Anharmonic effects at the Ni(100) surface

Using molecular dynamics with embedded-atom potentials, we investigate the role of anharmonicities at the (100) surface of Ni. In accord with recent low-energy electron diffraction (LEED) measurements [Y. Cao and E. Conrad, Phys. Rev. Lett. 65, 2808 (1990)], we observe an anomalously large thermal expansion of the surface, with a concomitant rapid increase of the mean-square amplitudes of vibration, and an extraordinary attenuation of the LEED intensities. These anomalies appear at a temperature of about 900 K, well below the onset of disordering. We find no evidence of anomalous softening of the surface phonons. Our results suggest that the observed anharmonic behavior results solely from enhanced vibrational motion.