Yoshihiko Okamoto

Yoshihiko Okamoto

Winner in 2018 | 20th Winner

Associate Professor, Department of Applied Physics, Nagoya University

Exploration of Novel Physical Properties and Functions of Transition Metal Compounds Based on the Unique Electronic and Crystal Structures

Abstract When Prize Awarded

Novel transition metal compounds with remarkable electronic properties, such as cuprite and iron-based superconductors, have opened up a new era of the condensed matter physics. In my Sir Martin Wood Prize lecture, I will present the results of material exploration of transition metal compounds using the crystal and electronic structure databases based on knowledge of solid-state chemistry, toward the discovery of such electronic properties and functions. We developed various materials including high-performance thermoelectric materials, candidate nodal line semimetals, metal-insulator transition systems, superconductors, and geometrically frustrated magnets and I will focus on the former two systems.

1. One-dimensional telluride Ta4SiTe4 as a high-performance thermoelectric material.

Thermoelectric cooling is a promising candidate for the next-generation of refrigeration technologies in replacing vapor compression cooling using gaseous refrigerants. However, there is currently no bulk material with a high enough performance to reach a practical level in the low temperature region. We found that Ta4SiTe4 and its substituted compounds show high thermoelectric performance at low temperature. Thermoelectric power of Ta4SiTe4 whisker crystals reaches S = -400 μV K-1 at 100-200 K, while maintaining low resistivity of ρ ~ 2 mΩ cm. These S and ρ give a larger power factor of P = S2/ρ of 80 μW cm-1 K-2 than those in Bi Te-based practical materials at room temperature, indicating that Ta4SiTe4 is a promising candidate for the low temperature applications of thermoelectric cooling. This very large P is probably caused by the very small spin-orbit gap opening on the strongly one-dimensional electronic bands at the Fermi energy.

2. CaAgP and CaAgAs as a candidate nodal-line semimetal.

In recent years, Dirac, and Weyl semimetals, which are zero-gap semiconductors with linear dispersion bands at the zero-gap points, have attracted broad interest as candidate systems for realizing topologically nontrivial states in bulk materials. In contrast, some systems are theoretically indicated to have a nodal line, where the linear dispersion bands cross on a line in the momentum space. We found that CaAgP and CaAgAs are promising candidates for the nodal-line semimetal. First principles calculation results indicate that the both compounds are ideal nodal-line semimetals, where the Dirac points form a ring at the Fermi energy. We synthesized polycrystalline samples and single crystals of CaAgP and CaAgAs and found that they have a ring-torus Fermi surface related to the nodal ring by physical property measurements of them.