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Current-voltage Characteristics of NdFeAsO0.85F0.15 and NdFeAsO0.85 Superconductors
Yong Liu,YiSheng Chai,Hyeong-Jin Kim,G.R. Stewart,김기훈,Zhi-An Ren,Zhong-Xian Zhao 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.2
The vortex phase diagrams of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors are determined from an analysis of resistivity and current-voltage (I-V ) measurements in magnetic fields up to 9 T. A vortex glass to liquid transition can be identified only in the oxygen-deficient NdFeAsO0.85, for which the I-V curves can be well scaled onto liquid and glass branches consistent with the vortex glass theory. With increasing magnetic field, the activation energy, U0, deduced from the Arrhenius plots of the resistivity based on the thermally-activated flux-flow model decays more quickly for NdFeAsO0.85F0.15 than for NdFeAsO0.85. Moreover, the irreversibility field, Hirr, of NdFeAsO0.85 increases more rapidly than that of NdFeAsO0.85F0.15 with decreasing temperature. These observations imply strong vortex pinning effects in the oxygen-deficient NdFeAsO0.85, presumably caused by enhanced defects and disorders. We infer that the observation of a vortex glass to liquid transition in NdFeAsO0.85 may be also related to the enhanced defects and disorder in the specimen.