SERIES EXPANSION FOR THE J(1)-J(2) HEISENBERG-ANTIFERROMAGNET ON A SQUARE LATTICE

We have developed series expansions about the Ising Limit for the grou ndstate energy, magnetization, susceptibility, and energy gap of the f rustrated J(1)-J(2) antiferromagnet. We find that the Neel order vanis hes at J(2)/J(1) similar or equal to 0.4 and collinear order sets in a round J(2)/J(1) similar or equal to 0.6, in broad agreement with other recent work. We also explore the nature of the phase diagram for the spin-anisotropic case.