SPIN-PEIERLS AND SPIN-LIQUID PHASES OF KAGOME QUANTUM ANTIFERROMAGNETS

The ground state of the spin-1/2 nearest-neighbor Heisenberg quantum antiferromagnet on the Kagome lattice probably lacks spin order; therefore, conventional spin-wave analysis breaks down. To ascertain the ground state, we instead use a systematic 1/n expansion with SU(n) fermions. Two distinct states occur in the large-n limit, depending on the size of the biquadratic interaction J approximately. When J approximately = 0, there are an infinite number of degenerate ground states consisting of disconnected dimers. At finite n, however, this degeneracy is broken by local resonance. In contrast, a globally resonating chiral spin-liquid phase with no spin-Peierls modulation is the likely large-n ground state at sufficiently large J approximately. For intermediate values of approximately J, a phase transition from the dimer state to the chiral phase occurs as the temperature increases. At a higher temperature, there is a second transition to a paramagnetic state. We comment on the possibility that these phases are experimentally realized by the nuclear magnetic moments of a second layer of He-3 atoms lying on a graphite surface.