Quantum phase transitions in the Shastry-Sutherland model for SrCu2(BO3)(2)

We investigate quantum phase transitions in the frustrated antiferromagneti c Heisenberg model for SrCu2(BO3)(2) by using the series expansion method. It is found that a novel spin-gap phase, adiabatically connected to the pla quette-singlet phase, exists between the dimer and the magnetically ordered phases known thus far. When the ratio of the competing exchange couplings alpha(= J'/J) is varied, this spin-gap phase exhibits a first- (second-) or der quantum phase transition to the dimer (the magnetically ordered) phase at the critical point alpha(c1) = 0.677(2) [alpha(c2) = 0.86(1)]. Our resul ts shed light on some controversial arguments about the nature of quantum p hase transitions in this model.