Disorder induced collapse of the electron phonon coupling in MgB$_{2}$ observed by Raman Spectroscopy

TL;DR

This study uses Raman spectroscopy to show how disorder in MgB$_{2}$ affects its electron-phonon coupling and superconducting gaps, revealing a transition to an isotropic state at 19 K.

Contribution

It provides experimental evidence linking disorder-induced changes in MgB$_{2}$ to the collapse of electron-phonon coupling and gap convergence.

Findings

Electron-phonon coupling constant decreases from 1.22 to 0.80 with disorder.

Correlation between $T_c$ and $E_{2g}$ mode frequency observed.

Transition to an isotropic superconducting state at 19 K confirmed.

Abstract

The Raman spectrum of the superconductor MgB$_{2}$ has been measured as a function of the Tc of the film. A striking correlation is observed between the $T_{c}$ onset and the frequency of the $E_{2g}$ mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an isotropic state at 19 K. The value of the electron phonon coupling constant is found to be 1.22 for films with T$_{c}$ 39K and 0.80 for films with T$_{c}\leq$19K.