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

We study the phase diagram of the body-centered solid on solid (BCSOS) model with an extended interaction range using transfer matrix techniques, pertaining to the (100)surface of single-component fee and bcc crystals. The model shows a 2X2 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 tricritical Ising behavior may be a possible explanation for the non-Ising exponents found in those models.