip resonator technique

Abstract

The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth λ(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 λ(T). This technique is applied to studying the low-temperature dependence of λ(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 λ(T) inc-axis YBa2Cu3O7−δ films can be interpreted as either a weak exponential or as a power law. In addition, the measured value of λ(0) is found to be strongly dependent on the form of the temperature dependence for λ(T) used in fitting the data. Best fits over the entire temperature range are obtained with a BCS temperature dependence having values for 2Δ(0)/kBTc strictly less than 3.5, consistent with our measurements of the temperature dependence of λ(T) at low temperatures in YBa2Cu3O7−δ.