MULTIPHONON EFFECTS IN THE ONE-PHONON CROSS-SECTION OF AL

We present a theoretical analysis of the phonon linewidth and energy s hift observed with neutron spectroscopy in the one-phonon resonances. To take full account of the anharmonicity of the crystal we use a; mol ecular-dynamics approach to evaluate the atomic displacement-displacem ent correlation functions. The atoms in the crystal interact with a se miempirical many-body potential and anharmonic effects of all orders a re taken into account. The linewidths are evaluated for Al at two diff erent temperatures, 80 and 300 K, in the high-symmetry directions of t he crystal. Our results reproduce well the experimental data. The larg er damping of the longitudinal phonons compared to that one of transve rse phonons is fully explained in terms of anharmonic effects. Our ana lysis also confirms the anomalous behavior of the energy shift for lon gitudinal phonons in the [111] direction.