Effective model Hamiltonian downfolded from first principles

First-principles methods are tools for accurately predicting materials' electronic structures and properties. One application of highly accurate first-principles calculations is to derive effective models for describing the low-energy physics in many-electron systems. This dissertation is focused on deriving effective models from first principles for many-body systems using downfolding methods and improving the state-of-the-art downfolding framework. For applications to many-body systems, I explained the nanoscale electric field effect on monolayer 1T'-WTe2 electronic structure with layer-dependent spin-momentum locking, using a downfolded tight-binding Hamiltonian. To improve the downfolding methods, I generalized the density matrix downfolding method to incorporate the spin-orbit coupling so that both electron correlations and relativistic effects can be treated on equal footing. I also demonstrated in hydrogen chains that the density matrix downfolding method based on highly accurate first-principles many-body wave function techniques can detect emergent low- energy degrees of freedom. Another recent ongoing work is a comprehensive comparison between the widely used state-of-the-art downfolding method with highly accurate first-principles many-body wave function calculations. It will provide insights into how to correct the double counting error in density functional theory based downfolded models. Future directions of downfolding include systematically improving models using machine learning and potential applications in systems with multiple degrees of freedom. Downfolding from first principles will enhance our understanding of how the interplay between charges, spins, lattice, and their interactions give rise to the zoo of phases in correlated materials.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Chang, Yueqing

Contributors dc:contributor

• Wagner, Lucas K
• Hughes, Taylor L
• Cooper, S. Lance
• Kahn, Yonatan F

• Condensed matter physics
• First-principles calculations
• Downfolding

Statement dc:rights

• Copyright 2022 Yueqing Chang

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/117712

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F117712