Kuwahara Hideki

We have grown single crystals of mixed-valence pseudobrookite titanate MgTi2O5-Ti3O5 systems (Mg1-xTi2+xO5) and have systematically investigated their physical properties as a novel canonical mixed-valence system which shows a successive change from the nonmagnetic band insulating phase (MgTi2O5) to the charge-ordered spin-singlet dimer phase (Ti3O5). In comparison with Al1-xTi2+xO5 [R. Takahama, Phys. Rev. Mater. 4, 074401 (2020)2475-995310.1103/PhysRevMaterials.4.074401], we have clarified that, in the lower x region of Mg1-xTi2+xO5, the physical properties are greatly affected by the change of electron density with a variation of x and we have found that the doped electrons enhance the optical conductivity along a Ti-Ti ladder direction. However, we have also clarified that, in the higher x region, almost all Ti3+ ions produced by an increase in x contribute not to an increase in itinerant electrons but to a development of Ti3+-Ti3+ dimer correlation. These observations indicate a crossover from the conductance along the ladder direction to a nontrivial conductance under the background of the dimer correlation.

Barium tantalate with a tetragonal tungsten bronze structure, Ba3Ta5O15, and compounds where Ba is partly substituted by various rare earths (R), Ba3-xRxTa5O15, were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta 5d orbital and the localized spins in the Eu 4f orbital.