We numerically investigate the phase diagram of two-dimensional site-diluted
coupled dimer systems in an external magnetic field. We show that this phase
diagram is characterized by the presence of an extended Bose glass, not
accessible to mean-field approximation, and stemming from the localization of
two distinct species of bosonic quasiparticles appearing in the ground state.
On the one hand, non-magnetic impurities doped into the dimer-singlet phase of
a weakly coupled dimer system are known to free up local magnetic moments. The
deviations of these local moments from full polarization along the field can be
mapped onto a gas of bosonic quasiparticles, which undergo condensation in zero
and very weak magnetic fields, corresponding to transverse long-range
antiferromagnetic order. An increasing magnetic field lowers the density of
such quasiparticles to a critical value at which a quantum phase transition
occurs, corresponding to the quasiparticle localization on clusters of local
magnets (dimers, trimers, etc.) and to the onset of a Bose glass. Strong
finite-size quantum fluctuations hinder further depletion of quasiparticles
from such clusters, and thus lead to the appearance of pseudo-plateaus in the
magnetization curve of the system. On the other hand, site dilution hinders the
field-induced Bose-Einstein condensation of triplet quasiparticles on the
intact dimers, and it introduces instead a Bose glass of triplets. A thorough
numerical investigation of the phase diagram for a planar system of coupled
dimers shows that the two above-mentioned Bose glass phases are continuously
connected, and they overlap in a finite region of parameter space, thus
featuring a two-species Bose glass. The quantum phase transition from Bose
glass to magnetic order in two dimensions is marked by novel universal
exponents.Comment: 15 pages, 16 figure