Acoustic modes in HeI and He II in the presence of an alternating electric field
Bogolyubov Institute for Theoretical Physics, National Academy for Sciences of Ukraine 14b Metrologichna Str., Kyiv 03680, Ukraine
Received October 4, 2019, published online March 24, 2020
By solving the equations of ordinary and two-fluid hydrody-namics, we study the oscillatory modes in isotropic nonpolar dielectrics He I and He II in the presence of an 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 shows that the field E changes the velocities of first and second sounds, propagating along E, by the formula uj ≈ cj + χjE02 (where j = 1, 2; cj the velocity of the j-th sound for E0 = 0, and χj a constant). We have also found that the field E jointly with a wave of the first (second) sound (ω, k) should create in He II hybrid acousto-electric (thermoelectric) density waves (ω +lω0, k+lk0), where l = ±1, ±2... The amplitudes of acousto-electric waves and the value of |u1-c1| are negligibly small, but should resonantly increase at certain ω and ω0. The first resonance apparently corresponds to the decay of a photon into two phonons with the momentum recoil to the whole liquid. Therefore, a narrow absorption line should be observed in the spectrum of the electromagnetic signal, like the Mössbauer effect.
Key words: first sound, second sound, spontaneous polarization, dielectric.