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The 3d2 vanadates RVO3 (R = Y or rare earth) display a variety of orbitally and/or
magnetically ordered states, and orbital exchange interactions as well as orbital
fluctuations are expected to be strong. We have studied the orbital excitations
via RIXS and infrared absorption, and investigated the orbital correlations via
ellipsometry. Using high-resolution RIXS we aim at the first unambiguous observation of a novel kind of elementary excitations in a solid, namely orbitons, propagating orbital excitations. We have observed orbital excitations in YVO3 and GdVO3 at the V L edge and the O K edge. Due to the excellent resolution of 60 meV, we have been able to resolve two different features at low energies, which we interpret as one-orbiton and two-orbiton excitations. The one-orbiton feature displays a clear momentum dependence. We discuss whether this reflects a matrix-element effect or the sought-after dispersion. Using ellipsometry, we study the excitations from the lower to the upper Hubbard band. We obtain a consistent description of the multi-peak structure in terms of the different d3 multiplets, solving the problems encountered by other groups. The optical spectral weight of the different absorption peaks shows a strong temperature dependence, which reflects nearest-neighbor spin-spin and orbital-orbital correlations. We compare our data with theoretical predictions based on either 'classical' orbital order or strong orbital fluctuations. |
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