Spin tunneling in thekagoméantiferromagnet

The collective tunneling of a small cluster of spins between two degenerate ground state configurations of the Kagom\'{e}-lattice quantum Heisenberg antiferromagnet is \mbox{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 $\exp(- \SSo / \hbar)$. 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 Kagom\'{e} lattice. For small, integer spins, our results suggest that tunneling strongly competes with \mbox{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.Comment: 38 pages (RevTex), 8 figures upon request PRB921