‘‘Spinon gas’’ description of theS=1/2 Heisenberg chain with inverse-square exchange: Exact spectrum and thermodynamics

The exact spectrum and thermodynamics of the S=1/2 Heisenberg chain with inverse-square exchange are explicitly obtained in closed form, and a description in terms of semionic spin-1/2 ``spinon'' excitations is developed. The unexpected degeneracies of the spectrum lead to a new representation of the k=1 SU(2) Wess-Zumino-Witten conformal field theory that exposes an infinite set of SU(2) symmetries of that model.