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内容記述 |
Since some 3D type perovkite compounds have been found in the experiments to<br />give gigantic dielectric constant, in contrast to the ordinary dielectrics, its<br />underlying microscopic mechanism has attractted much attention in the field of<br />solid state theory. As is well-known, the ferroelectric modes play an important roll<br />in the dielectric property, transportation and phase transition or these materials.<br />Many efforts were already performed on the connection of this ferroelectric mode<br />with the quartic or sextic anharmonic oscillators. ln experiment, glgantic<br />photo-enhancements of the electronic conductivity and the dielectric constant have<br />recently been observed in SrTiO3. lt was also pointed out that, this dielectric<br />enhancement remains to exist only under the ultraviolet (UV) illumination, while<br />vanishes as the illumination is turned off. As for the photo-induced electronic<br />conduction in SrTiO<sub>3</sub>, it is expected to be an alternative mechanism from that of the<br />ordinary field induced one in metallic systems. However, the microscopic origin of<br />these photo-induced phenomena has not yet been clarified theoretically. Thus, this<br />is just the motivation of the present study.<br /> We will give a short introduction to the soft mode theory in the<br />second chapter since our work will be mainly based on this theory.<br />The spatial structure and the electronic property of this 3D<br />perovskite. SrTi0<sub>3</sub>, are also stated. since we will focus our efforts<br />only on this compound in the present work. Some key points of the<br />previous studies on the fundamental properties of SrTiO<sub>3</sub>, are<br />summerized as well. =n Chapter 3, a detailed illustration is given<br />to the Super-Para-Electric (SPE) large polarons, from the set-up of<br />the cheoretic model to the numerical calculatlons, and ends up with<br />the impurity effect on this polaron. Combining the experimentally<br />observed dielectrlc and conductive properties with previous<br />theoretical researches on the model for this crystal, we give a further<br />investigation into the model and come up with a new model for the<br />photo excited state of the crystal. By which, we can adiabatically<br />obtain the corresponding energy surface and find all the metastable<br />states on it. With the investigation of each of such states, We can<br />find its connection with the dielectric and conductive property of<br />this crystal･ We also give an investigation into the most stable<br />quasiparticle state for the many electron system within the adeabatic<br />method and give a description to the possible lattice configurations<br />for the photo excited state of the electron and phonon coupling system.<br />Then it is followed by Chapter 4, the appllcations of this SPE polaron<br />theory in the interpretation of the photo-induced giant dielectric<br />constant and electronic conductivity. We will first discuss one of<br />the fundamental problems for phonons, phonon softening or phonon<br />hardening with the introduction of e-p coupling. Then we will apply<br />our SPE large polaron theory to the experimentally observed static<br />dielectric enhancement in srTiO<sub>3</sub>, clarifying the microscopic origin <br />of this photo-induced phase transition. In the following, we will<br />give a phenomenological interpretation to the experimentally reported<br />metallic conduction in SrTiO<sub>3</sub> by studying the translational property<br />of the polarons. In Chapter 5, we will discuss the relaxation process<br />of the lattice after the photo-excitation. It has already been studied<br />that for polymers, the formation of the polaron or exciton is an<br />ultra-fast process. <sup>[6l]</sup> In experiment, these states could be detected<br />and predicted from the spectra. However, by virtue of the molecular<br />dynamics theory, the information of the lattice configuration as well<br />as the electronic state can be traced so as to compare with the<br />energetic evolution. Then, the lattice relaxation process could be<br />recognized more clearly. We will first give a deschption to the <br />molecular dynamics theory for the e-p interacting system. Then, we<br />will apply this method to SrTiO<sub>3</sub> and give some detailed descriptions<br />about the relaxation process in this crystal. We will show the<br />formation of the SPE large polaron is an ultra-fast process of about<br />several picoseconds, and how the electron and phonon interaction<br />system releases its energy so as to reach its new stable state. The<br />influence of the electron and phonon interaction strength and the<br />size of the electron and phonon coupling system on the relaxation<br />process will be shown. A summary of all the conclusions of these<br />studies is given at Chapter 6. |
要旨・審査要旨 (265.9 kB)
