Interplay of electronic order and structural distortions in correlated materials
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2024
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Doctoral Thesis
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Abstract
In this thesis, I study correlated materials, specifically transition-metal oxides with a perovskite or double-perovskite structure. In the considered materials – CaFeO₃, Ba₂MgReO₆, KCuF₃, and LuNiO₃ – the transition metals have open d shells with strong local electron-electron interactions, which localizes these d electrons and makes the materials prone to symmetry breaking and electronic ordering. This electronic ordering is accompanied by lattice distortions and sometimes metal-insulator transitions, making this class of materials interesting for a range of applications, such as more energy-efficient transistors. The underlying phenomena are highly complex so a thorough understanding of the involved physics is necessary to optimize the materials for devices.
To better understand the role of the electrons and ions, I study the transitions in these materials with first-principles methods. The strong, local electron-electron interaction necessitates a treatment beyond density-functional theory (DFT) so I mainly use DFT+U and DFT+dynamical mean-field theory (DMFT) to describe magnetically ordered and paramagnetic materials. The projects here revolve around the competing ordered and disordered phases, the ordering mechanism when multiple competing or cooperative effects are at play, and the prediction of the strength of the local electron-electron interaction. Many of these projects have been carried out in collaboration with experimentalists and theorists within and outside my group.
In the first part of this thesis, I discuss the charge-disproportionating material CaFeO₃. I first demonstrate that the specific metal-insulator transition in this material can be described within a Hubbard model of the full d shell. Using DMFT, I map out the phase space of this model as a function of the local electron-electron interaction. Next, I calculate the local electron-electron interaction in CaFeO₃ in the constrained random-phase approximation. Here, I show that the local electronelectron repulsion is strongly screened by other electrons in the material, which in the context of the previously established phase diagram is a prerequisite for the insulating state in CaFeO₃. Finally, I give an outlook on first-principles DFT+DMFT calculations using the closely related rare-earth nickelate LuNiO₃ as an example.
In the second part, I consider another type of electronic order, local orbital order. Here, I first introduce the methodological project of constraining multipoles in DFT(+U). Multipoles provide a straightforward way to quantify orbital order, and constraining them allows us to investigate the energetics of orbital order without having to induce it with lattice distortions. I demonstrate this using the example of the prototypical orbitally ordering insulating material KCuF₃.
prototypical orbitally ordering insulating material KCuF₃. In the final two projects on orbital order, I investigate the material Ba₂MgReO₆ in depth. I first present my general DFT+DMFT study of Ba₂MgReO₆, which establishes that the insulating, paramagnetic phases are Mott insulating. In this project, I also compare the paramagnetic DFT+DMFT calculations to a recent DFT+U publication. Finally, in a joint experimental-theoretical study, I investigate the origin of the orbital order in Ba₂MgReO₆. I calculate the electronic susceptibility to quadrupolar order as a function of the temperature with DFT+DMFT, which together with other calculations in the publication shows that electronically a different ground state should emerge than observed experimentally. Therefore, I estimate the stabilization of the order by the electron-lattice coupling, which allows us to show that this stabilization is responsible for the experimental ground state.
In summary, the discussed projects have advanced the understanding of the correlated materials CaFeO₃ and Ba₂MgReO₆, and the methodological developments have opened up avenues for future research. This provides new insights into the interplay between structural distortions and electronic order, such as charge disproportionation and orbital order.
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03903 - Spaldin, Nicola A. / Spaldin, Nicola A.