Research themes

Japanese version is available here .

First-Principle calculation based on Quantum Monte Carlo

First-Principle Quantum Monte Carlo (FP-QMC) is an numerical method to solve the many-body Schrödinger equation using Monte Carlo integration. FP-QMC is one of the state-of-art first-principle methods, giving us the most exact electronic structure.

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However, FP-QMC cannot be easily performed unlike DFT because sophisticated knowledge about condense matter, many-body theory, and high-performance computing are necessary to make use of a FP-QMC code. I am currently developing new theories and implementations for FP-QMC in Sandro Sorella’s lab, who is one of the most prominent researchers in the field of FQ-QMC, at SISSA (Italy).

  • Submitted to Journal of Chemical Theory and Computation (2019).

Validation and verification of Density Functional Theory (DFT)

Recently, the materials informatics (MI) paradigm, in which novel materials are designed or searched for using techniques of information science and/or computational physics, has attracted much attention because of rapid improvements in computer and information science (including artificial intelligence, AI). The most important problem when applying AI to the field of materials is a lack of arranged databases for physical properties and functions, which is very different situation from board games and web services. Then, high-throughput ab initio calculations of physical properties are often performed to create large-scale databases for machine learning or data mining in MI.

Density functional theory (DFT) seems a promising framework in which to perform quantitative evaluations of physical properties. It is, however, sometimes unable to reproduce experimental results owing to the limitation such as a failure to take exchange-correlation effects into account, a lack of excited-state information, or the unavailability of an appropriate model. Therefore, it is very important to investigate whether or not DFT calculation can quantitatively predict a physical property even if the method has already been implemented in a DFT code.

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To validate the predictive power, we are collecting experimental data and comparing the experimental data and calculation results.

Application of first-principle calculation

The materials informatics (MI) paradigm has recently attracted much attention. It stimulates researchers to try to use high-throughput first-principle calculations for designing a novel material. However, computational physics and chemistry have originally been used for revealing mechanisms so far, which is still a valid strategy for designing a novel material. I am applying first-principle calculations to compounds recently synthesized by experimental groups.

Novel superconductors in layered titanium-oxypnictides

Almost all metals show zero resistivity at very low temperature, which is called a critical temperature (Tc). This phenomenon was discovered in Hg by Kamerlingh Onnes in 1911 and named as “superconductivity” later. A lot of researchers have been looking for novel high-Tc superconductors, ultimately room-temperature superconductors. Although several high-Tc superconducting families have been found, no room-temperature one has been discovered so far.

I am currently developing new theories and implementations for FP-QMC, but I started my career as an experimental researcher. My supervisors (Prof. Kageyama and Dr. Yajima) gave me a mission to find a novel superconductor in titaninum compounds. Fortunately, we found several novel superconductors such as BaTi2Sb2O and BaTi2Bi2O which are categorized as layered titanium-oxypnictides. They have still been studied by many groups because their analogies of cuprates and iron-arsenides high-Tc superconductors. Although several experiments such as NMR and muSR had revealed the superconducting mechanism, details of structural disorders occurring at low temperature had not been detected by X-ray or neutron diffraction measurements. I applied first-principle phonon calculations to the layered titaninum-oxpnictides to reveal the low-temperature structures.

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Collaboration with experimental groups

I am actively collaborating with experimental groups so that I can take advantage of my experience that I belonged to an experimental group and synthesized inorganic compounds. I advise how to use a DFT code or calculate electronic and/or phonon structures by myself.