Multi-gap superconductivity in MgB2: Magneto-Raman spectroscopy

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 and 78 cm−1 which we identify as the superconducting gaps in π- and σ-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.