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Posted: Jul 02, 2013
Universal bound states of fermions
(Nanowerk News) The behaviour of a few particles can have important ramifications for how a system of many particles behaves. In particular, bound states of two, three or more particles can determine how particles will cluster, i.e. how particles are correlated, in a gas or fluid. Examples include paired superfluids in dilute atomic vapours, and the clustering of nucleons in a nucleus. Indeed, for short-range interactions between particles, there is the prospect of universal correlations that are insensitive to the microscopic details and are thus relevant to a wide range of systems.
(left) Two species (,¯) of fermionic atom are confined in a two-dimensional layer of width d. (right) Bound states of three or more atoms can form above a critical mass ratio as the confinement width is varied.
Such bound states have been known to exist for bosonic atoms, which typically like to cluster, but Parish and Levinsen are the first to demonstrate the existence of a universal tetramer entirely composed of fermionic atoms. Fermions are famous for avoiding one another, with the so-called exclusion principle being responsible for the structure of the periodic table and the stability of matter. However, fermions may bind together if there is a lighter particle that dances around and effectively mediates an attraction between them, thus resulting in a tetramer of three heavy fermions and one light.
In principle, such a tetramer can be realised in experiment by confining atoms, such as potassium and lithium, to a two-dimensional layer with lasers. Varying the confinement can then lead to the formation of larger and larger clusters of fermions. Such a set-up could be used to tune the effective interactions and correlations in a system of many atoms, and this could ultimately be used to simulate other more complicated materials.