Spin-polarized atomic hydrogen: A magnetic gas (invited)

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Abstract
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.