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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 Cu-oxide, Bi-oxide, Mn-oxide and Ti-oxides has been renewed. In these materials the noble property said to comes from the strong inter-electron correlation or the electron lattice coupling. Among those materials BaBi0 3 (BBO) and it's related compounds BaPb 1-x Bi x O 3 (BPBO) and Ba 1-x 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 electron-phonon (el-ph) coupling is inferred to play very important role, and we will focus on this el-ph coupling. It can give two properties, one is superconductivity and other is charge density wave (CDW) type insulator. Very strong el-ph coupling will give two instabilities. If it is remain in the metallic state, it will give high-T c superconductor. But it often causes another instability like CDW state. So, these two basic possibilities come from el-ph coupling. In order to clarify strong el-ph 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 metal-oxygen 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 electron-phonon 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 Peierls-Hubbard 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 Peierls-Hubbard models with half-filled band electrons have been evaluated. Within this model, we introduce the adiabatic approximation for phonons, and the Hartree-Fock approximation for inter-electron coulombic interactions. The electron-hole correlation on the Bi atoms and the classical fluctuations of the oxygen sub-lattice 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 k-selection 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 Franck-Condon state to the self-trapped 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 self-trapped 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 mid-gap 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. |
要旨・審査要旨 / Abstract, Screening Result (299.3 kB)
