Optical absorption in the strong coupling limit of Eliashberg theory

TL;DR

This paper investigates the optical conductivity of superconductors in the strong-coupling limit, revealing universal absorption features dominated by bound states, distinct from weak-coupling behavior, and showing these features appear at intermediate coupling levels.

Contribution

It introduces a universal frequency dependence of optical absorption in the strong-coupling limit of Eliashberg theory, highlighting the dominance of bound states and their presence at intermediate coupling.

Findings

Universal optical absorption dominated by bound states

Significant differences from weak-coupling results

Features of strong-coupling limit appear at intermediate coupling

Abstract

We calculate the optical conductivity of superconductors in the strong-coupling limit. In this anomalous limit the typical energy scale is set by the coupling energy, and other energy scales such as the energy of the bosons mediating the attraction are negligibly small. We find a universal frequency dependence of the optical absorption which is dominated by bound states and differs significantly from the weak coupling results. A comparison with absorption spectra of superconductors with enhanced electron-phonon coupling shows that typical features of the strong-coupling limit are already present at intermediate coupling.