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Title:  First principles quantum Monte Carlo study of correlated electronic systems 
Author(s):  Zheng, Huihuo 
Director of Research:  Wagner, Lucas K. 
Doctoral Committee Chair(s):  Ryu, Shinsei 
Doctoral Committee Member(s):  Ceperley, David M.; Cooper, S. Lance; Hirata, So 
Department / Program:  Physics 
Discipline:  Physics 
Degree Granting Institution:  University of Illinois at UrbanaChampaign 
Degree:  Ph.D. 
Genre:  Dissertation 
Subject(s):  Quantum Monte Carlo
electronic structure strongly correlated system vanadium dioxide graphene effective model extended Koopmans' theorem 
Abstract:  The manybody correlation between electrons is the origin of many fascinating phenomena in condensed matter systems, such as high temperature superconductivity, superfluidity, fractional quantum Hall effect, and Mott insulator. Strongly correlated systems have been an important subject of condensed matter physics for several decades, especially after the discovery of high temperature cuprate superconductors. In this thesis, we apply first principles quantum Monte Carlo (QMC) method to several representative systems to study the electron correlations in transition metal oxides (vanadium dioxide) and low dimensional electronic systems (graphene and graphenelike two dimensional systems). Vanadium dioxide (VO2) is a paradigmatic example of a strongly correlated system that undergoes a metalinsulator transition at a structural phase transition. To date, this transition has necessitated significant posthoc adjustments to theory in order to be described properly. We apply first principles quantum Monte Carlo (QMC) to study the structural dependence of the properties of VO2. Using this technique, we simulate the interactions between electrons explicitly, which allows for the metalinsulator transition to naturally emerge, importantly without adhoc adjustments. The QMC calculations show that the structural transition directly causes the metalinsulator transition and a change in the coupling of vanadium spins. This change in the spin coupling results in a prediction of a momentumindependent magnetic excitation in the insulating state. While twobody correlations are important to set the stage for this transition, they do not change significantly when VO2 becomes an insulator. These results show that it is now possible to account for electron correlations in a quantitatively accurate way that is also specific to materials. Electron correlation in graphene is unique because of the interplay of the Dirac cone dispersion of pi electrons with long range Coulomb interaction. The random phase approximation predicts no metallic screening at long distances and low energies because of the zero density of states at Fermi level. It is thus interesting to see how screening takes place in graphene at different length scales. We addressed this problem by computing the structure factor S(q) and S(q, ω) of freestanding graphene using ab initio fixednode diffusion Monte Carlo and the random phase approximation. The Xray measured structure factor is reproduced very accurately using both techniques, provided that sigmabonding electrons are included in the simulations. Strong dielectric screening from sigma electrons are observed, which redshifts the pi plasmons resonance frequency at long distance and reduces the effective interactions between pi electrons at short distance. The short distance screening makes suspended graphene a weakly correlated semimetal which otherwise would be an insulator. The third piece of works is dedicated to studying the low energy excitation of manybody systems using extended Koopmans' theorem (EKT). The EKT provides a straight forward way to compute charge excitation spectra, such as ionization potentials, electron affinities from any level of theory. We implemented the EKT within the QMC framework, and performed systematic benchmark studies of ionization potentials of the second and thirdrow atoms, and closed and openshell molecules. We also applied it to compute the quasiparticle band structure of solids (graphene). For complex correlated systems, identifying relevant low energy physics degrees of freedom is extremely important to understanding the system's collective behavior at different length scales. In this sense, bridging the realistic systems to lower energy effective lattice models that involve fewer but important degrees of freedom is significant to understanding correlated systems. We have formulated three ab initio density matrix based downfolding (AIDMD) methods to downfold the ab initio systems into effective lattice models. We have demonstrated the successfulness of these methods by applying them to molecules (H2) and periodic systems (hydrogen chain and graphene). 
Issue Date:  20160712 
Type:  Text 
URI:  http://hdl.handle.net/2142/92794 
Rights Information:  Copyright 2016 Huihuo Zheng 
Date Available in IDEALS:  20161110 
Date Deposited:  201608 
This item appears in the following Collection(s)

Dissertations and Theses  Physics
Dissertations in Physics 
Graduate Dissertations and Theses at Illinois
Graduate Theses and Dissertations at Illinois