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Understanding the interplay of lattice, charge, orbital, and spin degrees of freedom in strongly correlated electron systems involving frustration remains one of the great tasks in modern solid state physics. The role of electronic and magnetic excitations for, e.g., the Mott metal-insulator transition is a key question in this respect. Resonant inelastic x-ray scattering (RIXS) has been established recently as a powerful experimental
technique to study these phenomena in complex solids, promoted by considerably improved energy resolution and detector sensitivity.
Here we combine high-resolution RIXS with cluster calculations to analyze the temperature, excitation energy, polarization, and momentum dependence of the magnetic and orbital excitations of the quasi-one-dimensional spin-Peierls and Mott insulating system TiOCl. The data allows to extract precise values for the crystal-field splitting in the room temperature as well as the spin-Peierls phase. The polarization dependence of the crystal-field levels is in very good agreement with the cluster calculations. From comparison to spinon theory we identify a low-energy feature as a two-spinon excitation, signalling the quasi-one-dimensional nature of TiOCl. The predictive power of a recently derived effective scattering operator for magnetic excitations in RIXS [1] is evidenced for the first time by simulating the momentum dependence of this excitation. Furthermore, indications are found that the spin-gap extracted previously from nuclear magnetic resonance is manifested in the RIXS data. [1] M. W. Haverkort, Phys. Rev. Lett. 105, 167404 (2010) |
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