Browsing by Author "Wu, MK"

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    Doping-driven structural phase transition and loss of superconductivity in MxFe1-xSe delta (M=Mn, Cu)
    (2010) Huang, TW; Chen, TK; Yeh, KW; Ke, CT; Chen, CL; Huang, YL; Hsu, FC; Wu, MK; Wu, PM; Avdeev, M; Studer, AJ
    In this paper, we report the results of detailed studies on Mn and Cu substitution to Fe site of beta-FeSe, namely, MnxFe(1-x)Se(1-delta) and CuxFe1-xSe1-delta (delta equals to 0.03-0.05 based on our neutron-diffraction refinements). The results show that with only 10 at. % Cu doping the compound becomes a Mott insulator. Detailed temperature-dependent structural analyses of these Mn- and Cu-substituted compounds show that the structural transition, which is associated with the changes in the building block FeSe4 tetrahedron, is essential to the occurrence of superconductivity in beta-FeSe.
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    Low temperature amorphization and superconductivity in FeSe single crystals at high pressures
    (2010) Stemshorn, AK; Tsoi, G; Vohra, YK; Sinogeiken, S; Wu, PM; Huang, Y; Rao, SM; Wu, MK; Yen, KW; Weir, ST
    In this study, we report low temperature x-ray diffraction studies combined with electrical resistance measurements on single crystals of iron-based layered superconductor FeSe to a temperature of 10 K and a pressure of 44 GPa. The low temperature high pressure x-ray diffraction studies were performed using a synchrotron source and superconductivity at high pressure was studied using designer diamond anvils. At ambient temperature, the FeSe sample shows a phase transformation from a PbO-type tetragonal phase to a NiAs-type hexagonal phase at 10 +/- 2 GPa. On cooling, a structural distortion from a PbO-type tetragonal phase to an orthorhombic Cmma phase is observed below 100 K. At a low temperature of 10 K, compression of the orthorhombic Cmma phase results in a gradual transformation to an amorphous phase above 15 GPa. The transformation to the amorphous phase is completed by 40 GPa at 10 K. A loss of superconductivity is observed in the amorphous phase and a dramatic change in the temperature behavior of electrical resistance indicates formation of a semiconducting state at high pressures and low temperatures. The formation of the amorphous phase is attributed to a kinetic hindrance to the growth of a hexagonal NiAs phase under high pressures and low temperatures.
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    Nonstoichiometry of LixCu2O2+delta single crystal and its relation to magnetic ordering
    (2010) Yeh, KW; Huang, TW; Ke, CT; Wu, PM; Zhao, L; Chao, WH; Lee, YC; Chen, CL; Wu, MK
    We report the first systematic study on the nonstoichiometry of Li/Cu ionic ratios x and oxygen defect concentrations delta in LixCu2O2+delta solid solution. Nonstoichiometric crystals with intentional Li-deficient (x=0.97) or Li-excessive (x=1.06) concentrations were prepared by slow-cooling and floating-zone methods. By applying different oxygen pressures during growth, different magnitudes of oxygen defects were obtained, revealing varied magnetic and electrical properties. Our results strongly suggest that Li/Cu and oxygen nonstoichiometry in these crystals are responsible for the varied temperatures of helimagnetic ordering. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483244]