Quantum Phase Transitions in the Shastry-Sutherland Model forSrCu2(BO3)2

We investigate quantum phase transitions in the frustrated antiferromagnetic Heisenberg model for $\mathrm{Sr}{\mathrm{Cu}}_2({\mathrm{BO}}_3{)}_2$ 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 $\alpha \left({=J}^{\prime }/J\right)$ 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 ${\alpha }_{c1\mathrm{}}=0.677\mathrm{}\left(2\right)$ [ ${\alpha }_{c2\mathrm{}}=0.86\mathrm{}\left(1\right)$]. Our results shed light on some controversial arguments about the nature of quantum phase transitions in this model.