Research Sector of Physics and Materials Design for High-Tc Superconductors
Our ultimate goal is to find room temperature superconductors. Superconductor, which holds zero resistance and diamagnetic property, benefits our daily life drastically, if they are realized at room temperature. So far, highest transition temperature (Tc) is below 0 oC and in an extreme environment.
We have been working on synthesizing new multi-functional materials as well as exotic superconductors. Along the way, we have found highest Tc intermetallic compound, MgB2, cuprate superconductor without CuO2 plane, Sr14-xCaxCu24O41 and many more. In the course of finding new high Tc superconductors, we are currently working on light element superconductors and synthesis of metastable material in high pressure/high temperature environment, especially in hydrogen compounds.
We search for and synthesize new superconductors. The compounds we are interested in include 3d transition metals with a strong electron correlation such as iron and 5d transition metals with a strong spin-orbit interaction such as platinum. We have discovered an iron-based superconductor 112-type CaFeAs2; and enhanced the superconducting transition temperature up to 47 K by substituting La and Sb. Other superconductors we have discovered include: (1) SrPtAs with a honeycomb structure, in which the emergence of chiral d-wave superconductivity has been discussed; (2) AuTe2, in which strong-coupling superconductivity emerges in the vicinity of structural instability. Our goal is develop new superconductors with higher transition temperature using various ideas of chemistry.
Research Sector of Quantum Physics for Superconductors
The objective of our group is to develop new materials that exhibit useful physical properties (functionalities) and to clarify the origin/mechanism of functionalities. Figure 1 is the bulk sensitive spin -resolved photoemission spectra of high quality CrO2. The combination of high quality CrO2 synthesized in our lab. and the spin-resolved PES constructed in our lab. enables us to observe intrinsic spin-resolved electronic states of CrO2: The electronic states near the Fermi level, which are responsible for the conducting properties, are derived only from electrons with up spin. The result provides reliable spectroscopic evidence for half metallic nature of CrO2. (H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).)
We (Ichioka, Onari, Adachi) theoretically study condensed matter physics by focusing especially on superconductivity and spin current. Main results in our research are as follows.
(1) We have clarified properties of unconventional superconductivity in the phenomena under magnetic fields (specific heat, magnetization, local electronic states, NMR spectrum, nuclear magnetism relaxation, small angle neutron scattering intensity, etc.) by theoretical calculations of spatial structure in the vortex states.
(2) We study mechanism of superconductivity and transport phenomena in the strongly correlated electron systems. We have clarified that the vertex corrections (higher-order many-body effects) are important in multi-orbital systems such as iron-based superconductors.
(3) We have done a systematic research on the spin Seebeck effect, i.e., the generation of spin currents by thermal energies, and clarified that a low-energy magnetic excitation termed magnon plays an indispensable role in the spin Seebeck effect.
We use density functional theory and manybody methods to study iron pnictide superconductors, quantum spin systems, organic charge transfer salts, metalorganic coordination complexes and strongly correlated oxide materials. The objective of our work is to understand the properties of complex materials and to improve the predictive power of theoretical tools in order to analyze the properties of hypothetical materials designed in the computer. We specially focus on understanding the electronic structure of unconventional superconductors, on energy mapping techniques to work out the Hamiltonian parameters of magnetic materials, on properties of organic and metalorganic complexes in the crystalline state, on the properties of 4d and 5d transition metal compounds with strong spin orbit coupling and on complex oxides with defects.
Research Sector of Light-Element Superconductors and Electronics
Our group has studied superconductivity induced by chemical doping of two-dimensional (2D) layered materials such as FeSe1-zTez, MoSe2, graphite and polycyclic aromatic hydrocarbon. Recently, pressure-induced high-Tc superconducting phase was discovered in (NH3)yCsxFeSe, which was prepared by intercalating Cs atoms into FeSe in liquid NH3. The (Tc) reached 49 K at 21 GPa (Scientific Reports 5, 9477 (2015)). Furthermore, we have investigated organic field-effect transistors (FETs) using aromatic hydrocarbon such as phenacene molecules. Above all, phenacene single-crystal FETs with ZrO2 gate dielectric showed excellent mobility reaching 18 cm2 V-1 s-1 (Scientific Reports 6<,21008 (2016)). In addition to these topics, electric field control of electronic properties has been intensively studied using 2D layered materials and graphene.
Research Sector of Synthetic Chemistry for Functional Materials
We have developed efficient synthetic methods and functional organic molecules based on organometallic chemistry. For example, polyaromatic hydrocarbons represent useful compounds for functional organic materials because of their unique redox properties and narrow energy gaps. Thus, we have designed new synthetic methodology to afford the target fused π-electron molecules by using transition metal catalysts, which is more efficient than previously reported conventional synthetic methods. Those compounds are also high potential materials for organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) owing to their strong intermolecular interaction and high carrier transporting ability. By using palladium-catalyzed copolymerization, we have develop new donor-acceptor type semiconducting polymers and high-performance OPV devices.
Research Sector of Theoretical Chemistry for Energy Storage Materials
The research interests in our group are mainly focus on exploring phase equilibria and phase transitions between liquids and solids, dynamics and structure of liquids, properties of crystalline and amorphous solids, and surface phenomena, with statistical mechanics, thermodynamics, and computer simulation as the "instrument". Recent studies cover the wide topics including microscopic mechanism of freezing and melting processes of water (recently published in Nature), polyamorphism of supercooled liquid water, water in confined geometry, new polymorph and phase transitions of high-pressure ice, properties of the “forbidden” (i.e. theoretically plausible but inexistent in reality) phases, novel theoretical approach to the stability of clathrate hydrates, and decomposition and formation of clathrate hydrates. These studies are linked to vast research fields such as life sciences, geology and astrogeology, meteorology, and energy resource development. Recently, we elucidated the adsorption mechanism of the molecules onto the surface of the methane hydrate for the first time by large-scale computer simulations using the K supercomputer and the supercomputers at RCCS. In this study, the molecules are found to be adsorbed on the surface by the entropic force instead of usual energetic stabilization process. (J. Am. Chem. Soc. 2015, 137 (37), 12079–12085.)
- Press release (In Japanese; http://www.okayama-u.ac.jp/tp/release/release_id334.html)
Phase boundaries between fluids may terminate at the critical point where the two distinct phases lose their identities; Solid-fluid phase boundaries, however, never end at the critical point. This is a common view on the critical point, which has been established by experimental observation and theoretical arguments, but now is no longer as definitive as it was since computer simulations suggested the possibility of solid-liquid critical points for substances confined in nanopores. Recently, we have revisited this problem and have obtained numerical evidence that strongly supports the existence of such critical points for water and argon, both confined in carbon nanotubes [K. Mochizuki and K. Koga, Proc. Natl. Acad. Sci. USA, 112, 8221 (2014)]. There we demonstrated liquid-solid phase equilibria and continuous freezing, located liquid-solid critical points by finite-size scaling analysis, and provided global phase diagrams of water and argon in carbon nanotubes.