Multiphonon effects in the one-phonon cross section of Al

We present a theoretical analysis of the phonon linewidth and energy shift observed with neutron spectroscopy in the one-phonon resonances. To take full account of the anharmonicity of the crystal we use a; molecular-dynamics approach to evaluate the atomic displacement-displacement correlation functions. The atoms in the crystal interact with a semiempirical many-body potential and anharmonic effects of all orders are taken into account. The linewidths are evaluated for Al at two different temperatures, 80 and 300 K, in the high-symmetry directions of the crystal. Our results reproduce well the experimental data. The larger damping of the longitudinal phonons compared to that one of transverse phonons is fully explained in terms of anharmonic effects. Our analysis also confirms the anomalous behavior of the energy shift for longitudinal phonons in the [111] direction.