c-axis Raman Scattering in MgB2: Observation of a Dirty-Limit Gap in the pi-bands

TL;DR

This study uses Raman scattering to investigate the superconducting gaps in MgB2, revealing that impurity scattering predominantly affects the pi-bands, distinguishing between clean and dirty limit behaviors.

Contribution

It provides the first detailed Raman analysis distinguishing the clean and dirty limit gaps in MgB2's sigma and pi bands.

Findings

Pi-bands exhibit dirty-limit superconductivity due to impurity scattering.

Sigma-bands show characteristics of a clean superconductor.

Different Raman polarization responses reveal the distinct gap behaviors.

Abstract

Raman scattering spectra from the ac-face of thick MgB2 single crystals were measured in zz, xz and xx polarisations. In zz and xz polarisations a threshold at around 29 cm^{-1} forms in the below Tc continuum but no pair-breaking peak is seen, in contrast to the sharp pair-breaking peak at around 100 cm^{-1} seen in xx polarisation. The zz and xz spectra are consistent with Raman scattering from a dirty superconductor while the sharp peak in the xx spectra argues for a clean system. Analysis of the spectra resolves this contradiction, placing the larger and smaller gap magnitudes in the sigma and pi bands, and indicating that relatively strong impurity scattering is restricted to the pi bands.