QUANTUM CORRECTION TO THE BKT TRANSITION FOR 2D EASY-PLANE ANTIFERROMAGNETS

We consider the quantum antiferromagnet with easy-plane exchange aniso tropy, namely, the antiferromagnetic XXZ model, on the square lattice. Its classical counterpart, compared to the planar model shows a reduc tion of the critical temperature T-BKT of the Berezinskii-Kosterlitz-T houless phase transition, that is a consequence of the thermal out-of- plane fluctuations. For the quantum system we use the pure-quantum sel f-consistent harmonic approximation to calculate how much the effectiv e exchange interaction is weakened as an effect of the pure-quantum pa rt of the fluctuations. One can then predict the further reduction of TB,with respect to the corresponding classical system. The theory work s well in a wide range of values of the easy-plane anisotropy. In the extreme case of the spin-1/2 model, the result is compatible with the estimate of T-BKT obtained by previous quantum Monte Carlo simulations . When the anisotropy is weak the theory leads to an unphysical ''isot ropization'' due to the use of the Villain spin-boson transformation. (C) 1996 American Institute of Physics.