Influence of an inter-chain coupling on spiral ground-state correlations   in frustrated spin-1/2 J1-J2 Heisenberg chains
  We investigate the influence of an inter-chain coupling on the spiral ground state correlations of a spin-1/2 Heisenberg model consisting of a two-dimensional array of coupled chains with nearest (J1) and frustrating next-nearest neighbor (J2) in-chain exchange couplings. Using the coupled-cluster method we calculate the transition point between the commensurate and the incommensurate (spiral) ground state as well as the quantum pitch angle of the spiral ground state. In addition, we provide a simple empirical formula which describes the relation between the quantum pitch angle and the frustration parameter J2/J1. 
Ronald Zinke, Stefan-Ludwig Drechsler, and Johannes Richter