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ResearchNano-Theory Field

Nano-Theory Coordinator: Taizo Sasaki

Understanding phenomena in the nanospace region, predicting new phenomena and creating novel nanostructured materials

Nanospace is a world in which common sense does not apply, where extremely small atoms are in motion, and electrons fly about in an even smaller space. Moreover, when huge numbers of these atoms and electrons act in coordination, they come to display behavior markedly different from those of single electrons and atoms. Ways of thinking and methods that are not bound by everyday common sense–namely, quantum mechanics and statistical mechanics–are essential for a proper understanding of the phenomena that occur there, and further, for devising new materials. Key activities in the field of nano-theory, which help achieve an understanding of the myriad phenomena emerging in nanospace, include building fundamental theories behind these novel behaviors by incorporating quantum mechanics and statistical mechanics, using our supercomputing facilities to obtain quantitative numerical predictions and develop new and efficient calculation methods. Besides providing interpretations of results obtained in other nanofield areas, we aim at invoking the outcomes of our research to predict as yet unearthed phenomena and to propose new materials featuring novel properties.

Joint research of large-scale first-principles calculations and experiments to develop high performance materials for next generation devices

In the MANA Nano-Theory Field, we are developing the large-scale first-principles calculation program called CONQUEST to calculate the positions of the enormous number of atoms in actual nanoscale devices or nanostructured materials and clarify the electronic states of these materials. Although the first-principles calculations of systems comprising a few thousand atoms are difficult with conventional techniques, first-principles structural optimization and molecular dynamics simulation of million-atom systems are possible with the CONQUEST code by utilizing a new theoretical technique called Order-N method.

In collaboration with a MANA experimental group (Nanostructured Semiconducting Materials Group, Group Leader: Naoki Fukuda), we are conducting research on Si/Ge core-shell nanowires, which are a promising material for next-generation vertical transistors. The properties of core-shell nanowires are expected to depend strongly on the size, interface between Si and Ge, impurity distribution and other factors, which could not be modeled before. It is now possible to predict the structure at atomic scale and the electronic properties by large-scale first-principles calculations. The figure shows an example of a distribution of a carrier when using p-type Si. At the same time, the experimental group can control the sizes of the core and shell regions, and the positions of impurities. Our aim is to develop high performance materials for next-generation devices through this joint theoretical and experimental research.

Calculated distribution of a carrier localized in the core region of Si/Ge core-shell nanowire

Elucidation of electronic state in vicinity of Mott transition and its application

In the strong electron-electron correlation regimes, we observe physical phenomena that are difficult to understand within the free-electron picture for conventional metals. For example, although electrons are particles with charge and spin degrees of freedom, charge-spin separated excited states appear in the substances called Mott insulators: charge excitation has a gap whereas spin excitation is usually gapless. It is difficult to explain such charge-spin separated excited states by a particle electron picture. Thus, we are engaged in a theoretical study from an unconventional viewpoint to clarify the electronic state in the strong correlation regimes like near-Mott insulators, and examine possibilities of new electronic devices by applying that unique electronic state.

Electronic state with small amount of doping in Mott insulator

Development of methods to control macroscopic and microscopic states by modeling of cooperative interactions

In the development of memory devices that make use of the bi-stability of materials, many attempts have been made to realize switching between bi-stable states by employing various types of stimuli, such as light, magnetic fields and pressure. To elucidate the mechanism of how changes in the electronic and spin states of atoms and molecules lead to changes in macro states, we study various mechanisms of many-body effects by modeling cooperative interactions. Examples include the discovery of a new type of phase transition in a two-dimensional spin-crossover system (upper figure) and the development of a quantum coherence control method for local magnetic moment (lower figure).

(Left) Ground state (6-fold degeneracy)
(Right) 6 state-clock mode

Control of local moments by external fields