First-Principle Calculations of Many-Body Effects in Solids Revealed by Positron Annihilation Experiments

Abstract

The advanced Density Functional Theory combined with Dynamical Mean-field Theory
(DFT+DMFT) approach offers a unique perspective for the investigation of complex
local electron correlation effects. The primary focus of this thesis is to explore the
reduced two-photon momentum distribution (RTPMD) and Fermiology of weak itinerant
ferromagnets using the advanced DFT+DMFT methods.
By comparing with positron annihilation experiments, the RTPMD of weak itinerant
ferromagnet ZrZn2 from DFT+DMFT method exhibits a significant improvement from
DFT results. The inclusion of DMFT self-energy leads to a renormalization of the flat
band of ZrZn2, resulting in the emergence of an additional Fermi neck that closely
aligns with experimental observations. This confirms the crucial role of local interactions
in determining the Fermi surface topology of the weak itinerant ferromagnet ZrZn2.
Simultaneously, it proved again that the positron annihilation technique is a sensitive
probe for the Fermiology of correlated electron systems.
The DFT+DMFT results for another weak itinerant ferromagnet, Ni3Al, exhibit
certain complexity. Due to limited investigations into this material, the information
of Hubbard U and Hund J is not clear. Therefore, DFT+DMFT calculations were
conducted using nine sets of different parameters. However, the outcomes indicate
that, while DFT+DMFT can alter the spectral function structure of Ni3Al, it does
not qualitatively improve the description of the RTPMDs. Additionally, results from
DFT+DMFT calculations employing fully localized double counting suggest that the
RTPMD of Ni3Al is not very sensitive to different choices of Hubbard U parameters.
Further exploration of this issue by considering a more comprehensive parameter space
or including interactions not include within the DMFT framework, such as non-local
interactions, may provide more deep insights.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Stephen B Dugdale (Supervisor)