Anisotropic second sound in superfluid 4He near T ? in the presence of a heat current

Abstract

In the presence of a homogeneous heat current the isotropy of superfluid 4He is broken and the propagation of second-sound waves becomes anisotropic. We calculate the angular dependence of the velocity and damping of second sound near Tλ within model F of Halperin, Hohenberg and Siggia to leading order in the renormalized couplings. The critical behavior is incorporated using results of the renormalization-group theory. Second-sound waves propagating parallel to the heat current become unstable if the heat current exceeds some critical value. The theory agrees with recent experiments.