Spin correlation functions in the large-α region of the square-lattice J1-J2 model

In the context of modified spin-wave theory, we perform a finite-temperature spin-wave calculation in the large-α region of the J1-J2 model, where α = J2/J1. The spin correlation functions for large separations are calculated and the correlation lengths at very low temperatures are given. The two-types of spin correlation functions with a spatial anisotropy are obtained, and each of them is expressed in terms of the two different correlation lengths. At α → ∞, the J1-J2 model on the square lattice with a lattice spacing a should physically be divided into the two disconnected square-lattice antiferromagnets with a lattice spacing √2a. At this limit, one of our two correlation functions vanishes and the other is correctly reduced to the one in the square-lattice antiferromagnet. Also the two correlation lengths are unified and are reduced to the one in the above square-lattice antiferromagnet. (author)