摘要
Thermodynamics of spin dimer system BaCu Si2 O6 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 mxy 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 mxy (h) of small coupling J in related systems such as Ni Cl2 -4SC (N H2) is explained with our new boson operator. © 2006 The American Physical Society.