Microwave spectroscopy of superfluid He II

Abstract

Experimentally observed with dielectric disc resonator technique the resonant microwave absorptionamplification in superfluid He II below λ-point has been interpreted theoretically as a phenomenon in electrically active dielectric medium with low-energy excitations which exist near the ground state of the He-He interatomic bond due to fine structure of spin subsystem in condensed helium phase. The experimentally registered microwave resonant absorption line with f res = 180.3 GHz at T = 1.4 K and f res = 150.0 GHz at T = 2.1 K is closely related to the standard value of roton gap ∆/k B = 8.64 K (179.36 GHz). The measured temperature dependence of the resonant absorption demonstrates an excellent agreement with the corresponding neutron diffraction data for ∆(T) known from different literature sources. From the common point of view the obtained resonant absorption provides a typical example of the microwave spectroscopy which occurs widely among molecular systems with rotational and vibrational degrees of freedom, but for the first time discovered on as simple atomic matter as a superfluid helium. We explain the phenomenon as an effect of spin-phonon interaction within superfluid He II phase, and our theoretical estimation gives an upper limit ~ 250 GHz for the microwave resonant response of the system at T = 0. We interpret the dielectric 4 He superfluid as an electrically active working substance for low temperature MASER, and clarify the atomic mechanism of microwave absorptionamplification in the condensed helium phases.