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Electronic and Optical Properties of ThreeDimensional Charge Density Wave System in BaBiO3
https://ir.soken.ac.jp/records/1288
https://ir.soken.ac.jp/records/1288014d75e6c9544ccca5ad425da5ff8c28
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本文 (6.9 MB)

Item type  学位論文 / Thesis or Dissertation(1)  

公開日  20100222  
タイトル  
タイトル  Electronic and Optical Properties of ThreeDimensional Charge Density Wave System in BaBiO3  
タイトル  
タイトル  Electronic and Optical Properties of ThreeDimensional Charge Density Wave System in BaBiO3  
言語  en  
言語  
言語  eng  
資源タイプ  
資源タイプ識別子  http://purl.org/coar/resource_type/c_46ec  
資源タイプ  thesis  
著者名 
Hasanuzzaman, S. M.
× Hasanuzzaman, S. M. 

フリガナ 
ハッサンジャーマン, シーク・モハメド
× ハッサンジャーマン, シーク・モハメド 

著者 
Hasanuzzaman, S. M.
× Hasanuzzaman, S. M. 

学位授与機関  
学位授与機関名  総合研究大学院大学  
学位名  
学位名  博士（学術）  
学位記番号  
内容記述タイプ  Other  
内容記述  総研大甲第372号  
研究科  
値  数物科学研究科  
専攻  
値  X1 放射光科学専攻  
学位授与年月日  
学位授与年月日  19990324  
学位授与年度  
値  1998  
要旨  
内容記述タイプ  Other  
内容記述  Since the advent of high T c superconductivity in copper oxides by Bednorz and Miiller, the interest in solid state properties of various oxides like Cuoxide, Bioxide, Mnoxide and Tioxides has been renewed. In these materials the noble property said to comes from the strong interelectron correlation or the electron lattice coupling. Among those materials BaBi0 3 (BBO) and it's related compounds BaPb 1x Bi x O 3 (BPBO) and Ba 1x K x BiO 3 (BKBO) are the objects of a special interest. BaBiO 3 becomes a superconductor if doped with Pb at the Bi sites (with a maximum of T c ～12K for x ～ 0.25), and also shows superconductivity if it is doped with K at the Ba sites (with a maximum of T c ～30K for x ～0.40). In these materials electronphonon (elph) coupling is inferred to play very important role, and we will focus on this elph coupling. It can give two properties, one is superconductivity and other is charge density wave (CDW) type insulator. Very strong elph coupling will give two instabilities. If it is remain in the metallic state, it will give highT c superconductor. But it often causes another instability like CDW state. So, these two basic possibilities come from elph coupling. In order to clarify strong elph coupling, which gives CDW, it is necessary to understand how electron couples with phonon, and how it effects on the ground state as well as excited state.<br /> In spite of this renewed interest, the basic electronic structure of the parent material BaBiO 3 is still not clarified sufficiently. These materials present several peculiarities with comparing to other perovskite type high T c superconductor compounds. It is a three dimensional system, does not contain any magnetic ions and absence of two dimensional metaloxygen plane. In the undoped phase of this material, a static charge density wave state with a periodic lattice distortion appears, opens up a gap at the Fermi level, and makes this compound a Peierls insulator. Furthermore, the superconductivity appears when this CDW order is destroyed by doping.<br /> It is well known that, there are two types of gap exists in this CDW type insulator BaBiO 3 , one is direct and another is indirect. In usual insulators, however the opening of optical gap (direct gap) and the appearances of indirect gap are often consider separately. The optical gap usually comes from the difference between the occupied and unoccupied atomic orbitals relevant to the valence and the conduction bands. While the indirect transition usually appears because of the weak electron phonon coupling, which slightly mixes up direct and indirect transitions.<br /> On the other hand, BBO is not an ordinary insulator, a strong electronphonon interaction is acting in this material, causes a Peierls distortion of the lattice, doubling the unit cell, opening up a wide direct gap, and makes the indirect transitions to appear. So both the direct gap and the appearance of the indirect transition have the same origin. For this reason, in our theory, we did not used the conventional perturbation approach, instead, we have developed a unified theory based on the extended PeierlsHubbard model.<br /> We have thus theoretically studied the electronic and optical properties of BaBiO 3 , as one of the typical material with three dimensional CDW state, in connection with nonlinear excitations. The ground and excited states of a three dimensional extended PeierlsHubbard models with halffilled band electrons have been evaluated. Within this model, we introduce the adiabatic approximation for phonons, and the HartreeFock approximation for interelectron coulombic interactions. The electronhole correlation on the Bi atoms and the classical fluctuations of the oxygen sublattice coordinates are also taken into account, so as to obtain exciton effect as well as thermal fluctuations of the lattice.<br /> By using our model, we at first clarified the near infrared and visible absorption spectra of BBO from a unified point of view. The direct transition corresponds to the excitation across the direct CDW gap (〓 2.OeV), and this gap arises due to the frozen part of the Peierls distortion. While the indirect part corresponds to the long tail in the infrared region of the absorption spectrum, and it is due to the excitation across the indirect CDW (〓 O.55eV). It arises due to the lattice fluctuations from the static Peierls distortion. This lattice fluctuation destroys the kselection rule, and makes the indirect transition possible. Our conclusion is that, the origins of both the direct gap and indirect transitions are the same, i.e., the strong coupling between the electron and the breathing motion of the oxygen atom. These theoretical results shown good agreement with recent optical experiments on BBO.<br /> Next, we have studied the nonlinear lattice relaxation process of exciton, and explained the origin of the photoinduced absorption in BaBiO 3 . The adiabatic potential energy surfaces that describe the nonlinear relaxation from the FranckCondon state to the selftrapped exciton (STE) state have been calculated within our unified theory. When the CT excitation is created by its threshold energy, the exciton is relax to a STE, and localized within the CDW gap. This localized selftrapped state partially canceled the charge density distribution of the uniform CDW ground state, and is return back towards the metallic state. It will gives a new absorption band with an energy of about a half of the energy gap (〓 0.9eV). This energy level could be observed in the photoinduced absorption measurement. The experimentally observed photoinduced reflectivity peaks at around midgap energy is assigned for the optical excitation from this localized state (STE) to the peak of the density of states of the conduction band.<br /> Finally, let us briefly discuss these nonlinear excitations from a somewhat different point of view. The collective excited states described in this work can never be created by ordinary thermal excitation from the ground state, because a much larger energy is required than the ordinary thermal energy such as at room temperature. It becomes possible only when the energy is supplied by photoexcitation. That is, as a combination of the photoexcitation and the subsequent lattice relaxations, we can clarify the multistable nature of the ground state, even when thermal excitation can never access it.  
所蔵  
値  有  
フォーマット  
内容記述タイプ  Other  
内容記述  application/pdf 