A NONLINEAR SPIN-WAVE THEORY OF QUASI-2D QUANTUM HEISENBERG ANTIFERROMAGNETS

Based on the Hartree-Fock approximation we propose a non-linear spin-wave theory of anisotropic 3D (or quasi-2D) quantum Heisenberg antiferromagnets, which reduces to the known spin-wave theories of isotropy when our anisotropy parameter delta takes special values. In the Hartree-Fock approximation the Dyson-transformed Hamiltonian is equivalent to the Holstein-Primakoff-transformed Hamiltonian truncated up to quartic operator terms. The spin-wave lifetime is obtained in the first-order approximation. For very small delta, the Neel temperature T(N) is much smaller.than the coupling constant J, in contrast with T(N) approximately J in the 3D isotropic case, so that our non-linear anisotropic spin-wave theory is suitable for a description of the ordering phase as well as the paramagnetic phase (up to J) of layer-like antiferromagnets. Applied to the antiferromagnetism of the cuprate La2CuO4, our quasi-2D non-linear spin-wave theory describes quite satisfactorily the existing experimental data of the Neel transition temperature, the correlation length above the Neel temperature, and staggered magnetization of the material if J = 1034 K and the anisotropy parameter is se to be 4 x 10(-5).