123 research outputs found
Role of heat and mechanical treatments in the fabrication of superconducting Ba0.6K0.4Fe2As2 ex-situ Powder-In-Tube tapes
Among the recently discovered Fe-based superconducting compounds, the
(K,Ba)Fe2As2 phase is attracting large interest within the scientific community
interested in conductor developments. In fact, after some years of development,
critical current densities Jc of about 105 A/cm2 at fields up to more than 10 T
have been obtained in powder in tube (PIT) processed wires and tapes. Here we
explore the crucial points in the wire/tape fabrication by means of the ex-situ
PIT method. We focus on scaling up processes which are crucial for the
industrial fabrication. We analyzed the effects on the microstructure of the
different heat and mechanical treatments. By an extensive microstructural
analysis correlated with the transport properties we addressed the issues
concerning the phase purity, the internal porosity and crack formation in the
superconducting core region. Our best conductors with a filling factor of about
30 heat treated at 800 C exhibited Tc = 38 K the highest value measured in such
kind of superconducting tape. The microstructure analysis shows clean and well
connected grain boundaries but rather poor density: The measured Jc of about 3
x 10^4 A/cm2 in self-field is suppressed by less than a factor 7 at 7 T. Such
not yet optimized Jc values can be accounted for by the reduced density while
the moderate in-field suppression and a rather high n-factor confirm the high
homogeneity and uniformity of these tapes
Why of (CaFeAs)PtAs is twice as high as (CaFePtAs)PtAs
Recently discovered (CaFePtAs)PtAs and
(CaFeAs)PtAs superconductors are very similar materials
having the same elemental composition and structurally similar superconducting
FeAs slabs. Yet the maximal critical temperature achieved by changing Pt
concentration is approximately twice higher in the latter. Using angle-resolved
photoemission spectroscopy(ARPES) we compare the electronic structure of their
optimally doped compounds and find drastic differences. Our results highlight
the sensitivity of critical temperature to the details of fermiology and point
to the decisive role of band-edge singularities in the mechanism of high-
superconductivity
Ab initio lattice dynamics simulations and inelastic neutron scattering spectra for studying phonons in BaFe2As2: Effect of structural phase transition, structural relaxation and magnetic ordering
We have performed extensive ab initio calculations to investigate phonon
dynamics and their possible role in superconductivity in BaFe2As2 and related
systems. The calculations are compared to inelastic neutron scattering data
that offer improved resolution over published data [Mittal et al., PRB 78
104514 (2008)], in particular at low frequencies. Effects of structural phase
transition and full/partial structural relaxation, with and without magnetic
ordering, on the calculated vibrational density of states are reported. Phonons
are best reproduced using either the relaxed magnetic structures or the
experimental cell. Several phonon branches are affected by the subtle
structural changes associated with the transition from the tetragonal to the
orthorhombic phase. Effects of phonon induced distortions on the electronic and
spin structure have been investigated. It is found that for some vibrational
modes, there is a significant change of the electronic distribution and spin
populations around the Fermi level. A peak at 20 meV in the experimental data
falls into the pseudo-gap region of the calculation. This was also the case
reported in our recent work combined with an empirical parametric calculation
[Mittal et al., PRB 78 104514 (2008)]. The combined evidence for the coupling
of electronic and spin degrees of freedom with phonons is relevant to the
current interest in superconductivity in BaFe2As2 and related systems
Transition from Mott insulator to superconductor in GaNbSe and GaTaSe under high pressure
Electronic conduction in GaMSe (M=Nb;Ta) compounds with the fcc
GaMoS-type structure originates from hopping of localized unpaired
electrons (S=1/2) among widely separated tetrahedral M metal clusters. We
show that under pressure these systems transform from Mott insulators to a
metallic and superconducting state with T=2.9 and 5.8K at 13 and 11.5GPa
for GaNbSe and GaTaSe, respectively. The occurrence of
superconductivity is shown to be connected with a pressure-induced decrease of
the MSe octahedral distortion and simultaneous softening of the phonon
associated with MSe-bonds.Comment: 10 pages, 5 figure
Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSeS
We report muon spin rotation (SR) and magnetization measurements under
pressure on FeSeS with x
.Above GPa we find microscopic coexistence of
superconductivity with an extended dome of long range magnetic order that spans
a pressure range between previously reported separated magnetic phases. The
magnetism initially competes on an atomic scale with the coexisting
superconductivity leading to a local maximum and minimum of the superconducting
. The maximum of corresponds to the onset of
magnetism while the minimum coincides with the pressure of strongest
competition. A shift of the maximum of for a series of single
crystals with x up to 0.14 roughly extrapolates to a putative magnetic and
superconducting state at ambient pressure for x .Comment: 10 pages, 6 figures, including supplemental materia
- …