Effect of four-spin interaction on magnetic excitation in ladder and 2D insulating cuprates

Abstract

We investigate the effect of the cyclic four-spin interaction J cyc on the magnetic excitation spectra in two-leg ladder and two-dimensional (2D) insulating cuprates. The dynamical spin-correlation function S( q, ω) is calculated by using the numerically exact diagonalization method. For the ladder system, we find that the dispersion of spin wave along momentum q =(0, π) to (π/2,π) flattens by introducing J cyc. This feature is consistent with the experimental results of recent inelastic neutron-scattering measurement. For the 2D system, we find that the J cyc reduces effectively the magnetic interactions between nearest-neighbor (NN) Cu spins by about 20%. Moreover, it is found that the dispersion along the magnetic zone boundary from q =( π,0) to (π/2,π/2) has a finite width. This behavior can be understood by considering that the J cyc gives effectively a ferromagnetic diagonal interaction between next NN Cu spins. In order to verify the effect of J cyc, the inelastic neutron-scattering experiment at high-energy region will be highly desirable for 2D cuprates.