EFFECTS OF PERTURBING SPINS ON THE PROPERTIES OF THE S=1/2 HEISENBERG-ANTIFERROMAGNET ON THE SQUARE LATTICE

Effects on ground-state properties of an extra spin coupled ferromagnetically or antiferromagnetically to the S = 1/2 Heisenberg antiferromagnet have been calculated for lattices of N= 8, 10, 16, 18, and 20 sites. For an antiferromagnetically coupled extra spin all spin-spin correlations and single-spin expectation values change discontinuously at a critical value, x(phi)(N), of the ratio of perturbing to unperturbed interaction strengths, x =J'/J. Our results provide evidence for a new type of zero-temperature phase transition on the infinite square lattice of possible relevance to the theory of high-T(c) superconductors.