Satoshi Yotsuhashi

Anisotropic thermoelectric properties in layered cobaltite AxCoO2 (A=Sr and Ca) thin films

We have fabricated epitaxial thin films of layered cobaltite AxCoO2 (A=Sr and Ca) on sapphire substrates by rf magnetron sputtering. The grown phase in the AxCoO2 films was found to be a monoclinic β-phase of primitive layered cobaltites and the epitaxial orientation of the film could be controlled by the surface plane of the substrates. The resistivity parallel to the CoO2 layers ρ‖ for the β-SrxCoO2 is as low as 2mΩcm at room temperature and shows metallic behavior. The ratio of perpendicular resistivity ρ⊥ to ρ‖ increases from 20 at room temperature to 90 at 3K. More isotropic nature was observed in the Seebeck coefficient. Parallel Seebeck coefficients S‖ of AxCoO2 are approximately 60μV∕K at room temperature and the perpendicular S⊥ are about half of S‖.

Photo-induced CO2 Reduction with GaN Electrode in Aqueous System

CO2 reduction with water and light illumination is realized using a gallium nitride (GaN) photoelectrode in which excited electrons induce CO2 conversion at the counter electrode. For the counter electrode, a copper (Cu) plate was chosen. The low affinity and wide gap of the nitride semiconductor enable us to create an electron–hole pair which has a sufficient energy for both CO2 reduction and water oxidation, in spite of the fact that a high energy for CO2 reduction is required. Within this system, the generation of formic acid (HCOOH) with 3% Faradic efficiency was confirmed by light illumination alone.

New Aspects of Quasi-Kondo Physics: Two-Level Kondo and Strongly Coupled Local Electron–Phonon Systems

New aspects of the two-level Kondo (TLK) effect and the strongly coupled local electron–phonon (LEP) system are studied by the Wilson numerical renormalization group method. It turns out that there exist strong-coupling fixed points for both systems which can be mapped on the antiferromagnetic pseudo-spin sd-model, if the bare coupling is chosen moderately strong. In the case of TLK model, it is found that there exsist two different strong-coupling regions one of which is found for the first time, and that there exists a zero-coupling region as well. In the case of LEP system, in which three low-lying phonon levels are taken into account, it is found that there exist a strong-coupling fixed point which corresponds to the effective potential of ionic motion with double-well character. An analysis of LEP system based on the poormans scaling with Lee algebra of SU(3) is also discussed.

Non-Fermi Liquid Behavior of U-impurity with f2-Singlet Ground State

It is shown by the Wilson numerical renormalization-group approach that a U-impurity with an f 2 -singlet ground state exhibits an unstable non-Fermi liquid (NFL) fixed point due to the balance between Kondo singlet and crystalline-electric-field singlet ground state. The puzzling behavior of R 1- x U x Ru 2 Si 2 (R=Th, Y and La, x ≤0.07) can be explained if we assume that the singlet ground state is realized and the Kondo temperatures in each orbital are of different orders of magnitude. For a wide region in parameter space, the system shows the NFL behavior as a transient phenomenon in a experimentally possible temperature range. Results for the temperature dependence of resistivity, magnetic susceptibility and the Sommerfeld coefficient with and without a magnetic field reproduce the behavior observed in Th 1- x U x Ru 2 Si 2 . Less anomalous properties of R 1- x U x Ru 2 Si 2 (R=Y and La) can be understood in the same scenario provided that these compounds have different sets of parameters.

Enhanced CO2 reduction capability in an AlGaN/GaN photoelectrode

Light illumination of a gallium nitride photoelectrode creates separate electron-hole pairs that drive water oxidation and CO2 reduction reactions. Here, we show enhanced photocurrent in an AlGaN/GaN device that consists of an unintentionally doped (uid-) AlGaN photoabsorption layer and an n+-GaN electrical-conduction layer. The production rate of formic acid by CO2 conversion in the uid-AlGaN/n+-GaN photoelectrode is about double that in the uid-GaN/n+-GaN device. This improvement is most likely due to the effect of internal bias in the uid-AlGaN layer generated by the polarization effect, which improves electron-hole separation.

Enhancement of transverse thermoelectric power factor in tilted Bi/Cu multilayer

We investigated transverse thermoelectric response in tilted Bi/Cu multilayer. Estimation through an equivalent circuit model showed that transverse thermoelectric power factor (PF) reaching 155 μW/cm K2 is possible with optimum device parameters. Finite element analysis confirmed the validity of the estimation using tensorial representation, which implicitly assumes homogeneous anisotropy. The Bi/Cu multilayer was fabricated by pressure injection of molten Bi into a periodically slitted Cu block. The measurement of the fabricated sample verified an enhancement in transverse PF up to 50.1 μW/cm K2, which is approximately 1.5 times greater than the conventional PF of constituent Bi, 34 μW/cm K2.

Control of Epitaxial Growth Orientation and Anisotropic Thermoelectric Properties of Misfit-Type Ca3Co4O9 Thin Films

We have investigated the control of crystal orientation in misfit-type layered cobaltite Ca3Co4O9 thin films by rf-planar magnetron sputtering and succeeded in growing epitaxial films with c-axis, a-axis and b-axis orientations normal to the substrate. Anisotropic transport properties (parallel to the CoO2 layers and perpendicular to the CoO2 layers) were measured using the b-axis-oriented epitaxial films with a layered structure perpendicular to the substrate surface. The resistivity parallel to the CoO2 layers (ρa) is about 8 mΩ cm at room temperature while that perpendicular to the CoO2 layers (ρc) is about 300 mΩ cm; the anisotropy is estimated to be about 40. Thermoelectric anisotropy is not considerably pronounced; the parallel Seebeck coefficient Sa is measured to be 110 µV/K and the perpendicular Sc is 40 µV/K at room temperature.

Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis

We report on a highly improved CO2 to HCOOH conversion system using a tandem photo-electrode (TPE) of InGaN and two Si p-n junctions. To improve its efficiency, narrow-band-gap InGaN was applied as the photo-absorption layer. In the TPE structure, the current matching between GaN-based photo-absorption layer and two Si p-n junctions is crucial for the improvement of the efficiency. The energy conversion efficiency for HCOOH production reached 0.97%, which is greater than average of global biological photosynthetic one.

Thermoelectric Properties of Electron-Doped KTaO3

To explore the possibility of large thermoelectric responses on the basis of d-electron orbital degeneracy, we investigated electrical and thermal transport properties of single crystals of electron-doped KTaO3. The electron-type carrier density (n) can be increased up to 1.4×1020 cm-3 (x = 0.009) by partially substituting K with Ba in the form of K1-xBaxTaO3. The power factor and dimensionless figure of merit at room temperature steeply increase with n, up to 14 µW cm-1 K-2 and 0.03 for x = 0.009, respectively. This suggests that K1-xBaxTaO3 is a potential thermoelectric material, provided that n can be further increased.

Enhanced Capability of Photoelectrochemical CO2 Conversion System Using an AlGaN/GaN Photoelectrode

We report significantly improved photosynthesis system based on AlGaN/GaN photochemical process. The resultant energy conversion efficiency is 0.13% which is the same level as that of real plants. The capability of this system is enhanced by high cathode potential due to the reduction of energy loss while utilizing the piezoelectric effect in the AlGaN/GaN heterostructure. The Faradaic efficiency of the CO2 conversion to organic materials is enhanced, accompanied by an increment in photocurrent by modification of the AlGaN/GaN photoelectrode structure and electrolytes. Furthermore, reaction products such as C2H4 and C2H5OH are generated by light illumination alone.