Dynamics in a two-leg spin ladder with a four-spin cyclic interaction

We study the two-leg Heisenberg ladder with four-spin cyclic interaction using the (dynamical) density-matrix renormalization-group method. We demonstrate the dependence of the low-lying excitations in the spin wave, staggered dimer order, and scalar-chirality order structure factors on the four-spin cyclic interaction. We find that the cyclic interaction enhances spin-spin correlations with wave vector around momentum (q(x), q(y)) = (pi/2, 0). Also, the presence of long-range order in the staggered dimer and scalar-chirality phases is confirmed by a delta-function peak contribution of the structure factors at energy omega = 0.