De Haas–van Alphen effect in MgB 2 crystals

Abstract

In this paper we review and expand our previous report concerning the observation of de Haas–van Alphen (dHvA) oscillations in single crystals of the high temperature superconductor MgB 2. The analysis of these oscillations provides an important and precise check of band-structure calculations, as well as a measure of the many body electron–phonon interactions, which enhance the quasi-particle effective masses and give rise to superconductivity. Orbits are observed on both the σ and π bands, with dHvA frequencies in good agreement with recent calculations. By comparing the measured quasi-particle effective masses with band masses calculated recently by three groups, we find that the electron–phonon coupling strength λ is a factor ∼3 larger on the 2D σ orbits than on the 3D π orbits, in accord with recent microscopic calculations. We are therefore able to verify two of the key ingredients in the two-gap model of superconductivity in this compound, namely the Fermi surface topology and the disparity in the electron–phonon coupling for the σ and π bands.