d-d Excitations in Bilayer Manganites Probed by Resonant Inelastic X-ray Scattering

摘要

We report a high resolution resonant inelastic x-ray scattering investigationof the bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x = 0.36$ and$0.5$. The momentum dependence along the crystallographic $(110)$ direction forenergy losses $1\,\rm{eV} \leq \Delta E \leq 15\,\rm{eV}$ has been measured indetail with the data analysis focusing on the energy loss region $1\,\rm{eV}\leq \Delta E \leq 5\,\rm{eV}$, which includes a strong peak located at $\DeltaE \approx 2\,\rm{eV}$. We observe a clear dispersion of up to $0.5\,\rm{eV}$ inthe measured $\textbf{q}$ range, which is direct evidence of the non-localcharacter of this excitation. Further, we found that the intensity in this lowenergy region strongly depends on both the reduced wave vector $\textbf{q} =(h, h, 0)$, $h = 0.1 - 0.5$, and temperature, i.e. different ordered phases.Results can be explained via an intersite $d-d$ charge transfer excitation,proposed for pseudo-cubic manganites, where the hopping rate is stronglyincreased (decreased) by ferromagnetic (antiferromagnetic) alignment ofneighboring in-plane Mn ion core spins.