Spectroscopy of prototypical thin film Mott materials

Authors: Philipp Scheiderer (Universitat Wurzburg)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2019

Abstract: The rich phase diagram of transition metal oxides essentially roots in the many body physics arising from strong Coulomb interactions within the underlying electron system. Understanding such electronic correlation effects remains challenging for modern solid state physics, therefore experimental data is required for further progress in the field. For this reason, spectroscopic investigations of prototypical correlated materials are the scope of this thesis. The experimental methods focus on photoelectron spectroscopy, and the test materials are the correlated metal SrVO 3 3 and the Mott insulator LaTiO 3 3 , both of which are fabricated as high quality thin films. In SrVO 3 3 thin films, a reduction of the film thickness induces a dimensional crossover from the metallic into the Mott insulating phase. In this thesis, an extrinsic chemical contribution from a surface over-oxidation is revealed that emerges additionally to the intrinsic change of the effective bandwidth usually identified to drive the transition. The two contributions are successfully disentangled by applying a capping layer that prevents the oxidation, allowing for a clean view on the dimensional crossover in fully stoichiometric samples. Indeed, these stoichiometric layers exhibit a higher critical thickness for the onset of the metallic phase than the bare and therefore over-oxidized thin films. For LaTiO 3 3 thin films, the tendency to over-oxidize is even stronger. An uncontrolled oxygen diffusion from the substrate into the film is found to corrupt the electronic properties of LaTiO 3 3 layers grown on SrTiO 3 3 . The Mott insulating phase is only detected in stoichiometric films fabricated on more suitable DyScO 3 3 substrates. In turn, it is demonstrated that a controlled controlled incorporation of excess oxygen ions by increasing the oxygen growth pressure is an effective way of p p doping the material which is used to drive the band filling induced Mott transition. Gaining control of the oxygen stoichiometry in both materials allows for a systematic investigation of correlation effects in general and of the Mott transition in particular. The investigations are realized by various photoelectron spectroscopy techniques that provide a deep insight into the electronic structure. Resonant photoemission not only gives access to the titanium and vanadium related partial density of states of the valence band features, but also shows how the corresponding signal is enhanced by tuning the photon energy to the L L absorption threshold. The enhanced intensity turns out to be very helpful for probing the Fermi surface topology and band dispersions by means of angular-resolved photoemission. The resulting momentum resolved electronic structure verifies central points of the theoretical description of the Mott transition, viz. the renormalization of the band width and a constant Luttinger volume in a correlated metal as the Mott phase is approached.

Subject Areas: Materials, Physics

Instruments: I09-Surface and Interface Structural Analysis

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