Incommensurate spin correlations in spin-1/2 frustrated two-leg Heisenberg ladders

We develop a low-energy effective theory for spin-1/2 frustrated two-leg Heisenberg spin ladders. We obtain a new type of interchain coupling that breaks parity symmetry. In the presence of an XXZ-type anisotropy, this interaction gives rise to a novel ground state, characterized by incommensurate correlations. In the case of a single ladder, this state corresponds to a spin nematic phase. For a frustrated quasi-one-dimensional system of infinitely many weakly coupled chains, this state develops true three-dimensional spiral order. We apply our theory to recent neutron scattering experiments on Cs2CuCl4.