  In this paper we investigate the quantum phase transition from magnetic Bose
glass to magnetic Bose-Einstein condensation induced by a magnetic field in
NiCl2.4SC(NH2)2 (dichloro-tetrakis-thiourea-Nickel, or DTN), doped with Br
(Br-DTN) or site diluted. Quantum Monte Carlo simulations for the quantum phase
transition of the model Hamiltonian for Br-DTN, as well as for site-diluted
DTN, are consistent with conventional scaling at the quantum critical point and
with a critical exponent z verifying the prediction z=d; moreover the
correlation length exponent is found to be nu = 0.75(10) and the order
parameter exponent to be beta = 0.95(10). We investigate the low-temperature
thermodynamics at the quantum critical field of Br-DTN both numerically and
experimentally, and extract the power-law behavior of the magnetization and of
the specific heat. Our results for the exponents of the power laws, as well as
previous results for the scaling of the critical temperature to magnetic
ordering with the applied field, are incompatible with the conventional
crossover-scaling Ansatz proposed by Fisher et al., [Phys. Rev. B 40, 546
(1989)], but they can all be reconciled within a phenomenological Ansatz in the
presence of a dangerously irrelevant operator.
