Spin-1/2 Heisenberg antiferromagnet on an anisotropic kagome lattice

We use the coupled-cluster method to study the zero-temperature properties of an extended two-dimensional Heisenberg antiferromagnet formed from spin-1/2 moments on an infinite spatially anisotropic kagome lattice of corner-sharing isosceles triangles, with nearest-neighbor bonds only. The bonds have exchange constants J(1) > 0 along two of the three lattice directions and J(2) = kappa J(1) > 0 along the third. In the classical limit, the ground-state (GS) phase for kappa < 1/2 has collinear ferrimagnetic (Neel') order where the J(2)-coupled chain spins are ferromagnetically ordered in one direction with the remaining spins aligned in the opposite direction, while for kappa > 1/2 there exists an infinite GS family of canted ferrimagnetic spin states, which are energetically degenerate. For the spin-1/2 case, we find that quantum analogs of both these classical states continue to exist as stable GS phases in some regions of the anisotropy parameter kappa, namely, for 0 < kappa < kappa(c1) for the Neel' state and for (at least part of) the region kappa > kappa(c2) for the canted phase. However, they are now separated by a paramagnetic phase without either sort of magnetic order in the region kappa(c1) < kappa < kappa(c2), which includes the isotropic kagome point kappa = 1 where the stable GS phase is now believed to be a topological (Z(2)) spin liquid. Our best numerical estimates are kappa(c1) = 0.515 +/- 0.015 and kappa(c2) = 1.82 +/- 0.03.