Abstract
Interaction of steps determines step distribution of a crystal in growth (melting) as well as in equilibrium. Thereby it controls the growth law and the crystal shape. Among various kinds of step interactions, the molecular potential of the rigid lattice, the elastic interaction and the interaction due to statistical fluctuation of steps are common and believed to be important. They depend on the step distance d as 〜d^<n+3> (n: power of the molecular potential), 〜d^<-2> and 〜d^<-2>, respecuvely. In a quantum system, a step is viewed as an oscillating string accompanied by superfluid flow. When there are many parallel steps, the oscillation spectrum is modified by the interference of the flow, and as a result the zero-point energy increases, leading to a 〜d^<-2> repulsion of steps.
