Polaronic effects in manganese oxides: Self-trapped electronic states in lanthanum manganate and sodium chloride.

다국어 초록 (Multilingual Abstract)

Self-trapped states occur in many insulating solids but are not especially well-understood. There is a need for better theoretical models and better experimental tools for exploring these states. This thesis provides models for two kinds of materials LaMnO₃ and NaCl, and predicts experimental effects which can be used to characterize such states.
LaMnO₃ is an insulating antiferromagnet which can be doped with holes over a wide concentration range, as in La₁₋<italic>_x</italic>Ca<italic>_x</italic>MnO₃.
Here I study the regime <italic>x</italic> <math> <f> ≪</f> </math> 1 where particularly interesting and simple behavior is predicted. The model has electronic and lattice-vibrational degrees of freedom chosen to represent the Mn ion outer electronic states and their interaction with oxygen motions in the three dimensional perovskite crystal structure. Four independent types of data are available to choose three adjusted parameters. Using electronic structure calculations, optical conductivity and Raman spectra for this choice the predicted magnitude of the static Jahn-Teller distortion agrees within 10–15% with neutron diffraction data.
I use the model to analyze and predict the self-localized states which form under optical excitation and under hole doping. In particular five types of behavior are analyzed: (1) the insulating nature of lightly doped LaMnO₃ due to the anti-Jahn-Teller polaron formation; (2) phonon broadening due to the exciton formation; (3) polaronic angle-resolved-photoemission-spectra (ARPES); (4) Raman spectra due to the Franck-Condon mechanism; (5) the self-trapped exciton in NaCl and its optical properties including the Franck-Condon effect using the first-principles Density Functional Theory (DFT) calculations. Experimental confirmation of the predicted behavior for LaMnO₃ will differentiate the Jahn-Teller model studied here from competing versions.
The results given here are novel in five ways. (1) Essentially exact analytical polaronic spectra of the two-orbital model Hamiltonian have been obtained. (2) Self-trapped exciton solution explains the broadening of the optical spectra. (3) <bold>k</bold>-dependent peak position and broadening of the ARPES spectra are predicted. (4) Resonant multiphonon Raman spectra were predicted and subsequently observed. (5) DFT first-principles calculations are used for the first time to study a self-trapped state in a real material, namely the self-trapped exciton in NaCl.