Roughening transitions on the helium crystal-superfluid interface

Abstract

We have studied the equilibrium shapes of large ⁴He crystals at two different roughening transitions (T_R1 = 1.2 K, T_R2 = 0.9 K) by a simple optical technique. The method used provides the temperature and angular dependences of the surface stiffness. The measured surface stiffness appears to be constant in the close vicinities of the roughening transitions (|T−T_R| ∼ 0.001−0.05T_R, ϕ ∼ 0.005−0.1), in strong disagreement with two current theories, viz., the phenomenological “mean field theory” and the lattice model calculations.