  Thermodynamics of spin dimer system BaCuSi_2O_6 is studied using a quantum
Monte Carlo calculation (QMC) and a bond-operator mean field theory. We propose
that a new type of boson, which, rather than being hard-core, allows up to two
occupancy at each site, is responsible for the Bose Einstein condensation of
field induced ordering. Its superfluid density is identified as the square of
the in-plane staggered magnetization m_{xy} in the ordered phase. We also
compare our QMC result of the spin Heisenberg model to those predicted by mean
field theory as well as by the simple hard core boson model for both large and
small intra-dimer coupling J. The asymmetry of the phase diagram of m_{xy}(h)
of small coupling J in related systems such as NiCl_2-4SC(NH_2)_2 is explained
with our new boson operator.
