Abstract
We present a microscopic study of the behavior of the order parameters near boundaries of a two-band superconducting material, described by the standard tight-binding Bardeen-Cooper-Schrieffer model. We find superconducting surface states. The relative difference between bulk and surface critical temperatures is a nontrivial function of the interband coupling strength. For superconductors with weak interband coupling, boundaries induce variations of the gaps with the presence of multiple length scales, despite non-zero interband Josephson coupling.
Highlights
The majority of the superconductors of current interest have multiple superconducting bands [1,2] with a widely varying strength of the interband coupling
We denote by Tc1 the bulk critical temperature, i.e., when the order parameter vanishes in the bulk
Since most of the superconductors of current interest are multiband, and there are experimental puzzles, such as the surface gap enhancement in ZrB12 [13,14,15], it is important to understand the boundary effects in multiband superconducting materials
Summary
The majority of the superconductors of current interest have multiple superconducting bands [1,2] with a widely varying strength of the interband coupling. These materials exhibit topological surface currents, the observation of which is searched as a smoking gun for topological superconductivity and can potentially be used to understand the nature of it. In the presence of surface superconductivity, it is important to study the gap ratio properties both in the bulk and near the boundaries. The series of experimental works [13,14,15] reported that, on the surface of ZrB12, the characteristics of the superconducting gaps are widely different compared to the bulk. Recent works [26,27] reported that enhanced superconductivity near the boundary is a generic property of the standard single-band Bardeen-Cooper-Schrieffer model. We focus on the limit of a clean ideal surface, with negligible single-particle interband scattering [28]
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