353 research outputs found
Rigid vortices in MgB2
Magnetic relaxation of high-pressure synthesized MgB bulks with different
thickness is investigated. It is found that the superconducting dia-magnetic
moment depends on time in a logarithmic way; the flux-creep activation energy
decreases linearly with the current density (as expected by Kim-Anderson
model); and the activation energy increases linearly with the thickness of
sample when it is thinner than about 1 mm. These features suggest that the
vortices in the MgB are rather rigid, and the pinning and creep can be well
described by Kim-Anderson model.Comment: Typo corrected & reference adde
Current-voltage characteristics of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors
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.Comment: 19 pages, 5 figure
Can antiferromagnetism and superconductivity coexist in the high-field paramagnetic superconductor Nd(O,F)FeAs?
We present measurements of the temperature and field dependencies of the
magnetization M(T,H) of Nd(O0.89F0.11)FeAs at fields up to 33T, which show that
superconductivity with the critical temperature Tc ~ 51K cannot coexist with
antiferromagnetic ordering. Although M(T,H) at 55 < T < 140K exhibits a clear
Curie-Weiss temperature dependence corresponding to the Neel temperature TN ~
11-12K, the behavior of M(T,H) below Tc is only consistent with either
paramagnetism of weakly interacting magnetic moments or a spin glass state. We
suggest that the anomalous magnetic behavior of an unusual high-field
paramagnetic superconductor Nd(O1-xFx)FeAs is mostly determined by the magnetic
Nd ions.Comment: 4 pages, 4 figure
- …