LAUSR

Experimentally observed with dielectric disk resonator technique, microwave absorption–amplification in liquid He II below $$\lambda $$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 four-electron He–He interatomic bond due to spin–spin and spin–phonon coupling. The experimentally registered microwave absorption line is $$f_0=180.3\,\hbox {GHz}$$f0=180.3GHz at $$T=1.4\,\hbox {K}$$T=1.4K and $$f_0=150.0\,\hbox {GHz}$$f0=150.0GHz at $$T=2.1\hbox {K}$$T=2.1K which strongly corresponds to the values of roton gap known from neutron diffractometry. Our theoretical estimation gives only an upper limit $$>250\,\hbox {GHz}$$>250GHz for resonant response of the system. We interpret the dielectric $$^4\hbox {He}$$4He superfluid as a working substance for low-temperature MASER and clarify the atomic mechanism of microwave absorption–amplification in the condensed helium phases.