I. Computational Material Sciences

I-1. Nano Sciences

An atom measures approximately 0.1 nanometers. Thus, materials or structures on the order of several nanometers are comprised of 10,000 to 100,000 atoms. Moreover, de Broglie wavelength (length scale representing the quantum nature) is generally on the order of a nanometer, although it depends on the circumstances. Therefore, controlling the behavior of electron waves according to differences in the nanostructures and nanomorphology can realize new electronic properties that are hidden in bulk materials. However, the science of the nanoworld cannot be elucidated without quantum theory of nanomorphology.

In our research group, we conduct research to understand the physical properties of light element nanomaterials such as carbon nanomaterials and to predict the functions of these nanomaterials. Additionally, we explore the emergence of new physical properties of semiconductor nanostructures through theoretical calculations based on quantum theory.

I-2. Science of Imperfection

There is no such thing as a perfect crystal. All materials have an imperfection of some sort. Although these imperfections are not abundant, they greatly influence the mechanical, electronic, and optical properties of a material. It is thus important to reveal roles of the imperfection based on reliable quantum theoretical calculations. Moreover, clarifying new aspects induced by the imperfection introduces new stimulus in condensed matter science and thus open new possibilities.

For example, semiconductor technology that supports our daily lives utilizes behavior of electrons in semiconductors as well as in insulators and metals. The behavior of electrons is described by quantum theory. Condensed matter physics based on quantum theory has indeed aided in the development of semiconductor technologies in the latter half of the 20th century. It has been recognized that imperfections in materials (e.g., atomic vacancies, point defects such as interstitial atoms, line defects such as dislocations, and planar defects such as stacking faults) affect the properties of resultant devices substantially. Thus, major topics in semiconductor technology research are to microscopically identify and remove such imperfections.

Although once considered a problem, imperfections can be advantageous as they can be utilized to add new functions to materials. Our group tackles the most important issues in materials science which include:

How do imperfections (defects and impurities), which inevitably exist in a material, affect the physical properties?
What types of new phenomena can be expected?
How does atomic migration (diffusion), which is an important factor in material growth and structure fabrication, emerge in materials?

We try to answer these questions by applying computational science techniques based on quantum theory to elucidate the influence of imperfections on the electronic properties. Moreover, we actively explore novel aspects of physics and chemistry that imperfections induce.

I-3. Surface and Interface Sciences

A surface is a border between a material and a vacuum, while an interface is a border between two different materials. A border offers new unprecedented insight because common knowledge of the bulk properties is not always valid at the border. Hence, numerous phenomena that occur at borders are waiting to be discovered.

All materials, including biological materials, are formed at surfaces or interfaces. For example, catalytic reactions occur on surfaces or at interfaces. Because all technologies use some type of material, materials are crucial elements of technologies. Hence, surface and interface sciences are very important, provide a firm framework for materials science, and are crucial from technological viewpoints.

Our group explores new properties and innovative functions on semiconductor surfaces as well at the interfaces of various materials. We use quantum theoretical approaches to clarify atomic reactions at surfaces and interfaces.

I-4 Exploring New Materials and Structures

Exploring attractive materials based on theoretical calculations is a major objective in computational materials science, and highly precise calculation techniques based on quantum theory are necessary to thoroughly explore these materials as well as to improve and develop innovative theoretical methods. The local-density approximation (LDA) and generalized gradient approximation (GGA) from the density functional theory provide high predictability, but have some limitations such as underestimating the effect of the electronic excitation energy (bandgap). However, applying the GW approximation (GWA) based on many-body perturbation theory can improve these approximations.

Using these LDA and GWA, we predicted a new material phase of silicon (Si) and germanium (Ge) [New Journal of Physics: 10, 083001 (2008)]. Figure 10 shows a newly discovered body-centered tetragonal lattice phase where each atom binds to four nearest-neighbor atoms similar to the diamond structure, but with different bond angles. Consequently, a slightly lower-density body-centered tetragonal lattice phase is formed. The total energy of this new phase is higher than the most stable diamond structure by only 0.1 eV per atom, and thus, it is highly probable that this structure exists in nature.

(Left) Atomic structure of a body-centered tetragonal lattice. (Right) Energy band structures of germanium (Ge). Horizontal axis shows the wave number (k). Solid lines and dots indicate the results of LDA and GWA, respectively.

The energy band structures of this new phase are particularly interesting. They significantly differ from those in the diamond structures; the bandgaps of the former are notably narrower than those of the latter. In particular, the bandgaps almost disappear in body-centered tetragonal Ge, suggesting Ge may be a semi-metal. Because four-folded covalent bonding materials are normally semiconductors, the metallization of body-centered tetragonal Ge would be a remarkable new finding.