Quantum Phase Transitions in the Shastry-Sutherland Model for SrCu₂(BO₃)₂

We investigate quantum phase transitions in the frustrated antiferromagnetic 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 plaquette-singlet phase, exists between the dimer and the magnetically ordered phases known thus far. When the ratio of the competing exchange couplings α( = J^′/J) is varied, this spin-gap phase exhibits a first- (second-) order quantum phase transition to the dimer (the magnetically ordered) phase at the critical point α_c1 = 0.677(2) [ α_c2 = 0.86(1)]. Our results shed light on some controversial arguments about the nature of quantum phase transitions in this model.