We investigate the thermal behavior of the (111) surface of silver, using phonon frequencies obtained from ab initio total energy calculations, and anharmonic effects treated within a quasiharmonic approximation. Our results reproduce the experimental observation of a large and anomalous increase in the surface thermal expansion at high temperatures [P. Statiris et al., Phys. Rev. Lett. 72, 3574 (1994)]. Surprisingly, we find that this increase can be attributed to a rapid softening of the in-plane phonon frequencies, rather than due to the anharmonicity of the out-of-plane surface phonon modes. This provides evidence for a new mechanism for the enhancement of surface anharmonicity. A comparison with Al(111) shows that the two surfaces behave quite differently, with no evidence for such anomalous behavior on Al(111).
@article{arxiv.mtrl-th/9511001,
title = {Unusually Large Thermal Expansion of Ag(111)},
author = {Shobhana Narasimhan and Matthias Scheffler},
journal= {arXiv preprint arXiv:mtrl-th/9511001},
year = {2008}
}