Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4

Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly reviewed. The high thermoelectric properties of this material are attributed to the substantially enhanced effective mass, which comes from the large entropy of Co$^{4+}$ in the low spin state. The large entropy confined in the CoO_2 block causes a spin-density-wave transition at 22 K upon Cu substitution for Co, which can be regarded as ``order from disorder''.Comment: 4 Pages, $ figures, Proceedings of The 2nd Hiroshima Workshop: Transport and Thermal Properties of Advanced Material

Hidden-charm scalar tetra-quark mesons

Decays of hidden-charm scalar tetra-quark mesons as the partners of D_{s0}(2317) which has successfully been assigned to the iso-triplet scalar tetra-quark [c,n][sbar,nbar]_{I=1} meson are studied. Because OZI-rule allowed strong decays are kinematically limited, their radiative decays are expected to be important.Comment: 4 pages, 0 figure, references added, typos corrected, description clarifie

High-temperature oxide thermoelectrics

We have evaluated the power factor of transition metal oxides at high temperatures using the Heikes formula and the Ioffe-Regel conductivity. The evaluated power factor is found to be nearly independent of carrier concentration in a wide range of doping, and well explains the experimental data for cobalt oxides. This suggests that the same power factor can be obtained with a thermopower larger than $2k_B/e$, and also suggests a reasonably high value of the dimensionless figure of merit $ZT$. We propose an oxide thermoelectric power generator by using materials having a thermopower larger than 300 $\mu$V/K.Comment: 5 pages, 2 figure

A new tetra-quark interpretation of X(3872)

A new tetra-quark interpretation of X(3872) is presented. In this model, X(3872) consists of two degenerate tetra-quark mesons, {[cn](cbar nbar) +- (cn)[cbar nbar]}_{I=0}, and, therefore, it is naturally understood that X(3872) decays into two different eigenstates of G-parity.Comment: 4 pages, 0 figures, RevTeX, description clarifie

X(3872) and Its Iso-Triplet Partners

Decays of X(3872) and its partners as hidden-charm axial-vector tetra-quark mesons are studied. As the result, it is seen that the iso-triplet partners of X(3872) can be broad, and therefore, higher statistics would be needed to find them.Comment: 4 page