| dc.description.abstract | Strong electron–electron interactions profoundly influence the properties of systems such as transition-metal, rare-earth, and actinide-based compounds, giving rise to a variety of exotic phenomena. In recent times, there has been a resurgence of interest in strongly correlated systems—especially those based on transition metals. An understanding of the electronic structure of these systems provides insights into the interesting properties observed in them. In this thesis, we have studied the electronic structure of strongly correlated systems using ab initio and model Hamiltonian approaches. A brief discussion of this is given in Chapter 1.
Single-impurity models [1] have traditionally been used to describe the electronic structure of transition-metal oxides [2, 3]. Within such an approach, a single transition-metal atom interacting with a cluster of oxygen atoms or with a ligand-derived p-band is usually considered. The neglect of the periodicity of the transition-metal sites is justified by their large spatial separation. However, it is seen that two transition-metal sites can have a non-negligible effective interaction via the oxygen atoms, resulting in substantial bandwidths of the transition-metal 3d-related bands. This has been shown to be particularly true in the case of the LaMO? (M = Mn–Ni) oxides, where the ground-state electronic and magnetic properties are well described within ab initio band-structure calculations [4].
To probe the relevance of ab initio approaches in describing the electronic structure of the LaMO? (M = Cr–Ni) compounds, we have carried out extensive analysis of various experimental electronic excitation spectra—namely, x-ray photoemission spectra, Bremsstrahlung isochromat spectra, and x-ray absorption spectra—in conjunction with band-structure calculations in terms of the partial densities of states obtained from LMTO-ASA calculations. In Chapter 2 of the thesis, we compare these experimental and calculated results, which establish a remarkably good agreement. This allows us to discuss the experimental spectra in terms of orbital characters as well as spin polarizations and obtain a detailed understanding of the electronic structure.
The gross electronic structure of transition-metal oxides depends primarily on three quantities:
the hopping interaction strength, t, between the transition-metal d and oxygen p orbitals,
the onsite Coulomb interaction strength, U, within the transition-metal 3d manifold, and
the charge-transfer energy, ?, between the oxygen p and the transition-metal d orbitals.
In view of the agreement between the band-structure and spectroscopic results for LaMO? compounds, in Chapter 3 we have extracted some of the parameter strengths by mapping [5] the results obtained from the spin-restricted ab initio band-structure calculations onto a nearest-neighbour tight-binding model. This provides us with estimates of the effective hopping interaction strength, t, and a relationship between ? and U in these compounds. The estimates of various interaction strengths compare well with those from spectroscopic data, wherever available. Since the parameter strengths extracted from experiments suffer from non-uniqueness, with a wide range of parameter values simulating the experimental spectra, we suggest that in various many-body calculations the hopping interaction strengths can be fixed to the values obtained here.
In the case of the compounds LaCrO?, LaMnO?, and LaFeO?, which have a magnetic ground state, we have carried out the analysis of the spin-polarized band-structure results corresponding to the experimentally observed magnetic structures. This analysis provides us with estimates of the intra-atomic exchange interaction strength, J, which plays an important role in the magnetic properties of these systems.
To understand the role of J on the electronic structure of transition-metal oxides, in Chapter 4 we study a multiband Hubbard-like model including J within the Hartree–Fock approximation. We have evaluated phase diagrams depicting different electronic and magnetic ground states in the U–? plane for different values of J. The specific model used corresponds to the two-dimensional lattice of NiO? corner-shared distorted octahedra found in La?NiO?. However, we believe that the generic features of the model can be used for discussing the electronic and magnetic properties of other transition-metal oxides as well. The inclusion of multiple d-orbitals and consequently the role of Hund’s coupling strength, J, yield a phase diagram with a mixture of first- and second-order transitions as well as reentrant behaviours.
In Chapter 5 of the thesis, we study the effect of degeneracy on the critical intra-atomic Coulomb interaction strength, U?, at which the metal–insulator transition takes place in a degenerate multiband Hubbard model for a finite system at various integral fillings using the Lanczos method [6]. In a 4-atom cluster with 3 degenerate p-orbitals at each atomic site, U? is found to be maximum near half-filling; the results further indicate a strong dependence of U? on degeneracy as well as on filling. The possible implications of these results in understanding the electronic structure of doped fullerenes are discussed. To investigate the effects of correlation at various fillings (integral and non-integral), we have studied the evolution of the spectral functions corresponding to the one-electron removal (photoemission) and the one-electron addition (inverse photoemission) spectra. The spectral features have been analysed in terms of their wavefunction characters. These results are also presented in Chapter 5.
In Chapter 6 of the thesis, we study the Auger spectra for an orbitally degenerate Hubbard model as a function of correlation and band filling. Previous work for a fully filled non-degenerate model [7] classifies the spectral features into two groups, arising from the two holes in the final state being either correlated or uncorrelated, corresponding to the holes being localized at the same atomic site or being delocalized at different sites, respectively. The spectral signatures of these two final states are expected to have an energy separation controlled by U/W, where W is the bandwidth. According to such an analysis, the results of Auger spectroscopy can be used to provide an estimate of U. However, calculated spectral functions for the partially filled degenerate model considered by us indicate multiple groups of spectral features apart from those present in the Auger spectra for the fully filled case [7]. The character of the various features indicates that the features dominating the Auger spectrum correspond to multi-electron excitations. Introducing the effects of the core hole in the initial state in the present model suggests major modifications in the spectral features arising from core-hole screening dynamics. Our calculations establish the surprising result that the core-hole potential suppresses the manifestations of correlation effects between the valence-level electrons, leading to the observation that the Auger spectra resemble the uncorrelated spectra even for large values of U. This method has been successfully applied to realistic systems such as Ni and LaCoO?, leading to a reinterpretation of Auger spectra from strongly correlated systems. | |
