Fujiwara Lab
Theoretical research and material design by first-principles electronic structure theory
Much attention has been focused on material designs with revealing and controlling the physical properties of each material by calculating its electronic structure and time-dependent behavior. Macroscopic behavior of materials can be understood by classical mechanics. However, we need to consider an effect of quantum mechanics to understand microscopic (less than nano-scale order) behavior.

The target of our laboratory is theoretical research and material design by first-principles electronic structure theory. First-principles electronic structure theory is the one that solves Schrödinger equation by using only a spatial position of nuclei without any empirical parameters and determines the electronic structure of realistic materials. First-principles electronic structure theory is almost a unique method which gives a quantitative prediction of electronic structure and physical properties on realistic materials without any empirical parameters, in contrast to model calculations with parameters determined by experiments.

We will establish the foundation and the methodology for electronic structure calculations in the following two aspects;

(1) Large Scale Electronic Structure Calculations
Establishing the quantum mechanical molecular dynamics simulation method and the technology of process simulator for nano-scale systems of semiconductors and metals with from ten thousands to ten millions atoms.
(2) Beyond LDA: Extension of the DFT to e.g. LDA+U, GW, LDA+DMFT and its Application to Strongly Correlated Electron Systems
Developping the novel method of the first principle electronic structure calculations with combination of one-electron band theory and many-electron theory.
Research Projects supported by external organizations
(1) CREST-JST project
    (In "Establishment of Next Generation Integration Simulation Technology")
    "Novel Methodology of Electronic Structure Calculations by Combining Several Different Aspects"
(2) Grant-in-Aid for Scientific Research on Priority Area
    "Development of New Quantum Simulators and Quantum Design"
(3) ELSES(Extra Large Scale Electronic Structure calculation)
Term:
First-principles electronic structure theory
The many-body Schrödinger equation should be solved in order to treat many-electron systems with quantum-mechanical approach. However, an approximate form of the many-body Schrödinger equation should be solved since we are not able to solve it exactly during finite time. Then, we assume that each electron moves in an averaged potential formed by all other electrons and nuclei. According to this assumption, the electronic structure of materials is determined without using any empirical parameters. This method is called "First-principles electronic structure theory".

The local density approximation (LDA) based on the density functional theory (DFT) is the most prevalent method among the first-principles electronic structure calculations. The LDA is based on the variational principles for functional of the electron density.

Stable structures and dynamical process of materials are also calculated by using ionic forces in each time calculated directly from First-principles electronic structure theory. This is called "First-principles molecular dynamics".

Strongly correlated electron systems
Strongly correlated electron systems are systems which have strong Coulomb interactions comparable to electron-hopping integrals. Typical examples of a strongly correlated electron system are transition metal oxides. Much attention has been focused on strongly correlated electron systems, since these systems show anomalous physical properties such as various spin, charge and orbital order, metal-insulator transition, Colossal Magneto-Resistance (CMR), High-temperature superconductivity and so on. The valence orbitals in strongly correlated electron systems are partially filled and well localized 3d or 4f orbitals and hence play important roles. These various physical properties in strongly correlated electron systems have been extensively used in recent development of device designs.

The local density approximation (LDA) based on the density functional theory (DFT) is a quit successful for electronic structure calculation of many real materials with weakly correlated materials. However, the LDA is hardly applicable to strongly correlated electron systems, since the LDA one-electron potential is orbital independent and hence takes into account the Coulomb interaction as an averaged term. In particular, LDA overestimates the width of 3d bands and underestimates the band gap for strongly correlated electron systems.

To understand the physical phenomena on strongly correlated electron systems, more sophisticated method with dynamical electron correlation effects are needed.