Mean-field theory of the spiral phases of a doped antiferromagnet

We develop a Schwinger boson-slave fermion mean-field theory for the t-t’-J model, where t’ represents same sublattice hopping. In an expansion for small hole concentration, δ, we show that doping favors a spiral distortion of the spins in which the antiferromagnetic order parameter spirals with a wave vector proportional to δ. In addition, we point out the novel possibility of a double spiral state which has isotropic spin correlations, is disordered for arbitrarily small δ, and exhibits a staggered chiral order. The relationship with resonating-valence-bond theory and the implications for neutron scattering experiments are discussed.