Boson Mean Field Theory of the Square Lattice Heisenberg Model

Abstract

Two-dimensional Antiferromagnetic Heisenberg model is investigated by a mean field theory, where order parameter corresponding to the resonating valence bond (RVB) state is retained. The spin is expressed by Schwinger bosons. As has been shown before, this method gives long-range Néel order at T =0 [D. Yoshioka: J. Phys. Soc. Jpn. 58 (1989) 32], and gives reasonable results at finite temperatures [A. Auerbach and D. P. Arovas: Phys. Rev. Lett. 61 (1988) 617]. In the present paper phase diagram in a magnetic field is given. Exact expressions for the specific heat, uniform susceptibility, and correlation length at low temperatures are given, too, which improve the results by Auerbach and Arovas. The results are compared with those by computer simulations, and good agreement is obtained.