Invited Paper SA+2D+AC+MI-WeM5
Resonant Inelastic X-ray Scattering on Low-Dimensional Correlated Transition Metal Oxides and Oxide Heterostructures

Wednesday, November 1, 2017, 9:20 am, Room 9

Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in / photon-out spectroscopic probe of the electronic structure of condensed matter with atomic and orbital sensitivity. It is a unique tool for studying excitations from the electronic ground state in correlated transition-metal oxides, being directly sensitive to lattice-, charge-, orbital- and spin-degrees of freedom. In this talk, we report RIXS investigations of the LaTiO₃ layers in (LaTiO₃)_n/(LaAlO₃)₅ superlattices undergoing a transition from Ti³⁺ to Ti⁴⁺ oxidation state upon reducing n and thickness as well as temperature-driven metal-insulator transitions in thin films of CaVO₃.

(LaTiO₃)_n/(LaAlO₃)₅ superlattices (SL) composed of a band-insulator (LaAlO₃) and a Mott-insulator (LaTiO₃) present an enhanced insulating character when n is reduced. We prepared a set of SLs (n=10, 5 and 2 unit cells) and investigated these with X-ray absorption spectroscopy (XAS) and RIXS. XAS shows a clear change in the Ti valence going progressively from the nominal Ti³⁺ (3d¹, n=10 u.c.) for bulk LaTiO₃ to an almost pure Ti⁴⁺ (3d⁰, n=2 u.c.). RIXS reveals two spectral developments when reducing the LaTiO₃ thickness n: 1) reduction of intra-t_2g / intra-e_g splitting and increase of t_2g to e_g separation and 2) increase of the charge transfer excitation spectral weight. The changes in the energy of the orbital levels observed as a function of n reveal a clear change of the local TiO₆ distortion. We suggest that an inverse Jahn-Teller effect, inducing the octahedra to assume higher symmetry, is responsible for the observed orbital energy shifts. This peculiar effect is partially caused by strain, triggering a 3d¹ → 3d⁰ electron transition at the interfacial Ti sites.

Bulk CaVO₃ is a correlated paramagnetic metal. Thin films of CaVO₃ undergo a metal-insulator transition (MIT) when the thickness is reduced below ca. 20 u.c.. Our XAS and RIXS measurements at the V L-edge across this dimensionality driven MIT in CaVO₃ reveal a large transfer of spectral weight from fluorescent to Raman modes upon entering the insulating state. We observe a large reduction in the charge excitation bandwidth and V-O covalence across the thickness and temperature-driven MIT. Further analysis of the charge modes suggests a bandwidth-controlled MIT, assisted by the presence of strong correlations.