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We analyze the stability of the spontaneously dimerized spin-liquid phase ofthe frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest tripletexcitation, corresponding to breaking of a singlet bond, is found to be stablein the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energycollective singlet mode, which is closely related to the spontaneous violationof the discrete symmetry. Both modes are gapped in the quantum disordered phaseand become gapless at the transition point to the Neel ordered phase(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and weargue that the disappearance of dimer order is related to the vanishing of thesinglet gap. We also present exact diagonalization data on a small (4x4)cluster which indeed show a structure of the spectrum, consistent with that ofa system with a four-fold degenerate (spontaneously dimerized) ground state.Comment: 4 pages, 4 figures, small changes, published versio

physical review. b, condensed matter

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Low-energy singlet and triplet excitations in the spin-liquid phase of the two-dimensionalJ1−J2model