Resonance excitation of single rotons in He II by an electromagnetic wave. Spectral line shape

Abstract

The amplitude-frequency characteristic of the spectral line of electromagnetic absorption in liquid helium is measured in the frequency range 40–200GHz at temperatures in the interval 1.4–2.75K. It is found that in the roton frequency region a narrow resonance absorption line on a broad pedestal is observed. The results are compared with data on the roton spectrum found in neutron scattering experiments in liquid helium. The narrow line is due to the creation of a single roton. It is shown that the momentum conservation law is satisfied on account of the transfer of momentum to the superfluid component. The analogy of this effect with the Mössbauer effect is pointed out.

Absorption of electromagnetic field energy by the superfluid system of atoms with a dipole moment

“…29), D % 8.4 K. These values are indeed close to the value of the temperature-dependent roton energy. 9 Due to the significant difference between the experiments on the absorption of electromagnetic field and slow neutron scattering, direct detection of the gapped branch of the spectrum in the neutron experiments is apparently impossible. This is due to several reasons.…”

“…2,3 Unexpectedly high electric activity of superfluid helium exhibited under different conditions has been reported in a series of experimental studies. [4][5][6][7][8][9][10] In particular, the absorption of electromagnetic radiation at a frequency close to the roton energy has been observed. Moreover, the observed absorption was anomalously narrow.…”

“…Moreover, the observed absorption was anomalously narrow. [7][8][9][10] To understand the effects observed in these and similar experiments, the interaction of manyparticle systems of Bose atoms in a coherent state with an electromagnetic field has to be investigated in detail.…”

Resonance excitation of slow rotons: Linewidth

“…In their experimental investigation of the interac tion of microwaves with superfluid 4 He, Rybalko et al [1] observed resonance absorption at the roton fre quency f 0 ~ 180 GHz. Since the momentum of a pho ton is much lower than that of a roton, their transfor mation into one another is possible only near the wall, where only the energy is conserved.…”

“…Rybalko et al [1] used "whispering gallery" modes disk dielectric resonator with the radius R ~ 1 cm immersed in liquid helium. The electric field in helium obeys the harmonic law E(t) = Ecos2πft.…”