Yoshihiko Takano

Determination of local atomic displacements in CeO1?xFxBiS2 system

We have used Bi and Ce L3-edges extended x-ray absorption fine structure measurements to study local structure of CeO(1-x)F(x)BiS2 system as a function of F-substitution. The local structure of both BiS2 active layer and CeO1-xFx spacer layer changes systematically. The in-plane Bi-S1 distance decreases (ΔRmax ∼ 0.08 Å) and the out-of-plane Bi-S2 distance increases (ΔRmax ∼ 0.12 Å) with increasing F-content. On the other hand, the Ce-O/F distance increases (ΔRmax ∼ 0.2 Å) with a concomitant decrease of the Ce-S2 distance (ΔRmax ∼ 0.15 Å). Interestingly, the Bi-S1 distance is characterized by a large disorder that increases with F-content. The results provide useful information on the local atomic displacements in CeO(1-x)F(x)BiS2, that should be important for the understanding of the coexistence of superconductivity and low temperature ferromagnetism in this system.

Coexistence of different electronic phases in the K 0.8Fe 1.6Se 2 superconductor: A bulk-sensitive hard x-ray spectroscopy study

We have studied electronic and magnetic properties of the K0.8Fe1.6Se2 superconductor by x-ray absorption and emission spectroscopy. Detailed temperature dependent measurements alongwith a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with high spin 3+Fe state and a minority phase with intermediate spin 2+Fe state. The effect of high temperature annealing suggests that the compressed phase with lower spin 2+Fe state is directly related with the high Tc superconductivity in the title system. The results clearly underline glassy nature of superconductivity in the electronically inhomogeneous K0.8Fe1.6Se2, similar to the superconductivity in granular phases.

Pressure dependence of superconductive transition temperature on KxFe2-ySe2

We measured the electric resistance of KxFe2-ySe2 which is said to have two SC dome. The measurement was performed at 1-14 GPa and at low temperature down to 1 K and the samples have different states of Fe-vacancy by annealing. From results of the measurements, we observed SC-like behaviour at over 11 GPa. The behaviour at higher temperature than Tc indicates that there are two SC phases.

Direct observation of double valence-band extrema and anisotropic effective masses of the thermoelectric material SnSe

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m

Anderson's impurity-model analysis on CeO1-xFxBiS2

_0

Unconventional superconductivity in the BiS

and an out-of-plane value of 0.71 m

_2

_0

-based layered superconductor NdO

, where m

_{0.71}

_0

F

is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe.