Quantum effects and broken symmetries in frustrated antiferromagnets

We investigate the interplay between frustration and zero-point quantum flu ctuations in the ground state of the triangular and J(1)-J(2) Heisenberg an tiferromagnets, using finite-size spin-wave theory, exact diagonalization, and quantum Monte Carlo methods. In the triangular Heisenberg antiferromagn et, by performing a systematic size-scaling analysis, we have obtained stro ng evidences for a gapless spectrum and a finite value of the thermodynamic order parameter, thus confirming the existence of long-range Neel order. T he good agreement between the finite-size spin-wave results and the exact a nd quantum Monte Carlo data also supports the reliability of the spin-wave expansion to describe both the ground state and the low-energy spin excitat ions of the triangular Heisenberg antiferromagnet. In the J(1)-J(2) Heisenb erg model, our results indicate the opening of a finite gap in the thermody namic excitation spectrum at J(2)/J(1) similar or equal to 0.4, marking the melting of the antiferromagnetic Neel order and the onset of a non-magneti c ground state. In order to characterize the nature of the latter quantum-d isordered phase we have computed the susceptibilities for the most importan t crystal symmetry breaking operators. In the ordered phase the effectivene ss of the spin-wave theory in reproducing the low-energy excitation spectru m suggests that the uniform spin susceptibility of the model is very close to the linear spin-wave prediction.