IWASAKI Laboratory
<Major Research Areas>Low-temperature solid properties, superconductive properties, thermoelectric material properties

Creation of highly functional materials
-Approach based on electronic transport phenomena-

Research activity

　　Energy-efficient technologies are urgently needed for global environmental preservation. Technology that uses waste heat energy without generating hazardous substances and technology for effective energy transportation and storage are two examples which are used for superconductors and thermoelectric semiconductors. Our laboratory analyzes the properties of materials from a physical viewpoint through experiments on electronic transport phenomena to create functional materials for practical use.

1Vortex state of high-Tc oxide superconductors

　　Oxide superconductors that have remarkably higher superconducting transition temperatures (Tc) than those of conventional superconductors are attracting attention from the viewpoint of application as well as that of basic physics. When electric current is applied to a superconductor, a magnetic field is induced and quantized magnetic flux is introduced into a sample material, causing a vortex state. It is also important to analyze the properties of the vortex system from the viewpoint of application. Anisotropy-the phenomenon of a material showing different properties depending on the viewing direction is the key parameter of our study. We have been investigating the vortex system, taking into consideration the vortex pinning effect, interactions between vortices and the thermal energy effect.

2Improving the conversion efficiency of thermoelectric energy

　　Since thermoelectric semiconductors are required to have a large Seebeck coefficient and good electrical conductivity with small thermal conductivity for higher energy conversion efficiency, materials have been developed according to these requirements. At our laboratory, we have developed devices that can evaluate the figure of merit of thermoelectric materials just by measuring their electrical resistance, and use them to measure thermoelectric properties. In addition, we are working to improve the conversion efficiency of thermoelectric materials such as mesoscopic elements by using the novel thermoelectric phenomenon (nonequilibrium and non-linear stationary state) that we discovered in our research processes.

3Development of an experimental method for the analysis of material properties

　　The study of properties of functional materials requires analysis of the properties from various viewpoints. For that purpose, it is necessary to develop innovative experimental methods and systems. The development of the thermoelectric property measurement system mentioned in 2) above is one such example that resulted in the discovery of novel thermoelectric phenomena such as non-equilibrium and non-linear stationary states. Now we are developing a system to analyze the properties of thermoelectric materials.

Equipment

High-field superconducting magnet, SQUID magnetometer (MPMS), physical property measurement system (PPMS), equipment for single crystal growth by infrared heating, sputter equipment, hot-press sample preparation equipment, thermoelectric property measurement system, X-ray diffraction equipment, electron probe micro analyzer (EPMA)

The main research achievements in the past five years

1. T. Naito, H. Iwasaki, T. Nishizaki and N. Kobayasi, Phase Transition of Josephson Vortices Under High Magnetic Fields up to 30T in Heavily Overdoped YBa₂ Cu₃ O_{7- 8} Single Crystals, J. Low Temp. Phys. 159 168 (2010)
2. S. W. Kim, R. Tarumi, H. Iwasaki, H. Ohta, M. Hirano, and H. Hosono, Thermal conductivity and Seebeck coefficient of 12CaO×7Al₂ O₃ electride with a cage structure, Rhys. Rev. B, 80 075201 (2009).
3. H. Iwasaki, H. Morita, and Y. Hasegawa, Evaluation of thermoelectric properties in Bi-microwires by the Harman method, Jpn. J. Appl. Phys., 47, 3576 (2008).
4. H. Iwasaki, Thermoelectric property measurements of mesoscopic materials by the Harman method, J. Thermoelectrics Society of Japan, 4, 11 (2008).
5. K. Kato, M. Yamamoto, S. Ohta, H. Muta, K. Kurosaki, S. Yamanaka, H. Iwasaki, H. Ohta, and K. Koumoto, Effect of Eu substitution on thermoelectric properties of SrTi_0.8 Nb_0.2 O₃ , J. Appl. Phys., 102, 116107 (2007).