Cyclic Spin Exchange in Cuprate Spin Ladders

TL;DR

This paper studies how cyclic spin exchange affects excitations in cuprate spin ladders, revealing its significant role in matching experimental spectra and providing insights into the magnetic interactions in these materials.

Contribution

It demonstrates the importance of cyclic spin exchange in accurately describing the excitation spectrum of cuprate spin ladders using DMRG methods.

Findings

Cyclic spin exchange reduces the dispersion of two-triplet bound states.

Optimal coupling ratios for describing experimental data are identified.

Dynamical DMRG accurately reproduces the observed excitation spectrum.

Abstract

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}Cu_{24}O_{41} a consistent description of both the spin gap and the S=0 bound state requires $J_{cyc}/J_\perp \approx 0.20-0.27$ and $J/J_{\perp} \approx 1.25-1.35$. 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.