Thursday 3.12.2009, 15:15-16:15 HS P.


Orbitons and orbital correlations in RVO3 studied by RIXS; infrared absorption and ellipsometry

Prof. Markus Grüninger

Univ. Köln

The orbital degree of freedom plays an important role in the 3d2 vanadates RVO3 (R = Y or rare earth). These compounds 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. At the V L edge, we are able to resolve (i) the intra-t2g excitations, (ii) spin-flip excitations within the t2g shell, and (iii) excitations into the eg levels, in excellent agreement with our infrared data. At the O K edge, we find evidence for a bi-orbiton excitation. 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. Comparison with theoretical predictions based on either 'classical' orbital order or strong orbital fluctuations favors the latter.