|
|
|
|
Theory Group
Theoretical technique for analysis of atomic and molecular processes
|
|
|
The mission of the group is to carry out theoretical and computational researches in cluster, surface, and solid state physics on the microscopic basis in order to provide prescriptions for manipulating materials at the atomic level and to design novel materials with desired properties solely by computational method.
ĦResults in Fiscal Year
A. Semiconductor and Surface subgroup
This subgroup studies the dynamical atomic processes and a variety of physical processes observed in bulk semiconductors and solid surfaces. In fiscal 1995, the following subjects were studied.
1. Oxidation processes on Si(001) surface: This is an important problem of modern semiconductor device technology. Activation energies for oxygen molecule dissociation along several reaction paths were estimated by spin-polarized GGA.
2. Homoepitaxial crystal growth on Si(001) surface: In order to clarify the elementary processes of Si growth on the terrace of Si(001) surface. We studied the stability of several ad-Si clusters and the activation energies for various elementary processes.
3. Analysis of C-type defect structure on Si(001) surface: The analysis of STM images of Si(001) surface indicates the presence of three types of defect: A, B, and C. While the structures of A- and B-type defects have been identified, that of C-type defect is still unknown. As the C-type defect plays an important role in the initial process of oxidation on Si(001) surface, its identification is of great significance. A model of C-type defect has been proposed on the basis of a new concept, and its validity seems to be well verified.
4. Stability of hydrogenated silicon clusters Si6Hx: Our detailed analysis revealed that the stable structure of Si6Hx changes from compact types (for x7) to less compact ring types (for x7) as hydrogenation proceeds.
5. Adsorption of CO and N2 on the transition-metal surfaces: The processes of adsorption and dissociation of CO and N2 on the transition-metal surfaces were studied.
6. spin polarization in Si melt: It was demonstrated that the spin polarization can be induced occasionally in cases of Si-Si bond formation and bond breaking. The dynamical properties are appreciably affected by the spin polarization.
B. Transition-Metal Compound subgroup
The aim is to understand the basic properties of transition-metal compounds and to make some contribution to synthesis of new functionality materials.
1. Analysis of electronic states of perovskite type transition metal oxides: Theoretical analysis has been made to elucidate the basic properties related to the crystal structure, magnetism and transport of LaMO3 (M=Ti, V, ..., Ni, Cu), which has been intensively and extensively studied by Tokura Group. The local spin density approximation (LSDA) has proved to be effective not only for the study of the ground states properties (crystal structure, magnetic structure, etc.) but also for the study of the single-electron excitation spectrum. The orbital ordering is a new concept in this system and plays an important role for stabilizing the magnetic ordering (see Figure). It wa also demonstrated that some approximations beyond LSDA can actually give better descriptions for the properties of this system.
C. Exotic Materials subgroup
1. Analysis of electronic states of organic conductors: Organic solids DCNQI-(Cu, Ag, Li) constitute a unique category of materials possessing almost all sort of interesting phenomena encountered in the condensed matter physics, and have been actively studied experimentally for nearly a decade. On the other hand, the first principles calculation of organic solids with sufficient accuracy has been hardly available because of their complex atomic structures. The electronic structure calculations with high reliability were performed for several DCNQI-(Cu, Ag, Li) systems for the first time. Our calculation has provided several pieces of useful information, and is attracting strong attention as bringing forth a new progress in the study of organic solids.
2. Optical properties of oligosilanes: One of the unique aspects of oligomers is that we can tune the physical properties by changing the oligomer size. For example, we can control the photon energy for absorption and emission. Based on the detailed electronic structure calculations for oligosilanes whose chain length ranges from 2 to 16, we analyzed the optical properties observed by Tokura group.
Orbital ordering in LaMnO3: The hatched orbitals are occupied by electrons while the orbitals denoted by broken lines are empty. The related energy diagram is also shown in the lower panel. This orbital ordering stabilizes the A-type antiferromagnetic spin ordering.
Kiyoyuki Terakura
