The mechanism for the 3 x 3 distortion of Sn/Ge(111)

We show that two distinct 3 x 3 ground states - one non-magnetic, metallic and distorted; the other magnetic, semi-metallic (or insulating) and undist orted - compete in alpha-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(lll), local (spin) density approximation( LSDA) and GGA calculatio ns indicate, in agreement with experiment, that the distorted metallic grou nd state prevails. The reason for the stability of this state is analysed, and traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a Ge-Ge bond directly und erneath. (C) 2000 Elsevier Science B.V. All rights reserved.