The effect of local correlations on the wave functions and experiment-theory comparisons within strongly and weakly correlated materials

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Many-body theories such as dynamical mean-field theory (DMFT) provide a description of the
local electron correlation effects that are missing in current density functional theory (DFT)
calculations. This thesis presents DFT with DMFT (DFT+DMFT) studies for various materials in
which the predicted description of the local electron correlations yields improved agreement with
the experiment for certain quantities.
Here, the methodology of the newly developed ELK-TRIQS interface is presented, including
how to calculate DFT+DMFT wave functions which can be used to calculate DFT+DMFT wavefunction-dependent quantities. This is first illustrated by calculating the electron localisation
function (ELF) in monolayer SrVO3 and CaFe2As2, which provides a means of visualising their
chemical bonds. Monolayer SrVO3 ELFs are sensitive to the charge redistribution between the
DFT, one-shot DFT+DMFT and fully charge self-consistent DFT+DMFT calculations. In both
tetragonal and collapsed tetragonal CaFe2As2 phases, the ELF changes weakly with correlation
induced charge redistribution of the hybridised As p and Fe d states.
The magnetic Compton profiles (MCPs) of Ni and the Compton profiles of V, using both DFT
and DFT+DMFT, are presented. For Ni, the theoretical MCPs were calculated using the full
potential linear augmented plane wave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver, along with the Korringa-Kohn-Rostoker (KKR) method with the
perturbative spin-polarised T-matrix fluctuation exchange approximation DMFT solver. The spin
magnetic moments decrease with the intra-atomic Coulomb repulsion U, which is also reflected
in the corresponding MCPs. The total magnetic moment obtained from the superconducting
quantum interference device measurements can be reproduced by intermediate values of U. However, the MCP shape still disagrees with the experiment. The spectral function reveals that the
minority X2 Fermi surface pocket shrinks and gets shallower with respect to the DFT calculations.
For V, The addition of DMFT improves the Compton profile directional differences significantly,
but these differences are fairly independent of the U values used. There are improvements in
the spectral function with respect to the corresponding experimental quantities, except for the
N-hole ellipsoid Fermi surface pocket sizes which worsen. For both Ni and V, there is a vital need
to include the missing electron correlations beyond the DFT+DMFT picture.
With the increasing interest in superlattice systems described by DMFT, here the metalinsulator transition (MIT) of strongly correlated 3d electrons in SrVO3 superlattices is shown
to be due to quantum confinement. By producing excellent agreement between experiment and
theoretical spectroscopic quantities, the underlying physics in these systems is captured by DMFT.
New light is shed on the microscopic mechanism of the MIT and previously reported anomalous
subband mass enhancement, both of which arise as a direct consequence of the quantization of
V xz(yz) states in the SrVO3 layers. Therefore, quantum confinement can tune the strength of
electron correlations.
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Date of Award11 May 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorStephen B Dugdale (Supervisor)

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