NEEL TEMPERATURE OF ORTHORHOMBIC LA2CUO4

We have studied the Neel temperature of undoped La2CuO4 in the orthorh ombic state using the quantum Heisenberg model and double-time-tempera ture spin Green's function theory. In the Tyablikov decoupling approxi mation, we find that the orthorhombic phase transition is necessary to keep La2CuO4 having 3D-AF ordering at finite temperatures. When the i nterlayer couplings J(1) in the a-b plane and J(2) in the b-c plane ar e much smaller than the antiferromagnetic coupling J(0) in the a-c lay er, the Neel temperature can be expressed in asymptotic form: k(B)T(N) = 2 pi J(0)/In[16J(0)/(J(1)-J(2))].