Investigating the effect of electronic correlation on the physical properties of late 3d transition-metal based system using ab-initio methods (PhD)

Abstract

The aim of the thesis is to understand how the electronic correlations are affecting various physical properties of late 3d transition-metal based systems like LaCoO3, FeSi and CoSi. The thesis presents a consistent study of these systems by using different ab-initio methods such as DFT, DFT+U, G0W0 and DFT+DMFT with the self-consistently calculated values of U & J. Each of these ab-initio methods deal differently with the different levels of electronic correlation existing in a material. The thesis shows an interesting study of LaCoO3 and CoSi where they have different strength of correlations while having same Co 3d orbital. At the same time, FeSi and CoSi exhibits same structure and just differ by one electron, but still their properties are differently affected by electronic correlation. The present thesis is divided into seven chapters. In chapter one, we discuss the history and development of the ab-initio methods for studying the properties of a material in condensed matter physics. This historical aspect is followed with the introduction of on-site Coulomb interaction U & Hund’s like exchange interaction J. For performing the electronic structure calculations these two parameters play an important role. After this, we reasoned out the need of these parameters and then followed by the description of first-principles based methods developed for calculating these parameters. After this chapter, we provide an overview of the theoretical methods such as DFT,DFT+U, GW, DFT+DMFT and semiclassical transport theory used for the investigations carried out in chapter two. Chapter 3 provides a systematic study of the electronic structure of LaCoO3 by using DFT,DFT+U and DFT+DMFT methodologies. The objective of this chapter is to see what level of electronic correlation is existing in the electronic structure of the compound. In this chapter we compared the density of states (DOS) as calculated from DFT, DFT+U and DFT+DMFT separately with photoemission (PES) and inverse photoemission spectra (IPES), respectively. The chapter brings out the difference between theoretical methods with their individual limitations in describing the experimental spectral attributes of the occupied and unoccupied states of LaCoO3. For the investigation, U is calculated using cDFT method ∼ 6.9 eV for Co 3d and using this U value J is calculated as ∼ 1.18 eV. The study shows that DFT has failed in even creating the hard gap while the DOS distribution seems to be similar to the experimental attributes. However, DFT+U created the gap ∼ 1.8 eV but eventually failed in explaining the DOS distribution with PES mainly. However, DFT+DMFT created the gap ∼ 1.1 eV and successfully explained the DOS distribution for both PES and IPES. This indicates that the level of correlations as treated within DFT+DMFT seems appropriate to understand the electronic states of LaCoO3. In chapter 4, we studied the effect of correlations of Fe (and Co) 3d electrons in FeSi (and CoSi) using DFT and DFT+DMFT methods. This is because FeSi has been reported to possess unusual temperature-dependent properties which are the consequences of its electron-electron interactions. At the same time, CoSi exhibits the same structure as FeSi does and just differs by one electron. So, there is an expectation of seeing the similar unusual temperature-dependent behavior in CoSi. We compared the calculated spectral functions from both DFT and DFT+DMFT methods with the available experimental x-ray PES data of both compounds. The calculated spectral functions from DFT+DMFT are found to provide fairly good representation for the experimentally observed PES for both compounds. This motivated us to carry forward the investigation by using DFT+DMFT method. From the study of the temperature-dependent (100-800 K) DOS and momentum resolved spectral functions the influence of electronic correlations on the electronic states of FeSi are found be largely affected than CoSi. From momentum resolved spectral functions, the excitations have shown enhanced broadening with temperature rise in FeSi whereas an opposite behavior is observed in CoSi. Chapter 5 studies the effect of pressure and temperature on the role of electronic correlation Fe 3d electrons in FeSi. In this chapter, the we tried to understand the pressure-dependent experimental observations of resistivity and bandgap by using ab-initio methods. At first, we studied the DFT obtained partial DOS and then DFT+DMFT obtained total DOS for T = 100 K and T = 300 K. Both the calculations have revealed the widening of the bandgap with increase in pressure which is similar to other experimental observations for stoichiometric FeSi. However, they could not explain the experimental observation of insulator-to-metal transition when T < 50 K with the condition P ≥ 14.4 GPa for nearly stoichiometric FeSi samples. As it is generally seen that the samples prepared experimentally are often found to be off-stoichiometric. Considering this fact, we then studied the electronic structure of Fe1.02Si0.98 using DFT based KKR-CPA method. KKR-CPA calculations revealed that the impurity states have been generated in the gapped region around the Fermi level and they have half-metallic behavior. With pressure increment the closure of the energy gap (in one channel) appears to be responsible for the experimental observation for the Fe excess sample. Further, KKR-CPA calculations have suggested the presence of strong Hund’s coupling responsible for creation of net magnetic moment at Si site when Fe is in excess. CoSi having the crystal structure which belongs to the B20 type with P213 spacegroup lacks the inversion center. Such a symmetry causes the emergence of unconventional fermionic quasiparticles (QPs) in CoSi which has been shown experimentally and theoretically (studies based on DFT). Since interacting picture is quite important because this picture takes account of the electronic correlations differently. Therefore, it is very important to see its effect onto these newly found fermions. In chapter 6, we studied the spectral functions of bulk and (001) surface of CoSi using DFT, DFT + DMFT (T = 100 K) and G0W0 methodologies with and without inclusion of spin–orbit coupling (SOC), respectively. SOC and electronic correlations appear to modify the nature of bands involved at Γ point. The existence of both coherent and incoherent features indicates the presence of QP–QP interactions which is eventually affecting the lifetime (τ) of exotic fermionic QPs. As CoSi has been reported to have exotic fermions at three nodal points in the band structure such as two at Γ (G1 & G2) and one at R (R1). Thus, if the chemical potential is kept close to the energies of these nodal points then there is a possibility of capturing the unconventional charge carriers’ contribution to electronic transport of CoSi. Based on this, the chapter 6 further studies the transport coefficients at these three nodal points for the temperature 40-300 K by using first principle based DFT method. At last in chapter 7, we summarize the thesis, with a brief overview of the significant conclusions drawn and give directions for future work.