Quasiclassical theory of superconductivity: a multiple interface geometry(II)

TL;DR

This paper introduces a new quasiclassical method for analyzing multiple coherent reflections at interfaces in superconducting structures, deriving boundary conditions and transfer matrices to study complex multi-layer systems.

Contribution

It develops a novel boundary condition framework for quasiclassical Green's functions at interfaces, enabling analysis of multi-channel scattering and superfluid density in layered superconductors.

Findings

The method accurately models interface scattering effects.

Superfluid density is highly sensitive to interface imperfections.

Low-temperature response can switch from paramagnetic to diamagnetic.

Abstract

A new method which allows one to study multiple coherent reflection/transmissions by partially transparent interfaces, (e.g., in multi-layer mesoscopic structures or grain boundaries in high-Tc's), in the framework of the quasiclassical theory of superconductivity is suggested. It is argued that in the presence of interfaces, a straight-line trajectory transforms to a simple connected 1-dimensional tree (graph) with knots, i.e. the points where the interface scattering events occur and pieces of the trajectories are coupled. For the 2-component trajectory "wave function" which factorizes the matrix Gor'kov Green's function, a linear boundary condition on the knot is formulated for an arbitrary interface, specular or diffusive (in the many channel model). From the new boundary condition, we derive: (i) the excitation scattering amplitude for the multi-channel Andreev/ordinary reflection/transmission processes; (ii) the boundary conditions for the Riccati equation; (iii) the transfer matrix which couples the trajectory Green's function before and after the interface scattering. To show the usage of the method, the cases of a film separated from a bulk superconductor by a partially transparent interface, and a SIS' sandwich with finite thickness layers, are considered. The electric current response to the vector potential (the superfluid density $\rho_s$) with the $\pi $ phase difference in S and S' is calculated for the sandwich. It is shown that the model is very sensitive to imperfection of the SS' interface: the low temperature response being paramagnetic ($\rho_s <0$) in the ideal system case, changes its sign and becomes diamagnetic ($\rho_s > 0$) when the probability of reflection is as low as a few percent.