DISORDERED FLAT PHASE AND PHASE-DIAGRAM FOR RESTRICTED SOLID-ON-SOLIDMODELS OF FCC(110) SURFACES

We discuss the results of a study of restricted solid-on-solid model H amiltonians for fee (110) surfaces. These models are simple modificati ons of the exactly solvable body-centered solid-on-solid model, and ar e able to describe a (2x1) missing-row reconstructed surface as well a s an unreconstructed surface. They are studied in two different ways. The first is by mapping the problem onto a quantum spin-1/2 one-dimens ional Hamiltonian of the Heisenberg type, with competing (SiSjz)-S-z c ouplings. The second is by standard two-dimensional Monte Carlo simula tions. We find phase diagrams with the following features, which we be lieve to be quite generic: (i) two flat, ordered phases (unreconstruct ed and missing-row reconstructed); a rough, disordered phase; an inter mediate disordered flat phase, characterized by monoatomic steps, whos e physics is shown to be akin to that of a dimer spin state. (ii) A tr ansition line from the (2x1) reconstructed phase to the disordered fla t phase showing exponents that appear to be close, within our numerica l accuracy, to the two-dimensional Ising universality class. (iii) A c ritical (preroughening) line with variable exponents, separating the u nreconstructed phase from the disordered flat phase. Possible signatur es and order parameters of the disordered flat phase are investigated.