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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

Physics with dense positronium

D. B. Cassidy, Allen Paine Mills

Published Jul 11, 2007• 25 citations
Publisher

Abstract

Abstract Our recent experiment on Ps‐Ps spin exchange quenching represents the first time that two Ps atoms have been observed interacting with each other. The implication is that experimentation on various systems containing more than one Ps atom is now possible, including the Ps2 molecule, the Ps Bose‐Einstein condensate, the Ps atom laser and the annihilation gamma ray laser. Here we describe various aspects of such research, including the development of new techniques, work currently underway and some possible future experiments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssc.200675760

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Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.

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Abstract Our recent experiment on Ps‐Ps spin exchange quenching represents the first time that two Ps atoms have been observed interacting with each other. The implication is that experimentation on various systems containing more than one Ps atom is now possible, including the Ps2 molecule, the Ps Bose‐Einstein condensate, the Ps atom laser and the annihilation gamma ray laser. Here we describe various aspects of such research, including the development of new techniques, work currently underway and some possible future experiments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract We present the actual state of affairs and future perspectives in the study of a quantum system of a collection of positronium (Ps) atoms. The interaction of a Ps atom with other atoms and molecules and specially with another Ps atom is described in some detail as Ps‐Ps interaction should play a crucial role in the dynamics of an assembly of Ps atoms. Using a simple model‐exchange potential, we could describe the available experimental results of Ps scattering reasonably well. The present scenario of the observation of Ps2 molecule, Ps Bose‐Einstein condensate (BEC) and the annihilation laser from a Ps BEC is presented. Possibilities of a Ps BEC formation via laser cooling of Ps atoms and via Ps formation in cavities are considered and difficulties with each procedure discussed (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Although it can probably only exist within a confining potential, a positronium (Ps) Bose-Einstein condensate (BEC) would be interesting because it could be viewed as the ultimate member of Wheeler’s polyelectron series, e+, Ps, Ps−, Ps+, Ps2, …, Ps-BEC [1], not counting high density neutral e+-e− plasmas. A Ps-BEC could be a source of very cold Ps atoms for precision spectroscopy, for gravitation experiments, and as the gain medium for an annihilation gamma ray laser [2]. It would also be interesting to look for other effects that are non-linear in the positron density, such as the production of Ps+ ions and highly polarized positrons at a metal surface. Specifically, a process in which one thermalized bulk positron passes into the vacuum accompanied by an electron and a surface positron (surface pseudo-Ps) to form a Ps+ ion in vacuum, e+solid + Pssurf → Ps+vac + □E, is energetically allowed for a Ni(100) remoderator surface, for which + □E = 1.00 eV. The positive ions could be used in an experiment in which one positron undergoes a process in a material and the other positron reports on the process via its subsequent Ps emission, etc. Since the two positrons involved in the formation of Ps+ and Ps2 must have opposite spins, at sufficiently high implanted positron densities the minority positron spins will be significantly depleted by these processes, resulting in the enhancement of the spin polarization of the reemitted positrons which might be useful for spin polarized angle-resolved positronium emission spectroscopy of the electronic structure of solids and surfaces.
We review the proposal for Bose–Einstein condensation of positronium atoms. All of the ingredients necessary to achieve BEC of Ps atoms are currently available. Introduction In this volume several authors have discussed and described a variety of weakly interacting systems which might display BEC. In this short contribution we suggest that a dense gas of positronium (Ps) atoms in vacuum is a rather ideal but somewhat more exotic system that might be a very good candidate for observing a weakly interacting BEC. Recent investigations of the interactions of positrons ( e + ) and (Ps) with solids have led to extraordinary improvements in the kinds of low energy experiments we can do with the positron [1, 2]. All of the ingredients necessary to achieve BEC of Ps atoms are currently available. We envision a scenario where roughly N ≅ 10 5 Ps atoms are trapped in a volume v ≃ 10 −13 cm 3 and allowed to cool through the Bose transition temperature of 20 – 30K in a time of the order of nanoseconds. In the following we discuss the relevant interactions and describe how Ps BEC can be achieved. Single Positronium Physics Ps is comprised of an e + − e − bound in a hydrogenic orbit. Its mass, 2 m e , is extremely light compared to H, an important ingredient for achieving reasonable Bose condensation temperatures. Its binding energy (6.8 eV) is half that of H.
Bose-Einstein condensates (BECs) of positronium (Ps) have been of experimental and theoretical interest due to their potential application as the gain medium of a $\ensuremath{\gamma}$-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho-positronium (ooo-Ps) and para-positronium (ppp-Ps), whose annihilation lifetimes differ by three orders of magnitude. In this paper, we study the spinor dynamics and annihilation processes in the ppp-Ps/ooo-Ps system using both solutions of the time-dependent Gross-Pitaevskii equations and a semiclassical rate-equation approach. The spinor interactions have an O(4) symmetry which is broken to SO(3) by an internal energy difference between ooo-Ps and ppp-Ps. For an initially unpolarized condensate, there is a threshold density of $\ensuremath{\approx}{10}^{19}$ cm${}^{\ensuremath{-}3}$ at which spin mixing between ooo-Ps and ppp-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability.
Abstract In recent years degenerate quantum gases confined in optical lattices developed as a new field of research bridging the areas of quantum optics, atomic physics, and condensed matter physics. It offers the possibility of manipulating quantum many‐body systems with an unprecedented flexibility, so that they can be considered as analog simulators of condensed matter systems. Particularly interesting is the possibility of creating strongly correlated bosonic or fermionic systems. We review here recent numerical simulations, where quantum critical behavior is studied both in and out of equilibrium. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Positronium (Ps) atoms exist in different spin configurations that determine their fundamental properties, such as energy level structure, decay rate and response to a magnetic field. The nature of interactions between Ps atoms is also strongly influenced by their relative spin states; oppositely polarized Ps atoms may exchange particles and scatter into different states, or join together to form molecular positronium. Conversely, interactions between spin aligned Ps atoms cannot lead directly to changes of spin states or molecule formation, but at high densities/low temperatures a collection of such atoms may undergo a phase transition to form a Bose-Einstein condensate. Here I consider some of the physics involved in such interactions and the positron beam parameters required to conduct such experiments.