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

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