Numerical evidence for the spin-Peierls state in the frustrated quantum antiferromagnet

Abstract

We study the spin-1/2 Heisenberg antiferromagnet with an antiferromagnetic (Formula presented) (third-nearest-neighbor) interaction on a square lattice. We numerically diagonalize this “(Formula presented)-(Formula presented)” model on clusters up to 32 sites and search for ground-state properties as the frustration parameter (Formula presented)/(Formula presented) changes. For “larger” (Formula presented)/(Formula presented) we find enhancement of the incommensurate spin order, in agreement with various theoretical predictions. But for intermediate (Formula presented)/(Formula presented), the low-lying excitation energy spectrum suggests that this incommensurate order is short range. Analysis of the ground-state expectation of the order parameter and the symmetries of the first excited state suggest that the spin-Peierls state is the best candidate for the ground state of the (Formula presented)-(Formula presented) model in an intermediate (Formula presented)/(Formula presented) region. © 1996 The American Physical Society.