Acoustic modes in He I and He II in the presence of an alternating electric field

Abstract

By means of the solution of the equations of ordinary and two-liquid hydrodynamics, we study the oscillatory eigenmodes in isotropic nonpolar dielectrics He I and He II in the presence of a weak alternating electric field E = E0iz sin (k0z − ω0t). The electric field and oscillations of the density become "coupled," since the density gradient causes a spontaneous polarization Ps, and the electric force contains the term (Ps∇)E. The analysis indicates that the field E changes the velocities of first and second sounds by the formula uj ≈ cj +χj E 2 0 (where j = 1, 2, cj is the velocity of the j-th sound for E0 = 0, and χj is a constant). We have found that the field E jointly with a wave of the first (second) sound (ω, k) should create in He II hybrid acousto-electric (thermo-electric) density waves (ω + lω0, k + lk0), where l = ±1, ±2, . . .. The amplitudes of acousto-electric waves and a change in the velocity of the first sound should resonantly increase at definite frequencies ω and ω0. These solutions can be verified experimentally.