Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy

TL;DR

This study uses magneto-Raman spectroscopy to identify multiple superconducting gaps and collective modes in MgB2, revealing detailed insights into its superconducting and phononic properties.

Contribution

It provides the first detailed Raman spectroscopy analysis of multiple superconducting gaps, Leggett mode, and phonon effects in MgB2, advancing understanding of its superconductivity.

Findings

Identification of three superconducting features: a clean gap and two coherence peaks.

Observation of Leggett's collective mode related to phase fluctuations.

Detection of phonon frequency hardening and linewidth changes below Tc.

Abstract

Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78 cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.