Effects of biquadratic exchange on the spectrum of elementary excitations in spin ladders

TL;DR

This paper studies how biquadratic exchange interactions, especially ring exchange, affect the elementary excitations in spin ladders, revealing that moderate ring exchange reduces the spin gap and aligns theoretical spectra with experiments.

Contribution

It provides a detailed analysis of biquadratic exchange effects on spin ladder excitations, combining perturbation theory, numerical diagonalization, and exact solutions.

Findings

Moderate ring exchange significantly reduces the spin gap.

Equal bilinear exchange on legs and rungs matches experimental spectra.

The study offers insights into the role of biquadratic interactions in spin ladder physics.

Abstract

We investigate the influence of biquadratic exchange interactions on the low-lying excitations of a S=1/2-ladder using perturbation theory, numerical diagonalization of finite systems and exact results for ladders with matrix product ground states. We consider in particular the combination of biquadratic exchange interactions corresponding to ring exchange on the basic ladder plaquette. We find that a moderate amount of ring exchange reduces the spin gap substantially and makes equal bilinear exchange on legs and rungs consistent with experimentally observed spectra.