Fundamental Analysis Research Group

Dr. Yoshihiro ASAI	(Group Leader : Condensed Matter Theory, Computational Physics, Theoretical Chemistry)
Dr. Makoto FUSHIKI	(Fundamental Theories for Fluids, Boltzmann Equation)
Dr. Takashi MIYAKE
Dr. Minoru OTANI
Dr. Ferdi ARYASETIAWAN (Chiba Univ.)

PD

Dr. Tomczak Jan MARTIN
and others

Research Projects

Fundamental theories for novel simulations

It has been well recognized that numerical simulation is very useful in various research fields such as materials science, as well as in technological applications. However, many important problems remain unsolved. To solve these problems novel approaches, which can be very different from existing methods, are required. The goal of our research group is to solve these difficult problems and build solid theoretical grounds for novel simulation methods.

The electronic properties of technologically important materials such as semiconductors, metals, magnetic and optical materials are governed at the very basic by quantum mechanical laws manifesting themselves in, among others, electron correlations, spin degree of freedom, excitation and deexcitation processes. Theoretical understanding of these basic aspects is therefore a crucial step in the device design technology. Equally important, theoretical simulations, with their predictive power, play an essential role in designing new materials and understanding physical and chemical processes. This is illustrated, in particular, by the relatively young field of nanotechnology, which is in great need for theoretical understanding, supported by theoretical simulations. Currently available softwares in the market can barely meet these advances in the ever merging fields of physics and chemistry, as well as biology. Thus, the development of theoretical simulation tools which require the basic understanding of the above mentioned quantum mechanical processes is indispensible. It is with this background that the following research activities take their perspective.

*Transport properties through nanostructured materials.

Nanostructured materials such like a single molecule linked to macroscopic electrodes has been studied extensively both theoretically and experimentally in the hope that they might be useful for building up the smallest electric device ever existed. The field called molecular electronics, which is one of the most exciting fields in nanotechnology, is a growing and interdisciplinary field between many branches of physics, chemistry and engineering. It is expected that theories and numerical simulations are very powerful to analyze detail physical processes which accompany with electric current flowing through the nanostructured object. Even more, numerical simulations are expected to have predictive power, which should be quite useful because physics in nanostructured system is quite out of range of daily intuition. We have been working to build solid theoretical grounds for simulation technologies aimed to nanotechnology and nanoscience, with special attention to the dissipation processes accompanying the electric conduction and competition between interference effects and effects of discrete energy level structures, both of which are quite important but have not yet well understood.

*Researches on strongly correlated electrons, and electronic excited state in solid state materials and nanostructured materials.

First-principles approaches and model Hamiltonian approaches have been developing rather independently for many years. First-principles approaches can give detailedinformation about real materials but it is difficult to extend existing theories, notably the local density approximation (LDA) and the GW method, to treat systems with strong (onsite) correlations that pervade many new interesting materials known as correlated materials such as magnetic materials, perovskites, high-temperature superconductors, and many others. On the other hand, model Hamiltonian approaches have reached a very sophisticated level due to the much simpler Hamiltonian compared to the Hamiltonian of the real system. Model approaches, however, necessitate adjustable parameters, ie, they are not first-principles. One of the main themes of research in our group is to merge first-principles approaches and model Hamiltonian approaches in order to construct a truly first-principles approach but with sophistication of the many-body theory used in model approaches. The latest development in this direction is the merging of dynamical mean-field theory (DMFT), which is known to be very successful in describing systems with strong onsite correlations and able to describe the difficult problem of metal-insulator transition, with the GW method, which is the method of choice when calculating excited-state properties of materials from first-principles. The new theory, dubbed GW+DMFT, is still at its early stage of development but it has been shown to give encouraging results when applied to study the electronic structure of ferromagnetic nickel. We are now working on a practical implementation of this new scheme.

*Efficient numerical methods to solve Boltzmann equation.

Recent Selected Works

Theory of Inelastic Electric Current through Single Molecules,
Y. Asai, Phys. Rev. Lett. 93, 246102 (2004); Phys. Rev. Lett., 94, 099901(2005).

First Principles Approach to the Electronic Structure of Strongly Correlated Systems: Combining GW and DMFT,
S. Biermann, F. Aryasetiawan, A. Georges, Phys. Rev. Lett. 90, 086402 (2003).

Theory of Fano effects in an Aharonov-Bohm ring with a quantum dot,
T. Nakanishi, K. Terakura and T. Ando, Phys. Rev. B69, 115307 (2004).

System Size Dependence of the Diffusion Coefficient in a Simple Liquid,
M. Fushiki, Phys. Rev. E68, 021203 (2003).

First-principles Calculations of Tunneling Conductance,
H. Ishida, D. Wortmann and T. Ohwaki, Phys. Rev. B70, 085409 (2004).