Nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J(1)-J(2) model

We analyze the spectrum of the two-dimensional S = 1/2 frustrated Heisenber g model near the transition from thr spontaneously dimerized spin-liquid ph ase into the Neel ordered phase. Two excitation branches, the triplet magno n and the collective singlet mode, both become gapless at the transition po int. However, we find that the length scales associated with these modes ar e well separated at the quantum transition. While in the quantum disordered phase the singlet excitation has finite spectral weight and reflects the e xistence of spontaneous dimer order, near the transition point the size of the singlet bound state grows exponentially with the correlation length, an d hence the quasiparticle residue is exponentially small. Therefore the cri tical dynamics remains in the O(3) universality class in spite of the four gapless modes.