Observation of a Novel Double-Step Phase in Pb(110) Surface

We report the observation of a novel phase on the Pb(110) surface below 400 K using the high-resolution low-energy electron diffraction technique. The surface contains two layers of atoms separated by double steps. We suggest that the observed phase may be related to a class of disordered flat phases predicted recently by Rommelse and den Nijs. As the temperature approaches 415 K, the double-step surface thermally transforms into a Kosterlitz-Thouless rough surface in which multilayers of atoms are involved and the steps have predominantly the single-atomic-step height.