73 research outputs found
Large critical current density improvement in Bi-2212 wires through groove-rolling process
Recently the interest about Bi-2212 round wire superconductor for high
magnetic field use has been enhancing despite the fact that an increase of the
critical current is still needed to boost its successful use in such
applications. Recent studies have demonstrated that the main obstacle to
current flow, especially in long wires, is the residual porosity inside these
Powder-In-Tube processed conductors which develops in bubbles-agglomeration
when the Bi-2212 melts. Through this work we tried to overcome this issue
acting on the wire densification by changing the deformation process. Here we
show the effects of groove-rolling versus drawing process on the critical
current density JC and on the microstructure. In particular, groove-rolled
multifilamentary wires show a JC increased by a factor of about 3 with respect
to drawn wires prepared with the same Bi-2212 powder and architecture. We think
that this approach in the deformation process is able to produce the required
improvements both because the superconducting properties are enhanced and
because it makes the fabrication process faster and cheaper
Application potential of Fe-based superconductors
open4siIn this paper we report basic properties of iron-based superconductors and review the latest achievements in the fabrication of conductors based on these materials. We compare state-of-the-art results with performances obtained with low-T c and high-T c technical superconductors, evidencing in particular the most significant differences with respect to high-T c cuprate coated conductors. Although the optimization of preparation procedures is yet to be established, a potential range of applications for iron-based superconductors in the high field low temperature regime can be envisaged, where they may become competitors to RE-123 coated conductors.openPallecchi, Ilaria; Eisterer, Michael; Malagoli, Andrea; Putti, MarinaPallecchi, Ilaria; Eisterer, Michael; Malagoli, Andrea; Putti, Marin
Groove-rolling as an alternative process to fabricate Bi-2212 wires for practical applications
Bi2Sr2CaCu2O8+x (Bi-2212) superconducting long-length wires are mainly
limited in obtaining high critical currents densities (JC) by the internal gas
pressure generated during the heat treatment, which expands the wire diameter
and dedensifies the superconducting filaments. Several ways have been developed
to increase the density of the superconducting filaments and therefore
decreasing the bubble density: much higher critical currents have been reached
always acting on the final as-drawn wires. We here try to pursue the same goal
of having a denser wire by acting on the deformation technique, through a
partial use of the groove-rolling at different wire processing stages. Such
technique has a larger powders compaction power, is straightforwardly adaptable
to long length samples, and allows the fabrication of samples with round,
square or rectangular shape depending on the application requirements. In this
paper we demonstrate the capability of this technique to increase the density
in Bi-2212 wires which leads to a three-fold increase in Jc with respect to
drawn wires, making this approach very promising for fabricating Bi-2212 wires
for high magnetic field magnets, i.e. above 25 T
Experimental investigation of electronic interactions in collapsed and uncollapsed LaFe2As2 phases
The iron-based pnictide LaFe2As2 is not superconducting as-synthesized, but
it becomes such below Tc around 12 K upon annealing, as a consequence of a
structural transition from a phase with collapsed tetragonal crystal structure
to an uncollapsed phase. In this work, we carry out specific heat, Raman
spectroscopy and normal state electric and thermoelectric transport
measurements in the collapsed and uncollapsed LaFe2As2 phases to gain insight
into the electron interactions and their possible role in the superconducting
pairing mechanism. Despite clear features of strong electron-phonon coupling
observed in both phases, neither the low energy phonon spectra nor the
electron-phonon coupling show significant differences between the two phases.
Conversely, the Sommerfield constants are significantly different in the two
phases, pointing to much higher electron correlation in the superconducting
uncollapsed phase and confirming theoretical studies.Comment: In press on Physical Review
Experimental Evidence for Static Charge Density Waves in Iron Oxypnictides
In this Letter we report high-resolution synchrotron x-ray powder diffraction and transmission electron microscope analysis of Mn-substituted LaFeAsO samples, demonstrating that a static incommensurate modulated structure develops across the low-temperature orthorhombic phase, whose modulation wave vector depends on the Mn content. The incommensurate structural distortion is likely originating from a charge-density-wave instability, a periodic modulation of the density of conduction electrons associated with a modulation of the atomic positions. Our results add a new component in the physics of Fe-based superconductors, indicating that the density wave ordering is charge driven
High-energy ball milling and synthesi temperature study to improve superconducting properties of MgB2 ex-situ tapes and wires
MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means
of the ex-situ powder-in-tube method using either high-energy ball milled and
low temperature synthesized powders. All sample were sintered at 920 C in Ar
flow. The milling time and the revolution speed were tuned in order to maximize
the critical current density in field (Jc): the maximum Jc value of 6 x 10e4
A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with
powders milled for 144h at 180rpm. Vorious synthesis temperature were also
investigated (730-900 C) finding a best Jc value for the wire prepared with
powders synthesized at 745 C. We speculate that this optimal temperature is due
to the fluidifying effect of unreacted magnesium content before the sintering
process which could better connect the grains
Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films
FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3
and MgO substrates and structurally and electro-magnetically characterized in
order to study the influence of the substrate on their transport properties.
The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates
shows no influence on the lattice parameters of the films, whereas the type of
substrates affects the crystalline quality of the films and, therefore, the
superconducting properties. The film on MgO showed an extra peak in the angular
dependence of critical current density Jc({\theta}) at {\theta} = 180{\deg} (H
|| c), which arises from c-axis defects as confirmed by transmission electron
microscopy. In contrast, no Jc({\theta}) peaks for H || c were observed in
films on CaF2 and LaAlO3. Jc({\theta}) can be scaled successfully for both
films without c-axis correlated defects by the anisotropic Ginzburg-Landau
(AGL) approach with appropriate anisotropy ratio {\gamma}J. The scaling
parameter {\gamma}J is decreasing with decreasing temperature, which is
different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO
substrates.Comment: accepted for publication in SUS
Magnetic properties of spin diluted iron pnictides from muSR and NMR in LaFe1-xRuxAsO
The effect of isoelectronic substitutions on the microscopic properties of
LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La
NMR. It was found that Ru substitution causes a progressive reduction of the
N\`eel temperature (T_N) and of the magnetic order parameter without leading to
the onset of superconductivity. The temperature dependence of 139La nuclear
spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band
model. One band giving rise to the spin density wave ground-state, while the
other one is characterized by weakly correlated electrons. Fe for Ru
substitution yields to a progressive decrease of the density of states at the
Fermi level close to the one derived from band structure calculations. The
reduction of T_N with doping follows the predictions of the J_1-J_2 model on a
square lattice, which appears to be an effective framework to describe the
magnetic properties of the spin density wave ground-state.Comment: 6 pages, 8 figure
Calorimetric evidence for two phase transitions in BaKFeAs with fermion pairing and quadrupling states
Theoretically, materials that break multiple symmetries allow, under certain
conditions, the formation of four-fermion condensates above the superconducting
critical temperature. Such states can be stabilized by phase fluctuations.
Recently a fermionic quadrupling condensate that breaks the time-reversal
symmetry was reported in BaKFeAs [V. Grinenko
et al., Nat. Phys. 17, 1254 (2021)]. Evidence for the new state of matter comes
from muon-spin rotation, transport, thermoelectric, and ultrasound experiments.
Observing a specific heat anomaly is a very important signature of a transition
to a new state of matter. However, a fluctuation-induced specific heat
singularity is usually very challenging to resolve from a background of other
contributions. Here, we report on detecting two anomalies in the specific heat
of BaKFeAs at zero magnetic field. The
anomaly at the higher temperature is accompanied by the appearance of a
spontaneous Nernst effect, indicating broken time-reversal () symmetry.
The second anomaly at the lower temperature coincides with the transition to a
zero resistance state, indicating superconductivity breaking the gauge
symmetry. Our data provide calorimetric evidence for the phase formation
above the superconducting phase transition.Comment: 12 pages, 3 figures and Supplementary informatio
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