NEEL ORDER VERSUS SPIN LIQUID IN QUANTUM HEISENBERG ANTIFERROMAGNETS ON TRIANGULAR AND KAGOME LATTICES

We have realized a thorough quantum mechanical analysis of an extensiv e number of low lying levels of the Heisenberg Hamiltonian on small pe riodic samples. Striking qualitative differences appear between the sp ectra of the triangular lattice and those of the Kagome lattice. Symme tries and finite size scaling of the low lying levels of the triangula r antiferromagnetic point to an a la Neel symmetry breaking, and the d ynamical structure factors corroborate this conclusion. The Kagome ant iferromagnet spectra are qualitatively different: no signature of a sy mmetry breaking has been exhibited and the dynamical structure factors of the low lying levels display broad bands. The Heisenberg antiferro magnet on a Kagome lattice is a serious candidate for a spin liquid.