Current-voltage characteristics of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors

TL;DR

This study investigates the vortex phase diagrams and flux dynamics of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors, revealing vortex glass behavior only in the oxygen-deficient sample and highlighting the effects of defects on vortex pinning.

Contribution

It provides the first detailed analysis of vortex phase transitions and flux pinning differences between these two NdFeAsO-based superconductors.

Findings

Vortex glass to liquid transition observed only in NdFeAsO0.85.

Activation energy U0 decreases more rapidly with field in NdFeAsO0.85F0.15.

Irreversibility field Hirr increases faster in NdFeAsO0.85 with decreasing temperature.

Abstract

The vortex phase diagrams of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors are determined from the analysis of resistivity and current-voltage (I-V) measurements in magnetic fields up to 9 T. A clear vortex glass to liquid transition is identified only in the oxygen deficient NdFeAsO0.85, in which 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 resistivity based on the thermally activated flux-flow model (TAFF), 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 evidence the strong vortex pinning effects, presumably caused by the enhanced defects and disorders in the oxygen deficient NdFeAsO0.85. It is inferred that the enhanced defects and disorder can be also responsible for the vortex glass to liquid transition in the NdFeAsO0.85.