Weak Anharmonic Effects In Mgb2: A Comparative Inelastic X-Ray Scattering And Raman Study

We study anharmonic effects in MgB2 by comparing inelastic x-ray and Raman scattering together with ab initio calculations. Using high-statistics and high-q-resolution measurements we show that the E-2g-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 behavior inconsistent with all the anharmonic ab initio calculations of the E-2g mode at Gamma available in the 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 behavior of the Raman-peak energy. The Raman-E-2g-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.