I Theory of Mott Transition and Correlated Metals.- Classification Scheme of the Metal-Insulator Transition and Anomalous Metals.- The Mott Transition: Results from Mean-Field Theory.- Some Aspects of Spin Gap in One- and Two-Dimensional Systems.- Quasi-Particles in Two-Dimensional Hubbard Model: Splitting of Spectral Weight.- Almost Localized Fermions and Mott-Hubbard Transitions at Non-Zero Temperature.- Anomalous Physical Properties Around Magnetic and Metal-Insulator Transitions - A Spin-Fluctuation Theory.- Exact Diagonalization Study of Strongly Correlated Electron Models: Hole Pockets and Shadow Bands in the Doped t - J Model.- II Electronic Structure.- Electronic Band Structures of LaMO3 (M = Ti, V, Cr, ..., Ni, Cu) in the Local Spin-Density Approximation.- First-Principles Calculations of the Electronic Structure and Spectra of Strongly Correlated Systems: LDA + U Method.- Unrestricted Hartree-Fock Study of Perovskite-iype Transition-Metal Oxides.- Electronic Structure of Transition Metal Compounds.- Core-Level Spectroscopy in Early-Transition-Metal Compounds.- Systematics of Optical Gaps in Perovskite-iype 3d Transition Metal Oxides.- III Charge Transport and Excitations.- Optical Spectroscopy on the Mott Transition in Perovskiteiype Titanates.- Spectral Weight Transfer and Mass Renormalization in Correlated d-Electron Systems.- Charge Transport Properties of Strongly Correlated Metals near Charge Transfer Insulator to Metal Transition.- Infrared Studies of Kondo Insulator and Related Compounds.- IV Magnetic Response.- Magnetic Correlations in Doped Transition-Metal Oxides.- Spin and Charge Differentiation in Doped CuO2 Planes Observed by Cu NMR/NQR Spectra.- Orbital-Spin Coupling in V2O3 and Related Oxides.- Magnetic and Transport Properties of the Kondo Lattice Model with Ferromagnetic Exchange Coupling.- V New Materials.- Superconductivity, Magnetism and Metal-Insulator Transitions in Some Ternary and Pseudoternary 3d-, 4d-, and 5d-Metal Oxides.- NMR Studies of Superconductivity and Metal-Insulator Transition in Cu Spinel CuM2X4 (M = Rh, Ir and X = S, Se).- Index of Contributors.

In this thesis, both time-resolved, nonlinear optical spectroscopy and linear spectroscopy are used to investigate the interactions and dynamics of elementary excitations in strongly correlated electron systems. In the first part, we investigate the renormalization of magnetic elementary excitations in the transition metal oxide Cr2O3. We have created a non-equilibrium population of antiferromagnetic spin waves and characterized its dynamics, using frequency- and time-resolved optical spectroscopy of the exciton-magnon transition. We observed a time-dependent pump-probe line shape, which results from excitation induced renormalization of the spin wave band structure. We present a model that reproduces the basic characteristics of the data, in which we postulate the optical nonlinearity to be dominated by interactions with long-wavelength spin waves, and the dynamics due to spin wave thermalization. Using linear spectroscopy, coherent third-harmonic generation and pump-probe experiments, we measured the optical properties of the charge-transfer (CT) gap exciton in Sr2CuO2Cl2, an undoped model compound for high-temperature superconductors. A model is developed which explains the pronounced temperature dependence and newly observed Urbach tail in the linear absorption spectrum by a strong, phonon-mediated coupling between the charge-transfer exciton and ligand field excitations of the Cu atoms. The third-order nonlinear optical susceptibility within the Cu-O plane of Sr2CuO2Cl2 is fully characterized in both amplitude and phase, and symmetry based conclusions are made with respect to the spatial arrangement of the underlying charge distribution. Theoretical considerations ascribe a newly reported resonance in the third-order nonlinear susceptibility at 0.7 eV to a three-photon transition from the ground state to the charge-transfer exciton. An even parity intermediate state of Cudd character, is found to contribute to the transition. Finally, preliminary results of time-resolved pump-probe spectroscopy confirm that the CT exciton or one of its constituent parts couples strongly to phonons, and we suggest ultrafast thermalization with the lattice as the dominating mechanism underlying the dynamical properties.

Spectra are reported for Sc(II) 3d(1), Y(II) 4d(1), and La(II) 5d(1), and it is suggested that the experimental procedures used can be extended so as to observe many transition ions in one electron configuration.