Computational Material Sciences
Design of artificial magnet based on semiconductor quantum wire networks
Physical properties and functions of nanostructures can be substantially controlled by their shapes as well as their constituent elements. We have succeeded to design the artificial ferromagnet fabricated only by non-magnetic InAs semiconductor quantum wires. Moreover, we have found that our designed ferromagnetism can easily be controlled by only applying electric and magnetic field.
Electronic properties of Nano-Carbon Materials
Recently, an interesting complex consisting of fullerenes and nanotubes is synthesized. The material consists of a 1D array of fullerenes which are encapsulated by a carbon nanotube (occasionally called ``peapod"). Our first-principle calculations reveals that the space between the nanotube and the encapsulated fullerenes is a decisive factor to determine the energetics for the encapsulation process of the fullerenes in the nanotubes and the stability of resultant structures. It is also clarified that the electronic structures of peapods depend on the space and that they reflect electron states of the encapsulated fullerenes.
Computational Material Sciences
Nonlinear Optical Responses of Semiconductor Wannier-Stark Ladder
Superlattices where a static electric field is applied in the crystal growth direction are termed Wannier-Stark ladder (WSL), as is depicted in the figure. An exciton formed by photoabsorption is exclusively in a Fano-resonance (FR) state, not in a bound state. Nonlinear optical phenomena ascribable to the WSL exciton are investigated. An asymmetric Autler-Townes doublet, characteristic of excitonic FR, manifests itself in transient four-wave mixing (FWM) spectra. This is due to quantum interference between a microscopic polarization of FWM signals and an electronic phase of the FR state. Moreover, an optically coherent control of electronic states of WSL in terms of a combination of laser parameters with strength of the bias allows one to be accessible to the novel effect of the dynamic charge localization and delocalization, which is found further enriched by the Zener resonance of WSL.
Mechanism of Colossal Magnetoresistance in Manganite
Some manganites show a remarkable reduction of resistance in a magnetic field called, ¡Ècolossal magnetoresistance (CMR) effct¡É. The detailed mechanism of the CMR is still not known, but strong electron correlations and strong electron-lattice interaction are crucially important. In order to treat the strong electron correlations we have performed the complete active space self-consistent calculations on the manganite cluster Mn2O11. It has been found that the cluster is unstable with respect to symmetry breaking deformations (Polaron formation). We have obtained the stabilization energy that is close to the experimentally estimated value.
Computational Life Science
Mechanisms of Self-Cleavage of Ribozymes
RNA that is responsible for trans-fer of genetic information occa- sionally works as an enzyme (ribo- zyme). [The right is a reaction scheme of the self-cleavage of a ribozyme.]. Clarification of the ribozyme function is important in both science and medical applica- tion: The ribozyme is a possible tool for gene therapy. Our group has performed the first-principle quantum-theoretical molecular-dynamics calculations for the ribozyme for the first time and identified the reaction pathways and obtained the corresponding free-energy barriers.
Atoms and Electrons in Cytochrome c Oxidase
At the final stage of the respi-ration, a protein named cytochrome c oxidase shows structural change triggered by electron transfer, then exhibits a physiological functions, and eventually assists in the for- mation of ATP. Our group has explored the origin of the functions based on the quant-um-mechanical calculations. The right is distribution of a wavefunction of a nearly-free-electron state that we have found is closely related to atom-scale-mechanisms of the respiration processes.
Computational Nuclear Physics
Physics of Finite Quantum Many-Body Systems
Quantum systems such as nuclei, atoms, and molecules, form a group of isolated finite matter in nature, having a number of common characteristic features. Moreover, recent developments in the nano-science technique enable production of new artificial elements in these finite quantum systems.
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| Real-space calculation of deuteron breakup reaction
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Approaches of the computational science are very useful in studying these systems composed of a few to a few-hundreds of particles. Our current research includes nuclear structure and reaction; especially, microscopic theories of collective motion and prediction of novel structures in unstable nuclei.
We also study dynamics of electrons in finite systems (atoms, molecules, clusters, etc.), including electronic excitations, optical response under weak and intense laser field.
Quantum Simulation in Real-Time and Real-Space
Every matter in nature is a many-body system made of electrons and nuclei. The nucleus itself is a many-body system of nucleons as well.
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| First-principle calculation of photoabsorption in ethylene
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We are developing methods of quantum simulation to study dynamics of these fermionic many-body systems. The time-dependent density-functional theory is of current interest for the first-principle calculation of fermionic dynamics. Real-time and real-space technique has been developed to investigate dynamical properties of nuclei, atoms, molecules, and clusters. In addition, we develop a wave-packet method with the absorbing boundary condition for quantum scattering problems, applied to the linear response in the continuum, the few-body reaction models, etc.
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