Collective spin-excitations in Cu L and O K edge Resonant Inelastic X-ray Scattering on Sr$_{14}$Cu$_{24}$O$_{41}$.

Using Resonant Inelastic X-ray Scattering (RIXS) at the Cu$ L_{3}$ and O $K$ edge we studied the spin-ladder/spin-chain compound Sr$_{14}$Cu$_{24}$O$_{41}$ [1,2]. We observe collective spin-excitations from the ladders, which we assign to two-Triplon modes [3]. At the Cu $L_{3}$ resonance the dispersion of the modes was mapped out, obtaining excellent sensitivity over the full first Brillouin-zone, in contrast to Inelastic Neutron Scattering [4]. At the O $K$ edge site-sensitive hole-excitation studies of the chain- and ladder-subsystems were performed, giving insight into the character of the holes. [1] T. Vuletic et al., Physics Reports 428, 169-258 (2006). [2] A. Kotani and S. Shin, Rev. Mod. Phys. 73, 203 (2001). [3] K. P. Schmidt and G. S Uhrig. Mod. Phys. Rev. Lett. 90, 227204 (2003). [4] S. Notbohm et al., Phys. Rev. Lett. 98, 027403 (2007).