Quantum antiferromagnets with competing interactions

By applying the method of hierarchy of algebras, we derive exact thermodyna mic results of the square 4 x 4 lattice and of the linear ring of 16 s = 1/ 2 spins described by an isotropic Heisenberg Hamiltonian. Next-nearest-neig hbour interactions are taken into account. The entropy, the specific heat, magnetic susceptibility and the thermal average of the square of the total S-z for various values of the relative strength of the second neighbour int eractions are discussed. (C) 1999 Elsevier Science B.V. All rights reserved .