A novel nonlinear spin wave theory for the spin 1/2 antiferromagnetic Heisenberg model on a triangular lattice

Abstract

We extend the nonlinear spin wave theory (NLSWT) for the spin 1/2 antiferromagnetic Heisenberg model on a triangular lattice (TAFHM). This novel NLSWT considers the corrections one order higher in 1/S than the linear spin wave theory (LSWT). It also distinguishes in which circumstance the negative energy excitation, the sign of the breakdown of LSWT, shall be renormalized to be positive both by a boson normal ordering and a self-consistent iteration. We draw a phase diagram by testing the stability of various magnetic orders for different parameters. In particular, the incommensurate configuration is found unstable by our study. The new phase transition point (PTP) of the collinear configuration agrees well with various previous studies.