We consider the quantum phase transition between a Néel 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 $C$ and the magnetic susceptibility $\chi$ at low temperature $T$ in terms of a correlation length $\xi (T)$. Comparing with quantum Monte Carlo results for the $J\mathrm{\text{−}}Q$ model, which is a candidate for a deconfined Néel-VBS transition, we obtain an almost perfect consistency between $C$, $\chi$, and $\xi$. The corresponding expressions for magnon (triplet) excitations are not internally consistent, however, lending strong support for spinon excitations in the $J\mathrm{\text{−}}Q$ model.

• Received 12 October 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.207203