Penetration depth in conventional layered superconductors: A proximity-effect model

We propose a theory for thr penetration depth lambda(T) of superconducting bilayers and multilayers, which are composed of two dissimilar superconduct ing layers S and S' with arbitrary coupling strength, and with the S layer thickness (T-c,T-S > T-c,T-S') less than or comparable to its coherence len gth. Within the framework of the theory, we discuss the influences of the S ' layer parameters, and of the coupling strength between the S' and S layer s. We show that their variations lead to a variety of temperature dependenc es of lambda(T). Many of the basic features observed experimentally in conv entional SS' structures have been reproduced. The theory begins with an ext ension of the proximity-effect model developed by Golubov et al. [Phys. Rev . B 51, 1073 (1995)], which is based on the Usadel equations. It therefore applies to the dirty SS' system, and provides a description of the supercon ducting properties over the entire temperature range below T-c, the transit ion temperature of the system. We shall compare our model with a phenomenol ogical model developed within the proximity-effect theory which is applicab le near T-c. A brief discussion on the highly anisotropic systems in connec tion with the intrinsically layered high-T-c superconductors is also presen ted.