Associate Professor, Department of Applied Physics, Nagoya University
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.