Roughening and preroughening in the six-vertex model with an extended range of interaction.

We study the phase diagram of the BCSOS model with an extended interaction range using transfer matrix techniques, pertaining to the (100) surface of single component fcc and bcc crystals. The model shows a 2×2 reconstructed phase and a disordered flat phase. The deconstruction transition between these phases merges with a Kosterlitz-Thouless line, showing an interplay of Ising and Gaussian degrees of freedom. As in studies of the fully frustrated XY model, exponents deviating from Ising are found. We conjecture that tri-critical Ising behavior may be a possible explanation for the non-Ising exponents found in those models. PACS. 64.60 Fr – Equilibrium properties near critical points, critical exponents PACS. 64.60 Cn – Order-disorder and statistical mechanics of model systems PACS. 64.60 Kw – Multicritical points PACS. 68.35 Rh – Phase transitions and critical phenomena Typeset using REVTEX