PREROUGHENING, FRACTIONAL-LAYER OCCUPANCIES, AND PHASE-SEPARATION AT A DISORDERED FLAT METAL-SURFACE

We study the phase separation that a particle-conserving (''canonical' ') full-layered metal surface must undergo at temperatures between pre roughening and roughening. The separation is into two disordered flat (DOF) domains of opposite order parameter with a step between them, ea ch domain exhibiting a half-filled top layer. It is shown that both Gi bbs-ensemble simulation and canonical Monte Carlo plus finite-size sca ling, carried out on a specific lattice Hamiltonian model, demonstrate this phase-separation phenomenon microscopically. A number of existin g particle-conserving molecular-dynamics simulations for fcc(110) meta l surfaces are then analyzed, and it is found that some display clear, previously unnoticed evidence of this DOF phase separation. Its main signal is a plateau of layer occupancies vith temperature, around valu es close to 3/4 for the first surface layer, and around 1/4 for the ad atom layer. It is proposed that this unusual type of phase separation could be observable on sufficiently step-free metal surfaces.