Ichiro Terasaki

Distinct Fermi-momentum dependent energy gaps in deeply underdoped Bi2212.

We used angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2Ca(1–x)YxCu2O8 (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.

An oxide thermal rectifier

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.

An organic thyristor

Thyristors are a class of nonlinear electronic device that exhibit bistable resistance—that is, they can be switched between two different conductance states. Thyristors are widely used as inverters (direct to alternating current converters) and for the smooth control of power in a variety of applications such as motors and refrigerators. Materials and structures that exhibit nonlinear resistance of this sort are not only useful for practical applications: they also provide systems for exploring fundamental aspects of solid-state and statistical physics. Here we report the discovery of a giant nonlinear resistance effect in the conducting organic salt θ-(BEDT-TTF)2CsCo(SCN)4, the voltage-current characteristics of which are essentially the same as those of a conventional thyristor. This intrinsic organic thyristor works as an inverter, generating an alternating current when a static direct-current voltage is applied. Whereas conventional thyristors consist of a series of diodes (their nonlinearity comes from interface effects at the p-n junctions), the present salt exhibits giant nonlinear resistance as a bulk phenomenon. We attribute the origin of this effect to the current-induced melting of insulating charge-order domains, an intrinsically non-equilibrium phenomenon in the sense that ordered domains are melted by a steady flow.

Thermoelectric Properties of Bi2.3-xPbxSr2.6Co2Oy Single Crystals

A set of single crystals of the layered cobaltite Bi2.3-xPbxSr2.6Co2Oy (0 ≤x≤0.44) were prepared by a flux technique. The resistivity ρ and the thermopower S along the in-plane direction were measured, and the thermoelectric properties were compared with those of other oxides. The Pb substitution causes a decrease in ρ and an increase in S below 100 K, which improves the power factor S2/ρ by 3–4 times. The transport parameters exhibit an anomaly near 25–50 K, such as a plateau in S and an upturn in ρ, which we ascribe to the reduction of the density of states owing to a pseudogap.

Low thermal conductivity of the layered oxide ( N a , C a ) Co 2 O 4 : Another example of a phonon glass and an electron crystal

The thermal conductivity of polycrystalline samples of

SPECIFIC-HEAT EVIDENCE FOR STRONG ELECTRON CORRELATIONS IN THE THERMOELECTRIC MATERIAL (NA, CA)CO2O4

(\mathrm{N}\mathrm{a},\mathrm{C}\mathrm{a}){\mathrm{Co}}_{2}{\mathrm{O}}_{4}

CaCu3Ti4O12∕CaTiO3 composite dielectrics: Ba∕Pb-free dielectric ceramics with high dielectric constants

is found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18 mW/cm K at 280 K, and it does not change appreciably with the substitution of Ca for Na. A quantitative analysis has revealed that the phonon mean free path is comparable with the lattice parameters, where the point-defect scattering plays an important role. Electronically the same samples show a metallic conduction down to 4.2 K, which strongly suggests that

Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo 2 − x Cu x O 4

{\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}

Transport properties and electronic states of the thermoelectric oxide NaCo2O4

exhibits a glasslike poor thermal conduction together with a metal-like good electrical conduction. The present study further suggests that a strongly correlated system with layered structure can act as a material of a phonon glass and an electron crystal.

A Novel Heavy-Fermion State in CaCu3Ru4O12

The specific heat of