Introduction
PHYSICAL properties of solids are most determined by how electrons act on stage of the crystal lattice. This is one of the reason why investigations on the electronic structure of materials have special importance in order to elucidate the microscopic origin of the various physical properties of solids. The big goal of our research activities is to understand systematically the electronic strucutre of the strongly correlated electron systems, particularly 3d transition-metal oxides. Keeping this goal in our minds, we investigate various compounds which show unusual or exotic properties or have potential for industrial applications. Our major experimental tools are (angle-resolved) photoemission spectroscopy (ARPES), which can probe the electronic structure directly, and x-ray absorption spectroscopy (XAS) which can extract information complementary to ARPES.
Current Research Project
Unusual thermoelectric power of the perovskite-type Co oxide La1-xCoxO3
La1-xCoxO3, produced by Pr-substitution fo LaCoO3, is not a carrier-doped system because the valence of La and Pr are the
same 3+. However, it has recently been observed that the sign of the thermoelectric
power (namely the sign of carriers) reverses upon Pr-substitution. In addition,
the spin-state at finite temperatures changes upon Pr-substituteion. We
are seeking the microscopic origin of those behaviors in terms of the electronic
structure.
(Samples: Collaboration with Asai Group, University of Electro-communications)
Simaltaneous spin-state-metal-insulator transition of the perovskite-type
Co oxide Pr1-xCaxCoO3
x=0.5 sample of this compound shows a simaltaneous spin-state-metal-insulator
transition with decreasing temperature, which is never observed in a similer
compound La1-xSrxCoO3. We are trying to solve this puzzling problem from electronic structure
point of view.
(Samples: Collaboration with Itoh Group, Tokyo Institute of Technology)
Electronic structure of a double-perovskite-type oxide (Sr1-yCay)2FeReO6
Iron-based double perovskites have recently been intensively investigated in connection with the colossal magnetoresistive (CMR) manganese oxides and/or potential application to spintronics devices. Among them, a metallic Sr2FeReO6 shows a fairly large tunneling magnetoresistance (TMR) although upon Ca-substitution
for Sr, (Sr1-yCay)2FeReO6 shows a metal-insulator transition (MIT) at about y=0.4. Moreover, at
the Ca end, Ca2FeReO6 has an unusually high ferrimagnetic TC. The origin of this MIT and the high TC has not yet been resolved.
(Samples: Collaboration with Correlated Electron Research Center)