Large-S analysis of a quantum axial next-nearest-neighbor Ising model in one dimension

We study the large-S limit of a one-dimensional quantum spin model in a transverse magnetic field. For S=½, this is dual to the quantum Hamiltonian limit of the two-dimensional axial next-nearest-neighbor Ising model. The phase diagram has three regions; paramagnetic, ferromagnetic, and a modulated phase of period 2. The Peschel-Emery curve is discovered to be a disorder line. The long-distance correlation functions behave differently on the two sides of the line, and the correlation length goes to zero upon approaching it from either side