Stepped surfaces of hcp helium-4 crystals

We have studied melting-freezing waves on helium-4 crystal surfaces which are tilted by a small angle φ with respect to the c facets. We present the first experimental evidence that a crossover exists from stepped to rough behavior as a function of angle. We demonstrate this by the independent measurement of the two components ${\gamma }_{\mathrm{\parallel }}$ and ${\gamma }_{\mathrm{\perp }}$ of the surface stiffness. In the stepped regime, below the crossover angle ${\mathrm{\phi }}_{\mathit{c}}$≊3°, we also measured both elastic and entropic contributions to the step interactions.