Spontaneous dimer order, excitation spectrum and quantum-phase transitionsin the J(1)-J(2) Heisenberg model

We overview some recent work and present new results on the ground-state pr operties and the spectrum of excitations of the two-dimensional frustrated Heisenberg antiferromagnet. Spontaneous dimer order is present in the quant um disordered phase of this model. We study the stability and analyse the s tructure of the spectrum, including the two-particle singlet excitation bra nch throughout the disordered phase, as well as in the vicinity of the Neel critical point. The variation in the dimer order parameter is also given, and it is argued that near the critical point it reflects the presence of t he low-energy singlet bound state.