SYSTEMATIC TRENDS IN THE NORMAL ENHANCEMENT OF THE PHONON ANHARMONICITY AT THE SURFACE OF METALS

The temperature dependence of the Rayleigh wave frequency measured by He atom scattering for the surfaces Al(001), Al(111), and Cu(001) is compared with bulk data in order to establish the surface enhancement of phonon anharmonicity. A comparison is made also with recent HREELS data for the Cu(110) surface. In all cases the surface anharmonicity is found to be less than or at most of the order of twice the bulk anharmonicity, in agreement with recent calculations of the enhancement of the zero-point mean-square displacements.