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 T = 0 between a gapped dimer phase and a gapless Neel phase when the ratio x = J'/J of nearest- and next-nearest-neighbor interactions is varied, and locate the transition at x(c) = 0.691(6). Using longer series we are able to give more accurate estimates of the model parameters by fitting to the high-temperature susceptibility data.