Numerical study of magnetic field induced ordering in BaCuSi₂O₆ and related systems

Thermodynamics of spin dimer system BaCuSi₂O₆ 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 occupancies 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 the mean field theory as well as by the simple hard-core boson model for both large and small intradimer coupling J. The asymmetry of the phase diagram of m_xy(h) of small coupling J in related systems such as NiCl₂-4SC(NH₂) is explained with our new boson operator.