Andreev - Saint James reflections: a probe of cuprate superconductors

TL;DR

This paper reviews Andreev-Saint James reflections in superconductors, highlighting their application to cuprates and revealing insights into electron-hole symmetry and pseudo-gap phenomena across doping levels.

Contribution

It provides a comprehensive historical and experimental review of ASJ reflections in cuprates, emphasizing their role in understanding superconducting and pseudo-gap states.

Findings

ASJ reflections are observed across all studied cuprates.

No significant electron-hole asymmetry near the Fermi level in the superconducting state.

Pseudo-gap values from ASJ are smaller than those from other methods, indicating a lack of coherence.

Abstract

Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint James and de Gennes and by Andreev, and their spectroscopic features discussed in detail by Saint James in the early sixties. They are generally called Andreev reflections but, for that reason, we call them Andreev - Saint James (ASJ) reflections. We present a historical review of ASJ reflections and spectroscopy in conventional superconductors, and review their application to the High $T_c$ cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy range.