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Le Journal de Physique Colloques

THE MAGNETIZATION AND DENSITY OF SPIN POLARIZED ATOMIC HYDROGEN

Jook T. M. Walraven, Isaac F. Silvera

Published Jul 1, 1980• 1 citation
Publisher

Abstract

The magnetization and density distribution of spin polarized atomic hydrogen in a magnetic field is very sensitive for the presence of magnetic field gradients. This feature is discussed in relation to realistic experimental conditions, accounting for the specific Bose-Einstein character of the gas. To a limited extent comparison with experiment is possible.

DOI
10.1051/jphyscol:1980723

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The magnetization and density distribution of spin polarized atomic hydrogen in a magnetic field is very sensitive for the presence of magnetic field gradients. This feature is discussed in relation to realistic experimental conditions, accounting for the specific Bose-Einstein character of the gas. To a limited extent comparison with experiment is possible.
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 phenomenological description of spin-polarized hydrogen, suggested by the microscopic Hamiltonian, is proposed that includes two Bose particles which correspond in the low density limit to the two lowest hyperfine states. Experiments are expected to initially populate both states, and equilibration times will be long. When Bose condensation occurs in both states a spontaneous coherent magnetization perpendicular to the stabilizing field will appear ; that should be readily observable in a simple magnetic resonance experiment, Field inhomogeneities should not broaden the resonance signal in such an experiment, contrary to one's experience with uncondensed systems.
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.
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.