  The plaquette phase of the square lattice quantum dimer model is studied
using a continuous-time reptation quantum Monte Carlo method for lattices of
sizes up to 48x48 sites. We determine the location of the phase transition
between the columnar and plaquette phases to occur at V_c/J=0.60 +- 0.05 which
is significantly larger than inferred from previous exact diagonalization
studies on smaller lattices. Offdiagonal correlation functions are obtained.
They exhibit long-range order in the plaquette phase but not at the
Rokhsar-Kivelson point. We also observe significant finite-size corrections to
scaling for the transition between the plaquette phase and the critical
resonating valence bond liquid. This study demonstrates the importance of
understanding finite-size effects when considering critical properties of the
square lattice quantum dimer model.
