Comment on "Uniqueness of the complex diffraction amplitude in x-ray Bragg diffraction"

We propose a theory for thr penetration depth lambda(T) of superconducting bilayers and multilayers, which are composed of two dissimilar superconducting 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 length. 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 layers. We show that their variations lead to a variety of temperature dependences of lambda(T). Many of the basic features observed experimentally in conventional SS' structures have been reproduced. The theory begins with an extension 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 superconducting properties over the entire temperature range below T-c, the transition temperature of the system. We shall compare our model with a phenomenological model developed within the proximity-effect theory which is applicable near T-c. A brief discussion on the highly anisotropic systems in connection with the intrinsically layered high-T-c superconductors is also presented.