Series expansions for a Heisenberg antiferromagnetic model for SrCu2„BO3...2

We use a variety of series expansion methods at both zero and finite temperature to study an antiferromagnetic Heisenberg spin model proposed recently by Miyahara and Ueda for the quasi-two-dimensional material SrCu2(BO3)2. We confirm that this model exhibits a first-order quantum phase transition at T50 between a gapped dimer phase and a gapless Néel phase when the ratio x5J8/J of nearestand next-nearest-neighbor interactions is varied, and locate the transition at xc50.691(6). Using longer series we are able to give more accurate estimates of the model parameters by fitting to the high-temperature susceptibility data. @S0163-1829~99!09225-5#