2026-03-14T02:16:04Z https://ir.soken.ac.jp/oai

oai:ir.soken.ac.jp:00000674 2023-06-20T16:00:13Z 2:428:15

Dual Electron-phonon Coupling Model for Giant Photoenhancements of Dielectric Constant and Electronic Conductivity in SrTiO3 Dual Electron-phonon Coupling Model for Giant Photoenhancements of Dielectric Constant and Electronic Conductivity in SrTiO3 邱, 宇クイユ QIU, Yu 総合研究大学院大学博士（理学） Since some 3D type perovkite compounds have been found in the experiments to give gigantic dielectric constant, in contrast to the ordinary dielectrics, its underlying microscopic mechanism has attractted much attention in the field of solid state theory. As is well-known, the ferroelectric modes play an important roll in the dielectric property, transportation and phase transition or these materials. Many efforts were already performed on the connection of this ferroelectric mode with the quartic or sextic anharmonic oscillators. ln experiment, glgantic photo-enhancements of the electronic conductivity and the dielectric constant have recently been observed in SrTiO3. lt was also pointed out that, this dielectric enhancement remains to exist only under the ultraviolet (UV) illumination, while vanishes as the illumination is turned off. As for the photo-induced electronic conduction in SrTiO3, it is expected to be an alternative mechanism from that of the ordinary field induced one in metallic systems. However, the microscopic origin of these photo-induced phenomena has not yet been clarified theoretically. Thus, this is just the motivation of the present study. 　We will give a short introduction to the soft mode theory in the second chapter since our work will be mainly based on this theory. The spatial structure and the electronic property of this　3D perovskite. SrTi03, are also stated. since we will focus our efforts only on this compound in the present work. Some key points of the previous studies on　the fundamental properties of SrTiO3, are summerized as well. =n Chapter 3, a detailed illustration is given to the Super-Para-Electric (SPE) large polarons, from the set-up of the cheoretic model to the numerical calculatlons, and ends up with the impurity effect on this polaron. Combining the experimentally observed dielectrlc and conductive properties with previous theoretical researches on the model for this crystal, we give a further investigation into the model and come up with a new model for the photo excited state of the crystal. By which, we can adiabatically obtain the corresponding energy surface and find all the metastable states on it. With the investigation of each of such states,　We can find its connection with the dielectric and conductive property of this crystal･ We also give an investigation into the most stable quasiparticle state for the many electron system within the adeabatic method and give a description to the possible lattice configurations for the photo excited state of the electron and phonon coupling system. Then it is followed by Chapter 4, the appllcations of this SPE polaron theory in the interpretation of the photo-induced giant dielectric constant and electronic conductivity. We will first discuss one of the fundamental problems for phonons, phonon softening or phonon hardening with the introduction of e-p coupling. Then we will apply our SPE large polaron theory to the experimentally observed static dielectric enhancement in srTiO3, clarifying the microscopic origin of this photo-induced phase transition. In the following, we will give a phenomenological interpretation to the experimentally reported metallic conduction in SrTiO3 by studying the translational property of the polarons. In Chapter 5, we will discuss the relaxation process of the lattice after the photo-excitation. It has already been studied that for polymers, the formation of the polaron or exciton is an ultra-fast process. [6l] In experiment, these states could be detected and predicted from the spectra. However, by virtue of the molecular dynamics theory, the information of the lattice configuration as well as the electronic state can be traced so as to compare with the energetic evolution. Then, the lattice relaxation process could be recognized more clearly. We will first give a deschption to the molecular dynamics theory for the e-p interacting system. Then, we will apply this method to SrTiO3 and give some detailed descriptions about the relaxation process in this crystal. We will show the formation of the SPE large polaron is an ultra-fast process of about several picoseconds,　and how the electron and phonon interaction system releases its energy so as to reach its new stable state. The influence of the electron and phonon interaction strength and the size of the electron and phonon coupling system on the relaxation process will be shown. A summary of all the conclusions of　these studies is given at Chapter　6. 総研大甲第989号 thesis 2006-09-29 application/pdf application/pdf https://ir.soken.ac.jp/record/674/files/甲989_要旨.pdf https://ir.soken.ac.jp/record/674/files/甲989_本文.pdf https://ir.soken.ac.jp/records/674 eng