We measured and analyzed the transport properties of single-crystal ${\mathrm{NaCo}}_2{\mathrm{O}}_4$, which is a metallic transition-metal oxide consisting of a two-dimensional triangle lattice of Co. Reflecting the crystal structure, the resistivity is highly anisotropic between in- and out-of-plane directions, and the in-plane resistivity is as low as 200 $\mu \Omega$ cm at 300 K. Most strikingly, the in-plane thermoelectric power of ${\mathrm{NaCo}}_2{\mathrm{O}}_4$ is about 100 $\mu$V/K at 300 K, which is nearly ten times larger than that of typical metals. The large thermoelectric power and the low resistivity suggest that ${\mathrm{NaCo}}_2{\mathrm{O}}_4$ is a potential thermoelectric material.

The magnetic susceptibility of single-crystal ${\mathrm{CuGeO}}_3$, a linear ${\mathrm{Cu}}^{2+}$ (spin-1/2) chain compound, was measured. The susceptibilities in all the directions rapidly drop to small constant values with decreasing temperature below a phase transition temperature near 14 K. The magnetic-field dependence of the transition temperature quantitatively agrees with both theoretical predictions and experimental results for organic spin-Peierls systems. This Letter is the first report of an unambiguous determination of the existence of the spin-Peierls transition in an inorganic compound.

We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi ₂ Sr ₂ Ca _{(1– x )} Y _x Cu ₂ O ₈ (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increased with underdoping, a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near-nodal regime where a coherent peak in the spectrum can be observed, did not increase with less doping, a behavior not previously observed in the single particle spectra. We propose a two-gap scenario in momentum space that is consistent with other experiments and may contain important information on the mechanism of high–transition temperature superconductivity.

We have experimentally demonstrated thermal rectification as bulk effect. According to a theoretical design of a thermal rectifier, we have prepared an oxide thermal rectifier made of two cobalt oxides with different thermal conductivities, and have made an experimental system to detect the thermal rectification. The rectifying coefficient of the device is found to be 1.43, which is in good agreement with the numerical calculation.

The magnetic susceptibilities of polycrystalline ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$ ${\mathrm{GeO}}_3$ were measured to study the effects of nonmagnetic impurities on the spin-Peierls transition. The transition temperature drastically reduces upon Zn doping and the spin-Peierls state collapses around x=0.03. The Zn doping causes another phase transition in the samples with 0.02≤x≤0.08, which proved to be spin-glass-like. The present Letter provides the first study of the spin-Peierls system with impurities.