Study of two-particle response and phase changes in strongly correlated systems using dynamical mean field theory
Creators: name: Chakrabarti, Bismayan
Publisher: name: No Publisher Supplied
Year: 2017

description: The study of strongly correlated materials is currently perhaps one of the most active areas of  research in condensed matter physics. Strongly correlated materials contain localized electronic  states which are often hybridized with more itinerant electrons. This interplay between localized  and delocalized degrees of freedom means that these compounds have highly complex phase diagrams  which makes these compounds very challenging to understand from a theoretical standpoint.  Computer simulations have proved to be an invaluable tool in this regard with state of the art abinitio  simulation techniques harnessing the ever-increasing power of modern computers to produce  highly accurate descriptions of a variety of strongly correlated materials. One of the most powerful  simulation techniques currently in existence is Dynamical Mean Field Theory (DMFT). This thesis  describes this powerful simulation technique and its applications to various material systems, as well  as addressing some theoretical questions concerning particular implementations of DMFT.  This thesis is divided into two parts. In part 1, we describe the theory behind DMFT and its  amalgamation with Density Functional Theory (DFT+DMFT). In chapters 2 and 3, we provide the  basic theory theory behind DFT and DMFT respectively. In chapter 4, we describe how these two  methods are merged to give us the computational framework that is used in this thesis, namely  DFT+DMFT. Finally, we round o  part 1 of the thesis in chapter 5, which provides a description  of the Continuous Time Quantum Monte Carlo (CTQMC) impurity solver, which is at the heart of  the DFT+DMFT algorithm and is used extensively throughout this thesis. In part two of the thesis, we apply the DFT+DMFT framework to address some important problems in condensed matter physics. In chapter 6, we study the Magnetic Spectral Function of strongly correlated f-shell materials to understand two important problems in condensed matter physics, namely the volume collapse transition in Cerium and the valence   uctuating state ground state of  -Pu. In chapter 7, we study the contribution of lattice parameters and electronic entropy to study the decades-old problem of understanding the spin state transition observed in LaCoO3, where we show how lattice expansion, octahedral rotations and electronic entropy are all essential  in stabilizing the high-spin state at high temperature. In chapter 8, we switch to studying a more  theoretical problem by looking at the problems with using the highly popular constrained Random  Phase Approximation (cRPA) method to estimate the screening of local inter-electronic repulsion in  strongly correlated systems. We show that cRPA systematically underestimates screening in such  systems which makes it an unsuitable method for estimating the repulsion parameter (U) used in  impurity solvers. We then develop an alternate method to estimate the screening using the full  local polarization which overcomes many of these limitations. Chapter 9 contains all the conclusions  obtained in this thesis, followed by references and appendices.