Current Research Interests
Thermoelectricity can be used as an energy conversion technology from thermal energy into electrical energy or vice versa. Due to the increasing demand on the renewable energy development, the thermoelectric power generation would be a promising candidate for a waste heat energy harvesting. The thermoelectric performance is characterized by the dimensionless figure of merit, defined by ZT=S2σT/κ, where S, σ, T, κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. In order to improve the thermoelectric performance we should increase the power factor (S2σ) as well as decrease the thermal conductivity (κ), simultaneously. Currently, it is known that the ZT value can be improved by two ways. One way is to maximize the power factor by the quantum confinement effect and the other way is to minimize the thermal conductivity by scattering phonon with preserving electronic transport. From the intensive investigation of this research we’ll do the breakthrough research on the thermoelectric materials development for the waste heat power generation and for the environmentally friendly refrigeration system.
Magnetic refrigeration is a cooling technology using the magnetocaloric effect. While the thermoelectric cooling is operated by applying an electrical voltage, the magnetic refrigeration is driven by applying a magnetic field. With applying magnetic fields, randomly oriented magnetic spins align along the magnetic field’s direction, thereby decreasing the magnetic entropy and heat capacity. From the repeating magnetization and demagnetization processes with heat transfer from the material outside of the system, the irreversible magnetic entropy change gives rise to magnetic cooling. The issues of the magnetocaloric materials development for commercial uses are the higher critical temperatures than room temperature and the giant spin entropy change near the transition temperature. For the fundamental materials research, we’ll study the crystalline field effect, magnetic phase transition, and structural phase transition for various magnetic materials.
LOW DIMENSIONAL QUANTUM MAGNEISM
Low dimensional behavior of 2D or 1D electronic transport makes unconventional properties such as charge density wave, Peierls transition, Luttinger liquid, and so on. They are the result of strong electron-phonon coupling and electron-electron interaction. Prof. Rhyee proposed that the charge density wave or Peierls lattice distortion plays a key role in high thermoelectric performance. Understanding on those exotic phenomena is very important to find new thermoelectric materials as well as fundamental physics problem. Because those behaviors are related with metal to insulator (MIT) and/or metal to semiconductor transitions, the raised issues on this subject are metal insulator transition and structural distortion of lattices. Concerning the spin degrees of freedom, the spin density wave and spin Peierls behaviors should be investigated as a counterpart of charge degrees of freedom.
The research on the magnetic and unconventional superconductor is very important to understand the high temperature superconductivity. Even though much attention has been devoted to understand the coexistence of magnetism and superconductivity, it is far from the complete understanding. It is very surprising that the itinerant ferromagnetic superconductors show spin triplet state rather than spin singlet state, which is contrast to our common knowledge for the conventional superconductivity. It has not been fully understood the pairing mechanism of those compounds until now. We’ll carry out the research to understand the magnetic and electronic ground states, pairing mechanism, and the coexistence of superconductivity and quantum critical behavior by the developments of new materials.