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Acta Physica Sinica

POSSIBILITY OF GASEOUS SUPERFLUID AND ITS ACOUSTICAL PROPERTIES IN SPIN POLARIZED HYDROGEN ATOMS (H↓)

NI WAN-SUN, 南京大学物理系及声学研究所

Published Jan 1, 1983• 0 citations
Publisher

Abstract

The spin polarized hydrogen atoms (H↓) are treated as a weak interacting Bose-Einstein system. On the basis of Isihara's finite temperature theory and hard-sphere approximation, the possibility of gaseous superfluid and two modes of sound have been discussed. The importance of acoustical measurement technique in study on H↓ system is emphasized.

DOI
10.7498/aps.32.1227

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THE MODE OF SOUND IN GASEOUS SUPERFLUID FILM OF SPIN POLARIZED HYDROGEN ATOMS (H↓) ABSORBED ON HELIUM FILM

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The mode of sound in gaseous superfluid film of spin polarized hydrogen atoms (H↓) absorbed on helium film is discussed. Principally, it ia the fourth sound in two dimensions resulting from compressibility of the gaseous film and different from the usual third sound resulting from the variances of thickness of helium film.

Acta Physica Sinica•Jan 1, 1983•0 citations

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Low Temperature Physics•Dec 1, 2011•10 citations
The spin polarized hydrogen atoms (H↓) are treated as a weak interacting Bose-Einstein system. On the basis of Isihara's finite temperature theory and hard-sphere approximation, the possibility of gaseous superfluid and two modes of sound have been discussed. The importance of acoustical measurement technique in study on H↓ system is emphasized.
Spin-polarized atomic hydrogen continues to be one of the most promising candidates for Bose condensation of an atomic system. In contrast to liquid helium, hydrogen is gaseous and therefore its density can be changed to vary its behavior from the weakly to the strongly interacting Bose gas. Until now, efforts to Bose condense hydrogen have been thwarted by recombination and relaxation phenomena. After a long introduction to the subject, two promising approaches to observe quantum degenerate behavior are discussed: the microwave trap and a two-dimensional gas of hydrogen. Introduction Since the stabilization of atomic hydrogen as a spin-polarized gas (H↓), reported in 1980 [1], there has been a continuous effort to observe Bose–Einstein condensation (BEC) or other effects of quantum degeneracy in this Bose gas. This challenge has not yet been realized due to the instability of H↓ towards recombination to H 2 or relaxation among hyperfine states as the conditions for BEC are approached. Although this difficulty is formidable, hydrogen presents a unique opportunity to study BEC and the related superfluidity, as it remains a gas to T = 0 [2]. By comparison, 4 He, the only experimentally observed boson fluid, is a strongly interacting superfluid Bose liquid, with little flexibility for varying its density. As a result of the gaseous nature of hydrogen, its density, n , can be varied over several orders of magnitude with the possibility of studying BEC and its relationship to superfluidity, ranging from the weakly to the strongly interacting boson gas.
Possible acoustic modes in a superfluid 2D gas of hydrogen atoms adsorbed on the surface of liquid helium at T ≲ 0.1 K are considered depending on the oscillation frequency and times of energy and momentum transfers both between 2D subsystems of hydrogen atoms and ripplons and into the bulk liquid or substrate. Analogues of the usual and second sounds are realized in 2D hydrogen at high frequencies. In the case of weak coupling with the bulk liquid and substrate, ripplons provide an addition to the normal hydrogen component, which leads to a change in the speed of the second sound. In the most interesting range of low frequencies, an analogue of the fourth sound is realized, when ripplons and the normal hydrogen component are immobile and only the superfluid hydrogen component moves. In this case, when the rate of heat transfer into the bulk liquid is much lower than the sound frequency, oscillations of the temperature of hydrogen can be observed in phase with density oscillations. Methods for exciting acoustic modes are discussed.
Spin-polarized, atomic hydrogen at temperatures below 1K shows promise of becoming the first physically realizable example of a degenerate, weakly interacting Bose gas. Because of the hyperfine structure of the hydrogen atom the Bose condensed state is expected to exhibit novel magnetic properties such as spin wave excitations of the superfluid and possibly a coherent precessing, spontaneous magnetization. This paper discusses a variety of theoretical problems which arise in the study of atomic hydrogen gas. Theories are presented for spin relaxation of the hyperfine levels and for the recombination of H-atoms into H2 molecules. The problem of the binding of H atoms to a surface of superfluid 4He is reviewed together with the theory of surface superfluidity in the hydrogen film proposed by Edwards and Mantz. Finally we consider the problem of superfluidity in the bulk atomic hydrogen gas. We describe the theory of Siggia and Ruckenstein of double Bose condensation into the two lowest hyperfine states and speculate about the possible observable consequences of this phenomenon.
Atomic hydrogen (H) has recently been created in a long-lived state referred to as spin-polarized atomic hydrogen (H↓). A moderate density gas (n≃1017 atoms/ cc) has been produced at temperatures T≃400 mK. H↓ is predicted to be the only atomic substance that remains gaseous to T = 0 K; current experiments are consistent with this picture. Since the H↓ atoms behave as composite bosons the gas is expected to have a Bose-Einstein condensation (BEC) and related superfluidity at a sufficiently high density or low temperature. H↓ is expected to display spectacular static and dynamic properties. Since the gas is polarized, the dominant electronic magnetization, M⃗e, is proportional to the atomic density. In an inhomogeneous magnetic field M⃗e will also be highly inhomogeneous. BEC will be spatially localized and the static M⃗e should be an identifying feature of this state. The low-lying excitation spectrum should not only feature electronic and nuclear spin-transitions, but should also have magnetic translational modes of the gas. The experimental technique for stabilization of H↓ as well as many of the predicted magnetic properties are discussed.
A possibility of polarization of the gas of hydrogen-like atoms at zero temperature in the Bose–Einstein condensate is studied at equilibrium and nonequilibrium conditions. The Gross–Pitaevskii equation is generalized for a gas of atoms considering their electronic structure. It is shown that the wave function of the system of interacting bosons in a single-particle condensate at zero temperature looks like the Glauber coherent state. With the help of the generalized Gross–Pitaevskii equation the polarization properties of atomic gas in the Bose–Einstein condensation state are investigated. It is shown that the electron stationary states in atoms of a gas in the coherent state differ from the electron stationary states in free atoms. Because of this the superfluid system can be spontaneously polarized, even if the free atoms in the stationary states have no intrinsic electric dipole moment. The polarization properties of a system of double-level hydrogen-like atoms in the Bose–Einstein condensate are studied more comprehensively. The propagation of both sound and high-frequency small disturbances in the coherent system of polarized atoms is accompanied by oscillations of polarization. The applicability of the developed idea to the analysis of electrical phenomena in superfluid helium is discussed.