Interplay between evanescent scattering modes and finite dispersion in superconducting junctions

TL;DR

This paper reexamines assumptions about scattering in superconducting junctions, highlighting the importance of evanescent modes and finite dispersion, and reveals new mechanisms for gap closing linked to symmetries and constraints.

Contribution

It generalizes Beenakker's determinant formula to include energy-dependent scattering and evanescent modes, uncovering new gap closing mechanisms and symmetry considerations.

Findings

Evanescent modes are crucial in scattering with finite dispersion.

Two mechanisms preserve gap closing despite energy-dependent scattering.

New constraints on scattering matrices extend beyond probability conservation.

Abstract

Superconducting junctions are essential building blocks for quantum hardware, and their fundamental behaviour remains a highly active research field. The behaviour of generic junctions is conveniently described by Beenakker's determinant formula, linking the subgap energy spectrum to the scattering matrix characterising the junction. In particular, the gap closing between bound and continuum states in short junctions follows from unitarity of the scattering matrix, and thus, from probability conservation. In this work, we critically reassess two assumptions: that scattering in short junctions is approximately energy-independent and dominated by planar channels. We argue that strongly energy-dependant scattering follows from finite dispersion of the conductor electrons even when they spend little time within the scattering region, and show that evanescent modes play a central role when cross-channel scattering is important. By generalising Beenakker's equation and performing a mapping to an effective Hamiltonian, we show that the gap closing is linked to a chiral symmetry. While finite energy dependence in the scattering breaks the chiral symmetry, we show two distinct mechanisms preserving the gap closing, each connected to new types of constraints on energy-dependant scattering matrices beyond unitarity. If the dispersive mode is planar, the gap closing is still preserved through a time-dependant probability conservation analysis of the scattering process. If the dispersive channel is evanescent, we derive a constraint which, notably, cannot follow from probability conservation. We thus demonstrate that Andreev physics reveal a much wider variety of properties of normal-metal scattering than commonly expected. We expect that our findings will have an impact on the dissipative behaviour of driven junctions, and offer a new perspective on fundamental properties of scattering matrices.