# Triplet excitations in the frustrated spin ladder Li$_2$Cu$_2$O(SO$_4$)$_2$

## TL;DR

This study investigates magnetic excitations in the frustrated spin-1/2 two-leg ladder system Li$_2$Cu$_2$O(SO$_4$)$_2$ using various experimental techniques, revealing dispersive triplet excitations and their coupling mechanisms.

## Contribution

It provides the first detailed experimental and theoretical analysis of triplet excitations in this specific frustrated spin ladder compound.

## Key findings

- Dispersive triplet excitations observed above a 10.6 meV spin gap.
- Infrared spectroscopy detects spin excitations via a dynamic Dzyaloshinskii-Moriya mechanism.
- Theoretical calculations support the experimental findings with a dimerized spin Hamiltonian.

## Abstract

Magnetic excitations of the recently discovered frustrated spin-1/2 two-leg ladder system Li$_2$Cu$_2$O(SO$_4$)$_2$ are investigated using inelastic neutron scattering, magnetic susceptibility and infrared absorption measurements. Despite the presence of a magnetic dimerization concomitant with the tetragonal-to-triclinic structural distortion occurring below 125 K, neutron scattering experiments reveal the presence of dispersive triplet excitations above a spin gap of $\Delta = 10.6$ meV at 1.5 K, a value consistent with the estimates extracted from magnetic susceptibility. The likely detection of these spin excitations in infrared spectroscopy is explained by invoking a dynamic Dzyaloshinskii-Moriya mechanism in which light is coupled to the dimer singlet-to-triplet transition through an optical phonon. These results are qualitatively explained by exact diagonalization and higher-order perturbation calculations carried out on the basis of the dimerized spin Hamiltonian derived from first-principles.