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Journal of Applied Physics

Atomic hydrogen: A weakly interacting, magnetic Bose gas (invited)

A. J. Berlinsky

Published Mar 1, 1981• 17 citations
Publisher

Abstract

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.

DOI
10.1063/1.328915

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Journal of Applied Physics•Mar 1, 1981•17 citations
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.
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.
We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for the possibility of observing the Berezinskii–Kosterlitz–Thouless superfluid transition.
Under normal circumstances a gas of atomic hydrogen (H) is highly unstable with respect to recombination to molecular hydrogen, H2. We have succeeded in stabilizing a gas of spin polarised hydrogen (H↓) at T = 300 mK in magnetic fields up to 10 Tesla in a superfluid helium coated cell. Densities of at least 1016atoms/cc have already been achieved. H is found to have a surprisingly long life even in zero magnetic field. Evidence points to H↓ remaining in the gaseous form and not liquifying or condensing out on the surfaces to any large extent. The crucial experimental techniques used to stabilize H↓ will be discussed as well as some of the observed properties and expectations of this new Bose gas.
An experiment for studying a gas of atomic hydrogen at temperatures between 0.9 K and 1.3 K is described. The field dependence of the recombination rate of H atoms into H₂ molecules and the spin-exchange cross-section are investigated. Atoms are produced in situ in a low temperature discharge and are confined in a pyrex cell. The walls of this cell are covered with a film of superfluid ⁴He for preventing recombination. The detection and the study of the hydrogen atoms is done using the technique of pulsed magnetic resonance between the lowest two hyperfine levels of the 1s hydrogen atoms. The frequency is about 765.5 MHz and corresponds to a shallow minimum in the difference between the lowest two hyperfine levels, which occurs at 6481 kGauss. A simple model is also derived for explaining the field dependence of the recombination in the presence of a relatively large ⁴He density. In fact, due to the vapour pressure, the helium density will be above 10¹⁸ atoms/cm³ in the range of temperatures in which we are interested, while the density of hydrogen atoms will be about 10¹¹ -10¹² atoms/cm³. The relaxation mechanism assumed is spin-exchange. The parameters of the model are fit to the existing data. It is found in particular that the asymptotic behavior of the hyperfine populations depends sensitively on the initial populations.
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.