Interpretation of high pressure experimental data in mgb₂: a challenge to current theories

Abstract

We review various anomalies seen in Raman spectroscopy, superconducting transition temperature T_c and electrical resistance at room temperature and high pressure in the new high temperature superconductor MgB₂. In order to explore the possibility of the anomalies being caused by an electronic topological transition (ETT), we have carried out detailed electronic structure studies of the band structure under pressure. Our calculations are done in the presence of a frozen-in phonon displacement that mimics the effects of thermal vibrations of the B-atoms at room temperature. Our results clearly establish the occurrence of a phonon-assisted ETT near 18 GPa. The dimensionless electron-phonon coupling λ is also found to be anomalous in its pressure variation at the same pressure. We also indicate why a detailed explanation of the phonon Raman anomaly needs the inclusion of effects arising from a non-linear electron-phonon coupling.