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.
@article{arxiv.cond-mat/0703126,
title = {Anharmonic effects in MgB2? A comparative inelastic X-ray scattering and Raman study},
author = {Matteo d'Astuto and Matteo Calandra and Stephanie Reich and Abhay Shukla and Michele Lazzeri and Francesco Mauri and Janusz Karpinski and N. D. Zhigadlo and Alexei Bossak and Michael Krisch},
journal= {arXiv preprint arXiv:cond-mat/0703126},
year = {2009}
}