Thermodynamics of a gas of deconfined bosonic spinons in two dimensions

We consider the quantum phase transition between a Neel antiferromagnet and a valence-bond solid (VBS) in a two-dimensional system of S=1/2 spins. Assuming that the excitations of the critical ground state are linearly dispersing deconfined spinons obeying Bose statistics, we derive expressions for the specific heat and the magnetic susceptibility at low temperature T. Comparing with quantum Monte Carlo results for the J-Q model, which is a candidate for a deconfined Neel-VBS transition, we find excellent agreement, including a previously noted logarithmic correction in the susceptibility. In our treatment, this is a direct consequence of a confinement length scale Lambda which is proportional to the correlation length xi raised to a non-trivial power; Lambda ~ xi^(1+a) ~1/T^(1+a), with a>0 (with a approximately 0.2 in the model).