Strong electron correlations in complex materials computational tools and applications
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Date
2025
Publication Type
Doctoral Thesis
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Abstract
In this thesis, we use computational tools to study the physics of strongly interacting
electron systems, with a focus on complex transition metal oxides. These materials,
characterized by open transition metal d shells and strong electron-electron interac-
tions, exhibit localization and ordering effects that drive a variety of diverse and fasci-
nating physical phenomena.
The interplay of these interactions with orbital, spin, and lattice degrees of freedom can
give rise, for instance, to Mott insulators, where Coulomb repulsion localizes electrons
and opens a band gap. Such materials hold promise for device applications, enabling
high on-off ratios for transistors, and potentially surpassing the Shockley-Queisser ef-
ficiency limit in photovoltaics. High-temperature superconductors are another hall-
mark of strong electronic interactions, though their underlying mechanism remains
unknown. Materials with strong electronic interactions also show potential in cataly-
sis.
Quantitative, material-specific computations of strongly interacting electron systems
often require methods that go beyond density functional theory (DFT). To account for
the physics of strong electronic interactions, we utilize DFT+U and DFT+dynamical
mean-field theory (DFT+DMFT). Both approaches depend on the Hubbard interaction
parameter U , which encodes the strength of electron-electron interactions and must be
provided as an input. Our research also focuses on ab initio methods to compute this
parameter.
These projects are carried out in close collaboration with experimentalists and theo-
rists, both within and beyond the Materials Theory group.
First, we investigate the physics of SrCrO3 , a material from the relatively understudied
family of alkaline earth-chromates. The electronic and magnetic behavior of SrCrO3
has been a subject of debate, with reports in the literature describing both metallic
and insulating phases under varying conditions. In this work, we examine two distinct
mechanisms driving a possible metal-insulator transition in SrCrO3 . We demonstrate
that a Jahn-Teller distortion, strongly coupled with the magnetic order, induces a band
gap, and this effect is further amplified under tensile epitaxial strain. These findings
align with experimental observations of SrCrO3 thin films under strain. Additionally,
we explore charge disproportionation as an alternative pathway for the metal-insulator
transition. Together, these results offer fresh insights into the interplay between struc-
tural and electronic correlations in SrCrO3 .
This work also advances several methodological aspects in the study of materials with
strong electronic interactions. First, we compare DFT+U with the Hartree-Fock limit
of DFT+DMFT by solving the DMFT impurity problem within the Hartree-Fock ap-
proximation. To ensure consistency, Wannier functions are used for a unified def-
inition of the atomic orbitals in both approaches. The comparison confirms that
DFT+DMFT reduces to DFT+U in this limit. Next, we examine the similarities and
differences between the two most widely used methods for computing the Hubbard
U parameter: linear response theory (LRT) and the constrained random-phase ap-
proximation (cRPA). A quantitative comparison is conducted by applying these tech-
niques to the same set of orbitals, with results presented for two materials, Sr2 FeO4 and
KCuF3 . Finally, we investigate the explicit treatment of ligand states in DFT+DMFT.
We demonstrate that an approximate but realistic Hartree-Fock treatment of the lig-
and p states improves agreement with spectroscopic data for LaTiO3 , LaVO3 , and rare-
earth nickelates, while also reproducing the experimentally observed insulating behav-
ior of these compounds.
Overall, this thesis deepens our understanding of the interplay of spin, charge, and
orbital degrees of freedom in the alkaline-earth chromates, revealing how phenomena
such as Jahn-Teller distortions, charge disproportionation, and strain effects can affect
the transport and electronic properties of the material. Additionally, the methodolog-
ical advances in DFT+U , DFT+DMFT, and computation of the interaction parameter
provide versatile tools for exploring and optimizing materials with strong electronic
interactions, with implications for designing materials with tailored electronic, mag-
netic, and optical functionalities.
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ETH Zurich
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strongly correlated electron systems
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03903 - Spaldin, Nicola A. / Spaldin, Nicola A.