Tsunehiro Takeuchi

An oxide single-crystalline whisker with high thermoelectric properties at temperatures ( T ) higher than 600 K in air has been discovered. This whisker is assigned to Ca 2 Co 2 O 5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c -axis. Seebeck coefficient of the whiskers is higher than 100 µV·K -1 at 100 K and increases with temperature up to 210 µV·K -1 . Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit ( Z T ) of the Co-225 whiskers is estimated 1.2–2.7 at T ≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

The photoemission line shape near $(\ensuremath{\pi},0)$ in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ below ${T}_{c}$ is characterized by a sharp peak, followed at higher energy by a dip and hump. We study the evolution of this line shape as a function of momentum, temperature, and doping. We find the hump scales with the peak and persists above ${T}_{c}$ in the pseudogap state. We present strong evidence that the peak-dip-hump structure arises from the interaction of electrons with a collective mode of wave vector $(\ensuremath{\pi},\ensuremath{\pi})$. The inferred mode energy and its doping dependence agree well with a magnetic resonance observed by neutron scattering.