Philipp Scheiderer (PhD thesis):Spectroscopy of Prototypical Thin Film Mott Materials

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 e ects remains challenging for modern solid state physics, therefore experimental data is required for further progress in the eld. 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 SrVO₃ and the Mott insulator LaTiO₃, both of which are fabricated as high quality thin lms. In SrVO₃ thin lms, a reduction of the lm 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 e ective bandwidth usually identi ed 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 lms. For LaTiO₃ thin lms, the tendency to over-oxidize is even stronger. An uncontrolled oxygen di usion from the substrate into the lm is found to corrupt the electronic properties of LaTiO₃ layers grown on SrTiO₃. The Mott insulating phase is only detected in stoichiometric lms fabricated on more suitable DyScO₃ substrates. In turn, it is demonstrated that a controlled incorporation of excess oxygen ions by increasing the oxygen growth pressure is an e ective way of p doping the material which is used to drive the band lling induced Mott transition.
Gaining control of the oxygen stoichiometry in both materials allows for a systematic investigation of correlation e ects 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 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 veri es 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.

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