Researches
Researches
Study of molecular vibrational modes reflecting electronic states
and molecular configurations in functional solids:
Molecular vibrations and electronic spectra presented in textbooks focus on gases, which can be regarded as isolated molecules, and molecules surrounded by solvents. Crystals composed of molecules, on the other hand, require consideration of effects that have not been treated in detail, such as charge transfer between neighboring molecules, electron-electron repulsion between neighboring molecules, and substituent interference between neighboring molecules. However, if we can understand how charge transfer, electron-electron repulsion, and substituent interference affect spectra, we can derive principles that contribute to important functions of solids (conductivity, magnetism, optical properties, thermal properties, mechanical properties, etc.).
The analysis of band structure is useful in discussing solid-state properties. Typically, light with energies below infrared or above ultraviolet is used for spectroscopic analysis of band structure. By using near-infrared light, we are able to observe the complex behavior of the HOMO- and LUMO-derived bands, which contribute significantly to the physical properties, crossing each other under different conditions. We believe that this technique will be useful for the study of single-component metals and Dirac electronic systems.
Study of charge-to-charge interactions
in molecular superconductors
In a textbook on solid state physics, it is introduced that two electric charges pair in a superconductor. In molecular superconductors, the mechanism of charge pair formation has been studied from the viewpoint of similarity with cuprate superconductors. The formation of pairs by other conditions has been neglected. This may be due to the fact that only materials close to the molecular arrangement, in which one charge is shared by two molecules, have received attention.
The superconductors, where one charge can exist on either of two molecules, have not received much research attention. However, the number of superconductors belonging to this type is overwhelmingly large, so it can be said to be a research object that should not be neglected. “One charge can exist in either of two molecules” is technically called charge order or charge fluctuation. Using a molecular spectroscopic approach, we were able to measure the charge-charge repulsion force and the weak bonding force between molecules, and successfully draw a phase diagram with the parameters of repulsion and bonding forces on the horizontal axis and temperature on the vertical axis. Moreover, this phase diagram was found to be compatible with the temperature-pressure phase diagram.
This study allowed us to determine the intermolecular distances that allow molecular crystals to become insulators, superconductors, and metals. Therefore, it is possible to provide guidelines for the synthesis of conductive molecular materials. We are currently conducting research that includes conditions that are not ideal for generating charge ordering.
Controlling the electron-electron repulsion
and bonding forces in molecular crystals
The study of 02 revealed that there are many molecular crystals in which the direction of the electron-electron repulsion and the direction of the bonding force are perpendicular to each other. This observation prompted the conceptualization that, by utilizing such crystals, independent modulation (increase and decrease) of both repulsive and bonding forces could be achieved. The conventional approach of resizing the constituent molecules and ions within a crystal, known as chemical pressure application, often fails to afford independent control over these forces. Moreover, bending or isotropic compression of crystals lacks uniform and independent control over these forces. Therefore, we have adopted a physical method of stretching and compressing only in specific directions. This novel technique has enabled the realization of phenomena typically challenging in pressure research, exemplified by inducing superconductivity in metals through pressure.
