Nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J1-J2 model

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