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Low Temperature Physics

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

Yu. M. Poluéktov

Published May 1, 2014• 6 citations
Publisher

Abstract

Absorption of electromagnetic energy by a superfluid system is considered on the basis of a modified Gross-Pitaevskii equation, which takes into account relaxation and interaction with an alternating electric field, under the assumption that the atoms possess an intrinsic dipole moment. It is shown that the absorption may exhibit a resonance behavior only if the dispersion curve of the electromagnetic wave and that of the excitations in the superfluid system intersect. It is noted that such a situation is possible if the superfluid system has an excitation branch with an energy gap at small momenta. Experiments on the absorption of microwave radiation in superfluid helium are interpreted as evidence for the existence of such gapped excitations. A possible modification of the excitation spectrum of superfluid helium in the presence of an excitation branch with an energy gap is discussed on a qualitative level.

DOI
10.1063/1.4881395

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Absorption of electromagnetic energy by a superfluid system is considered on the basis of a modified Gross-Pitaevskii equation, which takes into account relaxation and interaction with an alternating electric field, under the assumption that the atoms possess an intrinsic dipole moment. It is shown that the absorption may exhibit a resonance behavior only if the dispersion curve of the electromagnetic wave and that of the excitations in the superfluid system intersect. It is noted that such a situation is possible if the superfluid system has an excitation branch with an energy gap at small momenta. Experiments on the absorption of microwave radiation in superfluid helium are interpreted as evidence for the existence of such gapped excitations. A possible modification of the excitation spectrum of superfluid helium in the presence of an excitation branch with an energy gap is discussed on a qualitative level.
We study the propagation of electromagnetic waves in the Bose-Einstein condensate of atoms with both intrinsic dipole moments and those induced by the electric field. The modified Gross--Pitaevskii equation is used, which takes into account relaxation and interaction with the electromagnetic field. Two cases are considered: 1) when the dispersion curves of the electromagnetic wave and of the condensate excitations do not intercross and 2) when the condensate excitations' spectrum has a gap and the two dispersion curves do intercross. In the second case the two branches hybridize. It is shown that propagation of sound waves can be accompanied by oscillation of the electromagnetic field. The impact is studied of the dipole-dipole interaction on the character of electromagnetic and acoustic waves' propagation in the Bose-Einstein condensate.
Here, it was shown that the chain of equations, which is similar to the Bogoliubov chain in the theory of classical gases, may be obtained for the quasi-averages of the products of field operators in the system of Bose particles. The closed system of dynamic equations, which accounts for both the one-particle condensate and pair correlations, was obtained at zero temperature for the case where it is sufficient to confine ourselves to taking into account the quasi-averages of one field operator and the products of two field operators. The spectrum of excitations in a spatially homogeneous state was studied, and it was shown that this spectrum has two branches: the sound branch and the branch with an energy gap at zero momentum. The possibility of the existence of quasiparticle excitations with an energy gap in superfluid helium was discussed in connection with experiments on the absorption of microwave radiation.
It is shown that the condensate of a degenerated Bose gas consisting of neutral atoms possesses electrical properties which differ from a trivial polarization of the atoms in the electric field.A notion of an isotropic quadrupole moment (IQM) of a neutral atom is introduced.A distribution of IQM reflects a specific spatial ordering in the condensate and produces a distribution of the electric potential.Small vibrations of the Bose gas are considered and a correction to the Bogoliubov spectrum of elementary excitations in the degenerated Bose gas is obtained.An additional term in the Gross-Pitaevskii equation which is responsible for such a correction is found and a new type of the nonlinear Schr ödinger equation (NSE) is constructed.Since the Bose condensate is akin to the superfluid component in He II, a manifestation of its electrical activity could have a relation to the electrical activity of the superfluid liquid observed experimentally.
It is shown that the condensate of a degenerated Bose gas consisting \nof neutral atoms possesses electrical properties which differ from a trivial \npolarization of the atoms in the electric field. A notion of an isotropic \nquadrupole moment (IQM) of a neutral atom is introduced. A distribution \nof IQM reflects a specific spatial ordering in the condensate and produces \na distribution of the electric potential. Small vibrations of the Bose gas are \nconsidered and a correction to the Bogoliubov spectrum of elementary excitations \nin the degenerated Bose gas is obtained. An additional term in \nthe Gross-Pitaevskii equation which is responsible for such a correction is \nfound and a new type of the nonlinear Schrodinger equation (NSE) is constructed. Since the Bose condensate is akin to the superfluid component \nin He II, a manifestation of its electrical activity could have a relation to the \nelectrical activity of the superfluid liquid observed experimentally.
In the present work, the influence of an alternating circularly polarized electric field on the energy spectrum of the He atom is studied. The calculations are performed by the method of the energy matrix diagonalization of an atom in the electric field. This method has allowed us to study the behaviour of the helium energy spectrum from the same numerical procedure under resonant and non-resonant excitations by the electric field. Based on the calculation results, we have found that the resonance effects take place not only in the vicinity of resonance, but they influence the shift directions of the Stark states even under non-resonant excitation. Additionally, we have established that the helium energy spectrum behaves consistently in the electric field. The results obtained have allowed us to clarify mechanisms of the influence of the resonance effects on the radiative characteristics of helium plasma.
Abstract We show that a non-local form of the Gross-Pitaevskii equation describes not only long-wave excitations, but also the short-wave ones in Bose-condensate systems. At certain parameter values, the excitation spectrum mimics the Landau spectrum of quasi-particle excitations in superfluid helium with roton minimum. The excitation wavelength, at which the roton minimum exists, is close to the inter-particle interaction range. We determine how the roton gap and the effective roton mass depend on the interaction potential parameters, and show that the existence domain of the spectrum with a roton minimum is reduced if one accounts for an inter-particle attraction.