SYMMETRY-BREAKING RELAXATION OF VACANCIES ON SI(111)2X1

We have investigated the properties of an isolated neutral vacancy on the Si(111) 2 x 1 surface using the density-functional-molecular-dynamics method. The lattice distortions around the vacancy, its formation energy and activation barrier for migration along the pi-bonded chains, as well as the nature and the energy position of the vacancy-induced surface states are studied. We find that the relaxations near the vacancy break the mirror-plane symmetry of the surface, with a charge-transfer disproportionation of two previously equivalent surface atoms, plus formation of a weak bond. It is suggested that these features could possibly be detected using scanning tunneling microscopy.