Preroughening Transitions in Surfaces

We introduce a new type of phase of crystal surface and interfaces. This disordered flat phase appears intermediate between the familiar flat and rough phases in the presence of short-range interactions of a type common in experiments. The surface remains flat on average although it contains a disordered array of steps. The preroughening transition into the disordered flat phase belongs to a new universality class. Finite-size-scaling calculations for the restricted solid-on-solid model confirm the existence of the disordered flat phase and the preroughening transition.