SURFACE STEPS AS INTERACTING FERMIONS - AN APPROACH TO THE DISORDERING OF SURFACES WITH SUBLATTICES

The problem of thermal structural disordering of the fee (110) crystal surface is formulated in terms of a system of interacting domain wall s, representing the elementary defects ((111) steps) of the flat surfa ce. The method is quite general and may be extended to other systems, such as the fee (111) surfaces. The theory makes use of the known (app roximate) mapping of the two-dimensional statistical mechanics problem on a one-dimensional fermionic hamiltonian, but here two fermionic ba nds, multiple-hopping and short-range finite interactions must also be included. The phase diagram of a recently proposed extended body-cent red solid-on-solid (BCSOS) model of noble-metal fee (110) surfaces is derived by means of a mean-field calculation for the ground state ener gy of the quantum hamiltonian which has the advantage of reproducing t he exact classical ground state structure. Good agreement with results previously obtained by Monte Carlo simulation on the BCSOS model is o btained, indicating that the present treatment is qualitatively, as we ll as semiquantitatively, correct as far as the phase structure is con cerned. Deconstruction, pre-roughening and roughening transitions are detected and the criteria for the occurrence of these disordering phen omena are recast in terms of microscopic step-step interaction paramet ers.