Field-induced condensation of magnons and long range order in BaCuSi2O6

A quasi-two-dimensional antiferromagnet with a square-lattice bilayer structure is studied with a bond operator formalism. The excitation spectrum and its temperature dependence, the quantum phase transition from the spin-singlet dimerized state to the Neel state, are studied. In particular the field-induced long range order is interpreted using Bose-Einstein condensation of magnons. The phase diagram as well as the temperature and magnetic field dependences of the specific heat and the staggered magnetization are studied. The critical exponents around the critical magnetic fields H-c1 and H-c2 are calculated to be about 1.5. These results agree well with recent experimental results for BaCuSi2O6.