The transition-metal oxides (TMOs) are a material class host to a number of intriguing and potentially technologically useful phenomena as a result of many-body correlation effects, from superconductivity, magnetic and orbital ordering, to ferroelectricity and metal-insulator transitions. Here, materials with similar structures and seemingly equivalent electronic configuration often exhibit wildly different properties as a result of strong competition between different ground states from the many degrees of freedom, whose balance can be further tuned through the use of pressure, doping, magnetic fields or temperature.
To investigate these materials, we make use of photoelectron spectroscopy (PES), probing elementary excitations possible in the material and thus providing linked information both about the ground state and possible excited states, closely related to the physical properties of a material such as its response to external fields. Angle-resolved PES (ARPES) provides additional momentum information and as a result, it is uniquely suited to investigate the character of the electronic structure of solids as it resolves the dispersion, meaningful in the independent-electron view where crystal momentum is a well-defined quantum number, but which can retain validity even in strongly correlated systems through the concept of quasiparticles.
While ARPES is a well-established technique, it is rarely used in the soft X-ray regime (SX-ARPES) due to significant experimental challenges posed. However, the higher energies in SX-ARPES allow it to be significantly more bulk-sensitive, an extremely important fact since the properties of the bulk material and its surface are often extremely, or worse, subtly different. Critically, this permits measurements on single crystals of TMOs, whose surfaces may show roughness or reconstruction, for example as a result of a polar surface compensation, but whose bulk properties are well-defined in contrast to thin films which are additionally subject to substrate effects.
We demonstrate on three rather different perovskite oxides, a three-dimensional class of TMOs, that is worthwhile to overcome these issues since it provides access to the true momentum-resolved bulk electronic properties of materials and allows filling noticeable gaps in literature of k-resolved electronic structure measurements for this class of compounds stemming from the impossibility of such measurements at lower energy. A commonality between the materials studied in this thesis is the absence of a strong electronic symmetry-breaking order, such as local-moment antiferromagnetism or charge ordering, that could suppress the existence of sufficiently long-lived quasiparticles to observe dispersion (or equivalently prevent a mobile photo-hole).
We first establish that SX-ARPES is indeed capable and suited to measure the bulk-representative electronic structure by measurements on the perfect cubic d1 perovskite ReO3. We present the first k-resolved electronic structure for this material which is rather well explained by band structure, especially close to the Fermi level. In particular, we show and quantify the impact of the significant spin-orbit coupling on the Fermi surface and bands. However, the oxygen bands are less well reproduced by calculations and are correctable by use of hybrid functionals, taken as a sign of spurious self-interaction effects - likely due to the large extent and density differences between delocalised Re 5d and more localised O 2p. We also show that there are signs of some hitherto unknown distortion in ReO3.
We then turn to LaNiO3, a metallic oxide in a family of formally d7 rare-earth nickelates which otherwise all undergo metal-insulator and antiferromagnetic (AFM) transitions as well as oxygen bond disproportionation, with a strong competition between these ground states and possible exotic resulting states in the phase diagram. We are able to resolve the dispersion of the eg quasiparticle spectrum along high symmetry cuts of this material as well as its Fermi surface, the latter of which is accurately reproduced by band theory calculations. We investigate the influence of the rhombohedral distortion present in the material through unfolding methods to better compare their influence to measurement, and show how significantly it affects the dispersion, confirming again the importance of single crystals. Its effects are shown to be similar to correlation-induced mass enhancement and their effects are untangled with the help of first DFT+U and later rhombohedral multi-band dynamical mean-field theory (DMFT) calculations. Local correlation effects prove to be the dominant influence on the spectrum, although certain k-dependent mismatches remain, pointing to a possible simultaneous importance of non-local mechanisms.
Finally, on the d6 system LaCoO3 that is close to a spin-state transition, we show that this method can also be applied to insulating oxides. Absent a Fermi surface, we naturally concentrate more on the full valence band, where we show that the observed dispersion is well-described by mean-field band methods in the low-spin (LS) regime of LaCoO3 provided that static energy corrections of DFT+U are accounted for (which show a good match to local LS many-body configuration interaction calculations), thus providing k-resolved evidence that one may effectively consider LS LaCoO3 a band insulator, despite possibly strong correlations. We further unveil clear evidence of crystal periodicity doubling by observation of a backfolded oxygen band, and show evidence of a significant asymmetry in the k-resolved lineshape in the valence band and lastly we take a look at the spin state of Co at the surface, which, contrary to prior results, appears to be the same as in the bulk, but which we show to be complicated by significant orbital-shape matrix element effects.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:91250 |
| Date | 14 May 2024 |
| Creators | Falke, Johannes |
| Contributors | Tjeng, Liu Hao, Laubschat, Clemens, Hansmann, Philipp, Technische Universität Dresden, Max-Planck-Institut für Chemische Physik fester Stoffe |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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