The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?

TL;DR

This paper investigates the optical properties of Ba_{1-x}K_xBiO_3 and suggests an unconventional coupling mechanism for superconductivity involving high-energy excitations beyond standard electron-phonon interactions.

Contribution

It introduces a model combining weak electron-phonon coupling with high-energy excitations to explain the superconducting behavior.

Findings

Normal state coupling  0.2

Superconducting state coupling  0.7

Evidence for high-energy excitation near 0.4 eV

Abstract

We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the far-infrared region using Migdal-Eliashberg theory and found it inconsistent with standard electron-phonon coupling: Whereas the superconducting state data could be explained using moderate coupling, \lambda=0.7, the normal state properties indicate \lambda \le 0.2. We have found that such behaviour could be understood using a simple model consisting of weak standard electron-phonon coupling plus weak coupling to an unspecified high energy excitation near 0.4 eV. This model is found to be in general agreement with the reflectivity data, except for the predicted superconducting gap size. The additional high energy excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is not standard electron-phonon.