Triplet excitations in the frustrated spin ladder Li2Cu2O(SO4)2

International audienceMagnetic excitations of the recently discovered frustrated spin-1/2 two-leg ladder system Li2Cu2O(SO4)2 are investigated using inelastic neutron scattering, magnetic susceptibility and in-frared 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 ∆ = 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