Anharmonic effects in MgB2? A comparative inelastic X-ray scattering and Raman study

TL;DR

This study compares inelastic X-ray and Raman scattering with ab-initio calculations to investigate anharmonic effects in MgB2, revealing that anharmonicity is not the primary factor influencing phonon behavior and challenging previous interpretations.

Contribution

The paper provides a comprehensive experimental and theoretical analysis showing anharmonicity is not the main cause of phonon behavior in MgB2, contradicting prior assumptions.

Findings

E2g mode linewidth is temperature-independent along Gamma-A

Raman peak energy decreases with increasing temperature, contrary to anharmonic predictions

Excellent agreement between measured and calculated phonon spectra

Abstract

We study anharmonic effects in MgB2 by comparing Inelastic X-ray and Ramanscattering together with ab-initio calculations. Using high statistics and high q resolution measurements we show that the E2g mode linewidth is independent of temperature along Gamma-A. We show, contrary to previous claims, that the Raman-peak energy decreases as a function of increasing temperature, a behaviour inconsistent with all the anharmonic ab-initio calculations of the E2g mode at Gamma available in literature. These findings and the excellent agreement between the X-ray measured and ab-initio calculated phonon spectra suggest that anharmonicity is not the main mechanism determining the temperature behaviour of the Raman-peak energy. The Raman E2g peak position and linewidth can be explained by large dynamical effects in the phonon self-energy. In light of the present findings, the commonly accepted explanation of the reduced isotope effect in terms of anharmonic effects needs to be reconsidered.