Division of Quantum Condensed Matter Physics


Kazuhiro Yabana, Professor

Dr. Kazuhiro Yabana graduated from Kyoto University in 1987, with a doctor degree in theoretical nuclear physics. He joined nuclear theory group in Niigata University and moved to University of Tsukuba in 1999. His research interests range from theoretical nuclear physics to atomic, molecular, optical, and condensed matter physics. He has been enthusiastic in developing computational approaches for quantum dynamics of many-fermion systems.

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About our group

Quantum Condensed Matter Physics Group is the division for studying nano material sciences, semiconductor device physics, and the physics of light-matter interactions with computer simulations based on quantum mechanics. Downsizing of semiconductor devices is still the most important and effective way to improve their performances. Today’s device sizes are in nano-meter scales, and the further progress has required utilization of new type of structures and/or novel nano materials instead of the conventional planer-type silicon-based devices. In nano-meter scales, it is essential to describe the behavior of electrons by quantum mechanics, and therefore computer simulations based on quantum mechanics become important to clarify the physics of nano materials and devices.

Research topics

Fig.1 shows the cross-sectional views of silicon nanowires for several diameters. The silicon nanowire is a promising candidate for future CMOS devices, and 10- to 20-nm diameter is estimated to be optimal for practical device applications. For these sizes of silicon nanowires, thousands of atoms, or a hundred thousand of atoms for longer wires, are contained in the systems. Recently, we have developed a real-space finite-difference program code for large-scale first-principles electronic structure calculations based on the density-functional theory. The real-space method is suitable for massively-parallel supercomputers, and we have achieved the electronic structure calculations for silicon nanowires and silicon nanodots of over 10,000 atoms by using 1024 nodes of the PACS-CS.

Fig.1 Cross sectional views of atomistic models of silicon nanowires for several diameters.

Fig.2 shows the atomistic structures of several types of defects in silicon nitride, which is a foundation of MONOS-type charge-trap memories. The structures are obtained by first-principles density-functional calculations. The memory operations are simulated by adding or removing electrons from the charge-neutral systems. From the calculations, we have found that redundant oxygen impurities cause degradation of the MONOS-type memories due to irreversible structural changes during P/E cycles.

Fig.2 Atomistic structures of defects in silicon nitride: (a) two substitutional oxygen atoms, (b) nitrogen mono vacancy, (c) one substitutional oxygen atom and nitrogen mono vacancy.

Other Researches

Interactions between materials and lasers are also important issues in both, fundamental physics and technological applications, points of views. Since the light-matter interactions are essentially the quantum phenomena, the computer simulations based on quantum mechanics are also useful for theoretical investigations on the physics of the light-matter interactions. By employing large-scale quantum simulations, we study the Floquet states of electrons and holes confined in semiconductor nano structures, photoinduced phase transitions in organic and inorganic strongly correlated systems.

Web sites

Computational Science Frontier (Japanese)

KOIZUMI Laboratory (Japanese)

Research Group for Theoretical Physics of Light-Matter Interactions