Anisotropic Optical Conductivity Accompanied by a Small Energy Gap in One-Dimensional Thermoelectric Telluride Ta4SiTe4

We investigated the optical properties of single crystals of one-dimensional telluride Ta4SiTe4, which shows high thermoelectric performance below room temperature. Optical conductivity estimated from reflectivity spectra indicates the presence of a small energy gap of 0.1-0.15 eV at the Fermi energy. At the lowest energy, optical conductivity along the Ta4SiTe4 chain is an order of magnitude higher than that perpendicular to this direction, reflecting the anisotropic electron conduction in Ta4SiTe4. These results indicate that coexistence of a very small band gap and anisotropic electron conduction is a promising strategy to develop a high-performance thermoelectric material for low temperature applications.Comment: 6 pages, 4 figure

Regular-Triangle Trimer and Charge Order Preserving the Anderson Condition in the Pyrochlore Structure of CsW2_2O6_6

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report the discovery of a unique phase transition that preserves the cubic symmetry of the beta-pyrochlore oxide CsW$_2$O$_6$, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation is an unprecedented self-organization of d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. Electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the localized nature of the 5d electrons characteristic of the pyrochlore oxides were found to play important roles in this unique charge-orbital-spin coupled phenomenon.Comment: 7 pages, 3 figure

Technological Innovation in Ceramics : Pottery and Porcelain to Newceramics

第5回シンポジウム「日本の技術革新―経験蓄積と知識基盤化―」 : 2009年12月16日・17日 : 国立科学博物館 上野本館セラミックスは硬くて耐久性があり、陶磁器製の食器や花器などに使用されてきた。近代産業の発展とともに、セラミックスの科学的解明が進み、現代の生活ならびに産業社会を支える重要な基盤材料として開発されてきた。土鍋の改良やセラミックス包丁の開発などに、その歴史をみることができる。さらに、炭化ケイ素などの人工原料の利用ならびに製造技術の発展によって、強度、耐熱性、耐食性などに優れたニューセラミックスが実現し、様々な産業分野で活躍している