NUMERICAL EVIDENCE FOR THE SPIN-PEIERLS STATE IN THE FRUSTRATED QUANTUM ANTIFERROMAGNET

We study the spin-g Heisenberg antiferromagnet with an antiferromagnet ic J(3) (third-nearest-neighbor) interaction on a square lattice. We n umerically diagonalize this ''J(1)-J(3)'' model on clusters up to 32 s ites and search for ground-state properties as the frustration paramet er J(3)/J(1) changes. For ''larger'' J(3)/J(1) we find enhancement of the incommensurate spin order, in agreement with various theoretical p redictions. But for intermediate J(3)/J(1), the low-lying excitation e nergy spectrum suggests that this incommensurate order is short range. Analysis of the ground-state expectation of the order parameter and t he symmetries of the first excited state suggest that the spin-Peierls state is the best candidate for the ground state of the J(1)-J(3) mod el in an intermediate J(3)/J(1) region.