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As far as I know, Einstein was the first to consider a system of massive Bose particles and describe their condensation. I remember reading the relevant paper, but it will take me some time to find the reference. EDIT (6/22/2016): I guess the paper is Sitzungsberichte der Preussischen Akademie der Wissenschaften (1924), 261


Positronium is an exotic species composed of positron and electron. Depending on the allignment of the spins of postron and electron, the total spin of the positron can be zero or 1 (for the ground state for which l=0) and are designated as: ^1S (para-positronium) and ^3S (orto-positronium)

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