科研之友
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摘要
We have investigated the J(1)-J(2) Heisenberg model with exchange anisotropy on a square lattice and focused on possible AF1-AF2 phase transition below the Neel point and its dependence on the exchange anisotropy, where AF1 and AF2 represent Neel state and collinear state, respectively. We use the double-time Green's-function method and adopt the random-phase approximation. The less the exchange anisotropy, the stronger the quantum fluctuation of the system will be. Both the Neel state and collinear state can exist and have the same Neel temperature for arbitrary anisotropy and spin quantum number S when J(2)/J(1) = 0.5. Under such parameters, the calculated free energies show that there may occur a first-order phase transition between the Neel state and collinear state for an arbitrary S when anisotropy is not strong.
