Stepped surfaces of hcp helium-4 crystals

Abstract

We have studied melting-freezing waves on helium-4 crystal surfaces which are tilted by a small angle \ensuremath{\varphi} 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 ${\ensuremath{\gamma}}_{\mathrm{\ensuremath{\parallel}}}$ and ${\ensuremath{\gamma}}_{\mathrm{\ensuremath{\perp}}}$ of the surface stiffness. In the stepped regime, below the crossover angle ${\mathrm{\ensuremath{\varphi}}}_{\mathit{c}}$\ensuremath{\approxeq}3\ifmmode^\circ\else\textdegree\fi{}, we also measured both elastic and entropic contributions to the step interactions.