FIRST-PRINCIPLES CALCULATIONS OF CHARGE AND SPIN-DENSITY WAVES OF ROOT-3-ADSORBATES ON SEMICONDUCTORS

We present ab-initio electronic structure results on the surface of ro ot 3 x root 3 adsorbates. In particular, we address the issue of metal -insulator instabilities, charge-density waves (CDWs) or spin-density waves (SDWs), driven by partly filled surface states and their 2D Ferm i surface, and/or by the onset of magnetic instabilities. The focus is both on the newly discovered commensurate CDW transitions in the Pb/G e(111) and Sn/Ge(111) structures, and on the puzzling semiconducting b ehavior of the Pb/Ge(111), K/Si(111):B and SiC(0001) surfaces. In all cases, the main factor driving the instability appears to be an extrem ely narrow surface state band. So far, we have carried out preliminary calculations for the Si/Si(111) surface, chosen as our model system, within the gradient corrected local density (LDA + GC) and local spin density (LSD + GC) approximations, with the aim of understanding the p ossible interplay between 2D Fermi surface and electron correlations i n the surface +adsorbate system. Our spin-unrestricted results show th at the root 3 x root 3 paramagnetic surface is unstable towards a comm ensurate density wave with periodicity 3 x 3 and magnetization 1/3. (C ) 1998 Elsevier Science B.V. All rights reserved.