Free energy and stiffness of and steps on a (001) surface of a cubic lattrice: Revival of the solid-on-solid model

We have determined the stiffness and free energy of <110> and <010> oriented steps on a (001) surface of a cubic lattice within the framework of a solid-on-solid model having nearest-neighbor (NN) and next-nearest-neighbor (NNN) interactions. The stiffness of <110> oriented steps depends on both the strength of the NN and NNN interactions, whereas the stiffness of the <010> oriented steps only depends on the NNN interaction. For a vanishing NNN interaction the dimensionless inverse stiffness of the <010> oriented steps reduces to the universal value of 1/[square root of 2] as obtained by the Ising model. As an example we apply this solid-on-solid model to the well-studied Cu(001) surface.