Temperature Effect in Surface Reconstruction of Clean and Hydrogen-Adsorbed W(001) Surface

Abstract

It is now well-known that the clean W(001) surface undergoes a reconstructive phase-transition at certain temperature T_c and exhibits a c(2×2) structure at low temperatures. From various experimental and theoretical sources, it is concluded that this surface reconstruction is a second order transition caused by spontaneous displacements of surface atoms along a direction parallel to the surface as depicted in Fig.1 [1]. The reconstructed structure of W(001)is somehow stabilized when hydrogen atoms are adsorbed, so long as the coverage of hydrogen is small [2], As to the mechanism of the phase-transition, several models have been proposed [3], but at present it seems that an accepted view point is to attribute the origin of driving transition to the competition between stabilizing band electronic force and opposing ion-core repulsive force. Among others, TERAKURA et al. gave an electronic theory of the surface reconstruction on W(001) [4] which could explain even quantitatively the relevant experimental facts for absolute zero T=0. As to temperature-variation of this transition, however, there are a few theories in which the temperature-effect is seriously considered. Probably this is due to the fact that the main source of information on the temperature-dependence of the “order parameter” is LEED experiments, which are much influenced by large surface vibration as far as the temperature-effect is concerned. Indeed, for instance, we can observe in Fig.2, with increasing temperature, strong exponential decay of the scattering intensity of the LEED patterns characteristic to the low-temperature phase [5]. If we look at Fig.2 as a temperature-dependence curve for the “order parameter” of the phase-transition, it would be quite unusual in contrast to the standard behaviour of the order parameter in the second order phase-transition: The transition-temperature is obscured by the exponential tail.