4 research outputs found
Upper critical fields well above 100 T for the superconductor SmFeAsOF with T = 46 K
We report specific heat measurements at magnetic fields up to 20 T on the
recently discovered superconductor SmFeAsOF. The B-T diagram
of a polycrystalline SmFeAsOF sample with T = 46 K was
investigated. The temperature dependence of B was extracted from the
specific heat curves, the corresponding B(T = 0) value derived from the
Werthamer-Helfand-Hohenberg formula being 150 T. Based on magnetization
measurements up to 9 T, a first estimation of the field dependence of the
inductive critical current J is given. Evidence for granularity is found.
The presence of a peak effect is reported, suggesting a crossover in the vortex
dynamics, in analogy to the behaviour observed in high T cuprates.Comment: Accepted for publication in Physical Review
Thermally activated energy and critical magnetic fields of SmFeAsOF
Thermally activated flux flow and vortex glass transition of recently
discovered SmFeAsOF superconductor are studied in magnetic
fields up to 9.0 T. The thermally activated energy is analyzed in two analytic
methods, of which one is conventional and generally used, while the other is
closer to the theoretical description. The thermally activated energy values
determined from both methods are discussed and compared. In addition, several
critical magnetic fields determined from resistivity measurements are presented
and discussed.Comment: Accepted by Superconductor Science and Technolog. 5 page, 4 figure
Evidence for Supercurrent Connectivity in Conglomerate Particles in NdFeAsO1-d
Here we use global and local magnetometry and Hall probe imaging to
investigate the electromagnetic connectivity of the superconducting current
path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High
resolution transmission electron microscopy and scanning electron microscopy
show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around
individual particles, and second phase neodymium oxide which may be responsible
for the large paramagnetic background at high field and at high temperatures.
From global magnetometry and electrical transport measurements it is clear
that there is a small supercurrent flowing on macroscopic sample dimensions
(mm), with a lower bound for the average (over this length scale) critical
current density of the order of 103 A/cm2. From magnetometry of powder samples
and local Hall probe imaging of a single large conglomerate particle ~120
microns it is clear that on smaller scales, there is better current
connectivity with a critical current density of the order of 5 x 104 A/cm2. We
find enhanced flux creep around the second peak anomaly in the magnetisation
curve and an irreversibility line significantly below Hc2(T) as determined by
ac calorimetry.Comment: 11 pages, 4 figure
Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared by the powder-in-tube method
We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully
fabricated by the powder-in-tube (PIT) method for the first time. Structural
analysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs
was obtained by this synthesis method. The transition temperature of the
SmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on
magnetization measurements, it is found that a globe current can flow on
macroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self
field, while a high Jc about 2x10^5 A/cm^2 is observed within the grains,
suggesting that a significant improvement in the globle Jc is possible. It
should be noted that the Jc exhibits a very weak field dependence behavior.
Furthermore, the upper critical fields (Hc2) determined according to the
Werthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very
encouraging application of the new superconductors.Comment: 14 pages, 6 figure