424 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
Superconductivity and local-moment magnetism in Eu(FeCo)As
We report the measurements of resistivity and magnetization under magnetic
fields parallel and perpendicular to the basal plane, respectively, on a
cobalt-doped Eu(FeCo)As single crystal. We
observed a resistivity drop at 21 K, which shifts toward lower
temperatures under external fields, suggesting a superconducting transition.
The upper critical fields near show large anisotropy, in contrast with
those of other '122' FeAs-based superconductors. Low-field magnetic
susceptibility data also show evidence of superconductivity below 21 K. Instead
of expected zero-resistance below , however, a resistivity reentrance
appears at 17 K under zero field, coincident with the magnetic ordering of
Eu moments. Based on the temperature and field dependences of
anisotropic magnetization, a helical magnetic structure for the Eu spins
is proposed. External magnetic fields easily changes the helimagnetism into a
ferromagnetism with fully polarized Eu spins, accompanying by
disappearance of the resistivity reentrance. Therefore, superconductivity
coexists with ferromagnetic state of Eu spins under relatively low
magnetic field. The magnetic and superconducting phase diagrams are finally
summarized for both and .Comment: 8 pages, 10 figure
Magnetic phase diagram of Fe1.1Te1-xSex: A comparative study with the stoichiometric superconducting FeTe1-xSex system
We report a comparative study of the series Fe1.1Te1-xSex and the
stoichiometric FeTe1-xSex to bring out the difference in their magnetic,
superconducting and electronic properties. The Fe1.1Te1-xSex series is found to
be magnetic and its microscopic properties are elucidated through Moessbauer
spectroscopy. The magnetic phase diagram of Fe1.1Te1-xSex is traced out and it
shows the emergence of spin-glass state when the antiferromagnetic state is
destabilized by the Se substitution. The isomer shift and quadrupolar splitting
obtained from the Moessbauer spectroscopy clearly brings out the electronic
differences in these two series.Comment: 6 pages, 9 figure
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
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