Andreev Bound States in Superconducting Films and Confined Superfluid He-3

Abstract

This paper reviews the confinement-driven phase transitions, and the prediction of superfluid phases with broken time-reversal or translational symmetry in thin films. The new phases are a result of particle-hole coherent Andreev scattering processes that create quasiparticle states with energies inside the superconducting gap. These states cause profound restructuring of the low-energy spectrum in the surface region of several coherence lengths $\xi_0$ with large spatial variations of the superconducting order parameter. In confined geometry, such as slabs, films, pores, or nano-dots, with one or more physical dimensions $D \sim 10\xi_0$, Andreev bound states can dominate properties of the superfluid phases, leading to modified experimental signatures. They can dramatically change the energy landscape, and drive transitions into new superfluid phases, that are typically unstable in the bulk superfluid. On the examples of single-component singlet d-wave superconductor, and triplet multi-component superfluid $^3$He I show how properties of condensed phases in restricted geometry depend on the order parameter structure. I will highlight the connection between Andreev bound states and confinement-stabilized phases with additional broken symmetries, describe recent progress and open questions in theoretical and experimental investigation of superfluids in confined geometry.