KagoméHeisenberg antiferromagnet: Elementary excitations and low-temperature specific heat

The ground-state properties of the Heisenberg antiferromagnet on a kagomé lattice with further-neighbor interactions are discussed. The classical ground states are described as spiral phases of an equivalent square lattice with a basis, and quantum fluctuations are incorporated by means of a rotationally invariant Schwinger-boson approach. The quasiparticle dispersion relations have the correct zero-mode structure, without the pathologies present in the standard spin-wave approximation. For the nearest-neighbor S=1/2 model the theory predicts an ordered ground state, although with a largely reduced magnetization of only 18% of the classical value. Implications of the results for the low-temperature thermodynamics of the stacked S=3/2 kagomé antiferromagnet ${\mathrm{SrCr}}_{8\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{4+\mathit{x}}$ ${\mathrm{O}}_{19}$ are also discussed.