Normal fluid density, second sound, and fourth sound in an anisotropic superfluid

In a superfluid with an anisotropic quasiparticle energy, the normal-fluid density is a diagonal tensor. The second and fourth sound velocities are distinctly anisotropic, so that heat-pulse or fourth-sound studies with an applied magnetic field H⃗ may permit identification of the direction of the gap axis with respect to H⃗. High-frequency sound velocities may display a weaker anisotropy.