104 research outputs found
Structural properties, defects and structural phase transition in the ROFeM (R=La, Nd; M=As, P) materials
The structural properties of the ROFeM (R=La, Nd; M=As, P) materials have
been analyzed by means of electron diffraction, high-resolution
transmission-electron microscopy (TEM) and in-situ cooling TEM observations.
The experimental results demonstrate that the layered ROFeM crystals often
contain a variety of structural defects, such as stacking faults and
small-angle boundaries. The in-situ TEM investigations reveal that, in
association with the remarkable spin-density-wave (SDW) instability near 150 K,
complex structural transitions can be clearly observed in both crystal symmetry
and local microstructure features.Comment: 17 pages, 6 figure
Superconductivity at 53.5 K in GdFeAsO1-delta
Here we report the fabrication and superconductivity of the iron-based
arsenic-oxide GdFeAsO1-delta compound with oxygen-deficiency, which has an
onset resistivity transition temperature at 53.5 K. This material has a same
crystal structure as the newly discovered high-Tc ReFeAsO1-delta family (Re =
rare earth metal) and a further reduced crystal lattice, while the Tc starts to
decrease compared with the SmFeAsO1-delta system
Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides ReFeAsO1-delta (Re = rare earth metal) without F-Doping
Here we report a new class of superconductors prepared by high pressure
synthesis in the quaternary family ReFeAsO1-delta (Re = Sm, Nd, Pr, Ce, La)
without fluorine doping. The onset superconducting critical temperature (Tc) in
these compounds increases with the reduction of Re atom size, and the highest
Tc obtained so far is 55 K in SmFeAsO1-delta. For the NdFeAsO1-delta system
with different oxygen concentration a dome-shaped phase diagram was found
Structural and critical current properties in polycrystalline SmO1-xFxFeAs
A series of polycrystalline SmO1-xFxFeAs bulks (x=0.15, 0.2, 0.3 and 0.4)
were prepared by the conventional solid state reaction. Resistivity,
susceptibility, magnetic hysteresis, critical current density and
microstructure of these samples have been investigated. It is found that
critical transition temperature Tc increases steadily with increasing fluorine
content, with the highest onset Tc=53 K at x=0.4. On the other hand, the
superconductivity seems correlated with lattice constants; that is, Tc rises
with the shrinkage of a-axis while resistivity increases with the enlargement
of c-axis. A global critical current density of 1.1x10^4 A/cm^2 at 5 K in self
field was achieved in the purest sample. A method of characterization of
inter-grain current density is proposed. This method gives an inter-grain Jc of
3.6x10^3 A/cm^2 at 5 K in self field, in contrast to the intra-grain Jc of 10^6
A/cm^2. The effect of composition gradients on the inter-grain Jc in
SmO1-xFxFeAs is also discussed.Comment: 18 pages, 7 figure
Crystal chemical simulation of superconductors on the basis of oxide and intermetallic layers
Simulation of 'hybrid' superconductors of 3d-, 4d- and 5d-transition elements
consisting of two different superconducting fragments located between
positively charged ions planes - B'O2 oxide planes and B2C2 intermetallic
layers - has been performed on the basis of the structure of Sr2Mn3As2O2
(A2(B2C2)(B'O2)). The oxide planes are similar to those of CuO2 in
high-temperature superconducting cuprates while the intermetallic layers - to
those of Ni2B2 in low-temperature superconducting borocarbides RNi2B2C and
Fe2As2 layers in high-temperature superconducting oxypnictides RFeAsO1-xFx.Comment: Title changed by the Editor of Supercond. Sci. Technol., published
versio
Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides
Early studies have found quasi-reversible magnetization curves in
polycrystalline bulk rare-earth iron oxypnictides that suggest either
wide-spread obstacles to intergranular current or very weak vortex pinning. In
the present study of polycrystalline samarium and neodymium rare-earth iron
oxypnictide samples made by high pressure synthesis, the hysteretic
magnetization is significantly enhanced. Magneto optical imaging and study of
the field dependence of the remanent magnetization as a function of particle
size both show that global currents over the whole sample do exist but that the
intergranular and intragranular current densities have distinctively different
temperature dependences and differ in magnitude by about 1000. Assuming that
the highest current density loops are restricted to circulation only within
grains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-sample
current densities, though two orders of magnitude lower are 1000-10000 A/cm2,
some two orders of magnitude higher than in random polycrystalline cuprates. We
cannot yet be certain whether this large difference in global and intragrain
current density is intrinsic to the oxypnictides or due to extrinsic barriers
to current flow, because the samples contain significant second phase, some of
which wets the grain boundaries and produces evidences of SNS proximity effect
in the whole sample critical current.Comment: 28 pages, 14 figure
X-ray absorption spectroscopy (XAS) investigation of the electronic structure of superconducting FeSex single crystals
X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x=1-0.8)
single crystals cleaved in situ in vacuum reveal characteristic Fe 4sp states,
a lattice distortion and the Se K-edge spectra point to a strong Fe 3d-Se 4p
hybridization giving rise to itinerant charge carriers. A formal charge of
~1.8+ for Fe and ~2.2- for Se were evaluated from these spectra in the FeSex
(x=0.88). The charge balance between Fe and Se is assigned itinerant electrons
located in the Fe-Se hybridization bond. As x decreases the 4p hole count
increases and a crystal structure distortion is observed that in turn causes
the Fe separation in the ab plane change from 4p orbital to varying
(modulating) coordination. Powder x-ray diffraction (XRD) measurements also
show a slight increase in lattice parameters as x decreases (increasing Se
deficiency)
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
Point-Contact Spectroscopy of Iron-Based Layered Superconductor LaOFFeAs
We present point-contact spectroscopy data for junctions between a normal
metal and the newly discovered F-doped superconductor
LaOFFeAs (F-LaOFeAs). A zero-bias conductance peak was
observed and its shape and magnitude suggests the presence of Andreev bound
states at the surface of F-LaOFeAs, which provides a possible evidence of an
unconventional pairing symmetry with a nodal gap function. The maximum gap
value meV was determined from the measured spectra,
in good agreement with the recent experiments on specific heat and lower
critical field.Comment: 5 pages, 4 figure
Superconductivity in Co-doped SmFeAsO
Here we report the synthesis and basic characterization of SmFe1-xCoxAsO
(x=0.10, 0.15). The parent compound SmFeAsO itself is not superconducting but
shows an antiferromagnetic order near 150 K, which must be suppressed by doping
before superconductivity emerges. With Co-doping in the FeAs planes,
antiferromagnetic order is destroyed and superconductivity occurs at 15 K.
Similar to LaFe1-xCoxAsO, the SmFe1-xCoxAsO system appears to tolerate
considerable disorder in the FeAs planes. This result is important, which
indicates difference between cuprare superconductors and the iron-based
arsenide ones.Comment: 11 pages, 3 figure
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