SPIN TUNNELING IN THE KAGOME ANTIFERROMAGNET

The collective tunneling of a small cluster of spins between two degenerate ground-state configurations of the kagome-lattice quantum Heisenberg antiferromagnet is studied. The cluster consists of the six spins on a hexagon of the lattice. The resulting tunnel-splitting energy DELTA is calculated in detail, including the prefactor to the exponential exp(-S0/HBAR). This is done by setting up a coherent-spin-state path integral in imaginary time and evaluating it by the method of steepest descent. The hexagon tunneling problem is mapped onto a much simpler tunneling problem, involving only one collective degree of freedom, which can be treated by known methods. It is found that for half-odd-integer spins, the tunneling amplitude and the tunnel-splitting energy are exactly zero, because of destructive interference between symmetry-related (+) instanton and (-) instanton tunneling paths. This destructive interference is shown to occur also for certain larger loops of spins on the kagome lattice. For small, integer spins, our results suggest that tunneling strongly competes with in-plane order-from-disorder selection effects; it constitutes a disordering mechanism that might drive the system into a partially disordered ground state, related to a spin nematic.