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Physica Scripta

Quasi-Atoms and Super-Atoms

J-P Connerade

Published Jan 1, 2003• 11 citations
Publisher

Abstract

The terms "quasiatom" and "superatom" are introduced. A quasi-atom, for example contains two ions of opposite charge, which behave like an atom, forming Rydberg states. A superatom also contains many atoms, but behaves in a simple way, and can be modelled within the central field approximation. Both are examples of many-body systems which emulate quasi-particle properties. Their behaviour is the opposite of quasi-particle breakdown in isolated atoms. However, it also probes the boundary between simple and complex behaviour, for species larger than atoms. Examples are given, ranging from shallow donor impurities in semiconductors, through ion-pair molecules, confined and endohedral atoms, to metallic clusters.

DOI
10.1238/physica.regular.068ac0025

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The terms "quasiatom" and "superatom" are introduced. A quasi-atom, for example contains two ions of opposite charge, which behave like an atom, forming Rydberg states. A superatom also contains many atoms, but behaves in a simple way, and can be modelled within the central field approximation. Both are examples of many-body systems which emulate quasi-particle properties. Their behaviour is the opposite of quasi-particle breakdown in isolated atoms. However, it also probes the boundary between simple and complex behaviour, for species larger than atoms. Examples are given, ranging from shallow donor impurities in semiconductors, through ion-pair molecules, confined and endohedral atoms, to metallic clusters.
Recent investigations of Rydberg atoms are described in an effort to give a picture of their properties, many of which can be understood from the behaviour of the Coulomb radial functions. The authors begin by a discussion of Coulomb wavefunctions, and progress to a description of the energy level structure. The behaviour of Rydberg atoms in electric fields is discussed to provide an understanding of the phenomenon and to show how electric field ionisation may be used as a tool in a wide variety of experiments. Unique opportunities to study the interaction with black-body radiation and vacuum fluctuations are discussed. A survey of the broad range of Rydberg atom collisions, with ground-state atoms and molecules, charged particles and other Rydberg atoms, is presented to show the different physical interactions responsible. This is followed by a brief summary of the study of autoionising Rydberg states, a stepping stone toward the elusive 'double Rydberg' atom. Finally typical experimental procedures are described.
Properties of some few-body systems which include one positively charged muon μ+μ^{+}μ+ and two electrons e−e^{-}e− are discussed. In particular, we consider the negatively charged muonium ion Mu−^{-}− (or μ+e2−μ^{+} e^{-}_{2}μ+e2−​) and four-body MuPs (or μ+e2−e+μ^{+} e^{-}_{2} e^{+}μ+e2−​e+) systems each of which has only one stable bound (ground) state. The problem of annihilation of the electron-positron pair(s) in the MuPs system is investigated. The hyperfine structure splitting of the ground state in the MuPs system evaluated with our expectation value of the muon-positron delta-function is Δ≈Δ\approxΔ≈ 23.05758 MHzMHzMHz. Another group of interesting four-body neutral systems investigated in this study includes the p+μ+e2−,d+μ+e2−p^{+} μ^{+} e^{-}_2, d^{+} μ^{+} e^{-}_2p+μ+e2−​,d+μ+e2−​ and t+μ+e2−t^{+} μ^{+} e^{-}_2t+μ+e2−​ `quasi-molecules'. These quasi-molecules are formed in large numbers when positively charged muons slow down in liquid hydrogen, or in liquid deuterium and/or tritium. The properties of these systems are unique, since they occupy an intermediate position between actual two-center molecules and one-center atoms.
Atomic physicists have traditionally studied the properties and interactions of small numbers of atoms and molecules. Condensed-matter physicists, on the other hand, are interested in the behaviour of larger numbers of such particles. In recent years, however the interests of these two groups of physicists have started to converge. Clusters, for example, bridge the gap between small numbers of atoms and the solid state, as do Bose-Einstein condensates – a new form of matter in which millions of atoms collapse into the same quantum state. Many of the laser cooling and trapping techniques that have helped to make Bose-Einstein condensation possible are also being used to explore the interface between atomic and bulk properties in another regime – in the new state of matter found in “optical lattices”.
Strong, long-range interactions between atoms in high-lying Rydberg states can suppress multiple Rydberg excitations within a micron-sized trapping volume and yield sizable Rydberg level shifts at larger distances. Ensembles of atoms in optical microtraps then form Rydberg superatoms with collectively enhanced transition rates to the singly excited state. These superatoms can represent mesoscopic, strongly interacting spins. We study a regular array of such effective spins driven by a laser field tuned to compensate the interaction-induced level shifts between neighboring superatoms. During the initial transient, a few excited superatoms seed a cascade of resonantly facilitated excitation of large clusters of superatoms. Due to spontaneous decay, the system then relaxes to the steady state having nearly universal Rydberg excitation density . This state is characterized by highly non-trivial equilibrium dynamics of quasi-particles—excitation holes in the lattice of Rydberg excited superatoms. We derive an effective many-body model that accounts for hole mobility as well as continuous creation and annihilation of holes upon collisions with each other. We find that holes exhibit a nearly incompressible liquid phase with highly sub-Poissonian number statistics and finite-range density–density correlations.
Abstract We propose a new approach to excite ion-pair states of ultracold dimers. The central idea is a two-step process where first long-range Rydberg molecules are formed by photoassociation, which are then driven by stimulated emission towards the ion-pair state, a process bearing features of a photo-induced harpooning reaction. We assess the feasibility of this approach through a detailed experimental and theoretical study on a specific system, p-wave-scattering dominated long-range Rydberg molecules in caesium, and discuss potential applications for the study of strongly correlated plasmas consisting of oppositely charged particles of equal or similar mass.