Surface charge density waves and the Mott insulators for √3 x √3 adlayers on (111) semiconductor surfaces

We discuss our recent studies on the lots-temperature instabilities of the root3 x root3? phase of tetravalent adatoms on (111) semiconductor surfaces. The cases of interest include the surface charge density wave (CDW) systems Pb/Ge(1 1 1) and Sn/Ge(1 1 1), as well as the Mott insulators Si/SiC(0 0 0 1) and K/Si(1 1 1):B. We have approached the problem in two ways: first, by employing a one-band model Hamiltonian of the Hubbard-Holstein type, in order to understand general features of the phase diagram as a function of the strength of electron-electron (e-e) and electron-phonon (e-ph) interactions; second, by performing realistic ab initio calculations within the local spin density approximation (LSDA) for the case of Sn/Si(1 1 1), and of a hypothetical root3 x root3 Si/Si(1 1 1) mimicking K/Si(1 1 1):B. The collinear LSDA calculation for both Sn/Ge(1 1 1) and Si/Si(1 1 1) predicts a spin density wave (SDW) state with a uniform magnetization m(z) = 1/3 and a small secondary CDW. We discuss and stress the likely important role played by e-e interactions in explaining the phenomenology of all these systems, as opposed to the secondary role played by the e-ph coupling, which would at most drive the lattice, for Pb-Sn/Ge(1 1 1), after the electrons have caused the transition. (C) 2001 Elsevier Science B.V. All rights reserved.