Wataru Kobayashi

A Novel Heavy-Fermion State in CaCu3Ru4O12

We have measured the susceptibility, specific heat, resistivity, and thermopower of CaCu 3 Ti 4- x Ru x O 12 and CaCu 3- y Mn y Ru 4 O 12 , and have found that CaCu 3 Ru 4 O 12 can be regarded as a...

Thermal rectification in bulk materials with asymmetric shape

We investigate thermal rectification in a bulk material with a pyramid shape to elucidate shape dependence of the thermal rectification, and find that rectifying coefficient R is 1.35 for this shape, which is smaller than R=1.43 for a rectangular shape. This result is fully duplicated by our numerical calculation based on Fourier’s law. We also apply this calculation to a given shape, and show a possible way to increase R depending on the shape.

Fast discharge process of layered cobalt oxides due to high Na + diffusion

Sodium ion secondary battery (SIB) is a low-cost and ubiquitous secondary battery for next-generation large-scale energy storage. The diffusion process of large Na+ (ionic radius is 1.12 Å), however, is considered to be slower than that of small Li+ (0.76 Å). This would be a serious disadvantage of SIB as compared with the Lithium ion secondary battery (LIB). By means of the electrochemical impedance spectroscopy (EIS), we determined the diffusion constant (D) of Na+ in thin films of O3- and P2-type NaCoO2 with layered structures. We found that the D values (~ 0.5–1.5 × 10−10u2005cm2/s) of Na+ are higher than those (< 1 × 10−11u2005cm2/s) of Li+ in layered LiCoO2. Especially, the D values of O3-NaCoO2 are even higher than those of P2-NaCoO2, probably because O3-NaCoO2 shows successive structural phase transitions from the O3, O’3, P’3, to P3 phases with Na+ deintercalation. We further found that the activation energy (ED ~ 0.4u2005eV) for the Na+ diffusion is significantly low in these layered cobalt oxides. We found a close relation between the relative capacity and the renormalized discharge rate ( = L2/DT, where L and T are the film thickness and discharge time, respectively).

Orbital Ordering of Intermediate-Spin State of Co3+ in Sr3YCo4O10.5

The orbital ordering and spin state in the room-temperature ferromagnet Sr 3 YCo 4 O 10.5 were investigated by a resonant X-ray scattering technique. A resonating signal with a σ→π scattering component was observed at ( h 00), where h =2 n +1 ( n : integer), near the 1 s → e g transition energy of Co. This provides direct evidence of not only e g orbital ordering but also the existence of the intermediate-spin (IS) state. Moreover, we suggest that the ferrimagnetism is induced by the e g orbital ordering of the IS state of Co 3+ . The peculiar e g orbital, spin-state, and magnetic orderings are also discussed.

Structure-property relationship in the ordered-perovskite-related oxide Sr3.12 Er0.88 Co4 O10.5

Synchrotron x-ray-diffraction patterns were measured and analyzed for a polycrystalline sample of the room-temperature ferromagnet Sr{sub 3.12}Er{sub 0.88}Co{sub 4}O{sub 10.5} from 300 to 650 K, from which two structural phase transitions were found to occur successively. The higher-temperature transition at 509 K is driven by ordering of the oxygen vacancies, which is closely related to the metallic state at high temperatures. The lower-temperature transition at 360 K is of first order, at which the ferromagnetic state suddenly appears exhibiting a jump in magnetization and resistivity. Based on the refined structure, possible spin and orbital models for the magnetic order are proposed.

Origin of colossal dielectric response of CaCu3Ti4O12 studied by using CaTiO3∕CaCu3Ti4O12∕CaTiO3 multilayer thin films

To elucidate the origin of the colossal dielectric response (CDR) of CaCu3Ti4O12 (CCTO), multilayer thin films of CCTO interposed in insulating CaTiO3 (CTO) were synthesized using a pulsed laser deposition technique. The capacitance C of CTO/CCTO/CTO films with different layer thicknesses is measured. After removing the capacitance of CTO by extrapolating C to zero CTO thickness, the real part of dielectric constant of CCTO is estimated to be 329–435, which is much smaller than the reported value for CCTO thin films. This fact indicates that the CDR of CCTO is extrinsic and originates from an internal barrier layer capacitor.

Nap1 stimulates homologous recombination by RAD51 and RAD54 in higher-ordered chromatin containing histone H1

Homologous recombination plays essential roles in mitotic DNA double strand break (DSB) repair and meiotic genetic recombination. In eukaryotes, RAD51 promotes the central homologous-pairing step during homologous recombination, but is not sufficient to overcome the reaction barrier imposed by nucleosomes. RAD54, a member of the ATP-dependent nucleosome remodeling factor family, is required to promote the RAD51-mediated homologous pairing in nucleosomal DNA. In higher eukaryotes, most nucleosomes form higher-ordered chromatin containing the linker histone H1. However, the mechanism by which RAD51/RAD54-mediated homologous pairing occurs in higher-ordered chromatin has not been elucidated. In this study, we found that a histone chaperone, Nap1, accumulates on DSB sites in human cells, and DSB repair is substantially decreased in Nap1-knockdown cells. We determined that Nap1 binds to RAD54, enhances the RAD54-mediated nucleosome remodeling by evicting histone H1, and eventually stimulates the RAD51-mediated homologous pairing in higher-ordered chromatin containing histone H1.

High-Temperature Metallic State of Room-Temperature Ferromagnet Sr1-xYxCoO3-δ

Resistivity, thermopower, Hall coefficient, and magnetization are measured for a set of polycrystalline samples of the room-temperature ferromagnet Sr 1- x Y x CoO 3-δ up to 800 K. A peculiar metallic state accompanied by a large effective magnetic moment of 3.40 µ B /Co is observed above 600 K, indicating that electrons on the Co 3+ ions are magnetic and itinerant. The magnetic Co 3+ ions survive at low temperatures, which causes the ferromagnetism below 335 K. We attribute this to the large CoO 6 octahedra stabilized in the A-site ordered structure.

Negative Thermoelectric Power Induced by Positive Carriers in CaMn3-xCuxMn4O12

We have found that the thermoelectric power of the ordered perovskite CaMn 3- x Cu x Mn 4 O 12 is negative at high temperature (-35 µV/K at 1000 K), although this material is a hole-doped conductor in which Cu 2+ is substituted for Mn 3+ . This is quantitatively explained by the extended Heikes formula, where entropy flows in a direction opposite to the charge.

Intrinsic rapid Na+ intercalation observed in NaxCoO2 thin film

Electrochemical properties are investigated for thin film electrode of NaxCoO2, which was fabricated by pulsed laser deposition. A high rate of 400 C was realized in the film together with a fine cyclability where its capacity after 1000 cycles remains 84.4% of the initial one. The NaxCoO2 thin film exhibits a clear sign of the structural phase transition induced by Na+ intercalation even at the high rate of 200 C. This observation indicates bulk nature of the rapid and highly-reversible Na+ intercalation in NaxCoO2.