Spin–Spin and Spin–Phonon Interaction as a Nature of Microwave Absorption in He II

Experimentally observed with dielectric disk resonator technique, microwave absorption–amplification in liquid He II below $\lambda$-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 four-electron He–He interatomic bond due to spin–spin and spin–phonon coupling. The experimentally registered microwave absorption line is $f_0=180.3\text{GHz}$ at $T=1.4\text{K}$ and $f_0=150.0\text{GHz}$ at $T=2.1\text{K}$ which strongly corresponds to the values of roton gap known from neutron diffractometry. Our theoretical estimation gives only an upper limit $>250\text{GHz}$ for resonant response of the system. We interpret the dielectric ${}^4\text{He}$ superfluid as a working substance for low-temperature MASER and clarify the atomic mechanism of microwave absorption–amplification in the condensed helium phases.