Stiffening Transition in Vicinal Surfaces with Adsorption

We study the vicinal surface with adsorption below the rougheningtemperature, using the restricted solid-on-solid model coupled with the Isingmodel. We calculate the step tension $\gamma$ and the step-interactioncoefficient $B$ by employing a variant of the density matrix algorithm. We finda ``stiffening transition'' at a temperature $T_{\rm s}$ where $B$ vanishes. At$T_{\rm s}$ surface free energy has the form $f(p)- f(0)= \gamma p +({const.})p^5 + ...$ ($p$: surface gradient), which differs from the well-known$p$-$p^3$ form.