Cyclic spin exchange in cuprate spin ladders -: art. no. 180404

We investigate the influence of a cyclic spin exchange J(cyc) on the one- and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density-matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by J(cyc) due to a repulsion between triplets on neighboring rungs. In (La,Ca)(14)Cu24O41 a consistent description of both the spin gap and the S=0 bound state requires J(cyc)/J(perpendicular to)approximate to0.20-0.27 and J/J(perpendicular to)approximate to1.25-1.35. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.