Fingerprinting superconductors by disentangling Andreev and quasiparticle currents across tunable tunnel junctions

TL;DR

This paper introduces a tunneling spectroscopy method that distinguishes different superconducting pairing symmetries by analyzing the decay rates of Andreev and quasiparticle currents at the atomic scale.

Contribution

The study demonstrates that the additivity of excess decay rates allows for clear separation of Andreev and quasiparticle contributions, enabling identification of unconventional superconducting states.

Findings

Excess decay rate additivity enables separation of currents.

Andreev reflection dominates in s-wave superconductors.

Distinct spectral fingerprints identify unconventional pairing symmetries.

Abstract

Tunneling Andreev reflection (TAR) spectroscopy offers a powerful new approach to fingerprint superconducting pairing symmetry at the atomic scale. By leveraging the exponential sensitivity of excess tunneling decay rate to Andreev reflection, TAR robustly distinguishes between s-wave, d-wave, and more complex order parameters, overcoming limitations of traditional conductance-based techniques. Here, using atomistic superconducting transport simulations, we show that the additivity of excess decay rate enables clear separation of Andreev and quasiparticle currents. In particular, we reveal how their competition as well as higher-order scattering processes shape both the decay rate spectra and their dependence on the coupling strength. We show that this phenomenology stems from the fact that Andreev reflection dominates mid-gap conductance for s-wave superconductors, it is suppressed for the d-wave, and it coexists with quasiparticle tunneling in sign-changing symmetries if the expectation value for the superconducting gap remains finite. These distinct spectral fingerprints pave the way for atomically resolved identification of unconventional superconducting states.