  The antiferromagnetic to valence-bond-solid phase transition in the
two-dimensional J-Q model (an S=1/2 Heisenberg model with four-spin
interactions) is studied using large-scale quantum Monte Carlo simulations. The
results support a continuous transition of the ground state, in agreement with
the theory of "deconfined" quantum criticality. There are, however, large
corrections to scaling, of logarithmic or very slowly decaying power-law form,
which had not been anticipated. This suggests that either the SU($N$) symmetric
noncompact CP^(N-1) field theory for deconfined quantum criticality has to be
revised, or that the theory for N=2 (as in the system studied here) differs
significantly from N -> infinity (where the field theory is analytically
tractable).
