Magnetic penetration depth measurements in cuprate superconductors

We examine recent results on measurements of the magnetic penetration depth in cuprate superconductors, with particular emphasis on our results obtained with the microstrip and parallel-plate resonator techniques. The results show that the magnetic penetration depth temperature dependence in YBa₂Cu₃O_7– is not consistent with a simple scaled weak-coupled BCS temperature dependence. We present three classes of interpretations of these measurements, and how they relate to other experimental studies of the cuprates. Finally, these interpretations are discussed in terms of some theories of cuprate superconductivity.