112 research outputs found
Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain
High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were
grown by Pulsed Laser Ablation on different substrates. By varying the film
thickness, Tc up to 21K were observed, significantly larger than the bulk
value. Structural analyses indicated that the a axis changes significantly with
the film thickness and is linearly related to the Tc. The latter result
indicates the important role of the compressive strain in enhancing Tc. Tc is
also related to both the Fe-(Se,Te) bond length and angle, suggesting the
possibility of further enhancement
From antiferromagnetism to superconductivity in Fe 1+y(Te1-x,Sex) (0 < x < 0.20): a neutron powder diffraction analysis
The nuclear and magnetic structure of Fe1+y(Te1-x,Sex) (0 < x < 0.20)
compounds was analyzed between 2 K and 300 K by means of Rietveld refinement of
neutron powder diffraction data. Samples with x < 0.075 undergo a tetragonal to
monoclinic phase transition at low temperature, whose critical temperature
decreases with increasing Se content; this structural transition is strictly
coupled to a long range antiferromagnetic ordering at the Fe site. Both the
transition to a monoclinic phase and the long range antiferromagnetism are
suppressed for 0.10 < x < 0.20. The onset of the structural and of the magnetic
transition remains coincident with the increase of Se substitution. The low
temperature monoclinic crystal structure has been revised. Superconductivity
arises for x > 0.05, therefore a significant region where superconductivity and
long range antiferromagnetism coexist is present in the pseudo-binary FeTe -
FeSe phase diagram.Comment: 33 pages, 4 tables, 13 figure
Transport and superconducting properties of Fe-based superconductors: SmFeAs(O1-x Fx) versus Fe1+y (Te1-x, Sex)
We present transport and superconducting properties - namely resistivity,
magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper
critical field - of two different families of Fe-based superconductors, which
can be viewed in many respects as end members: SmFeAs(O1-xFx) with the largest
Tc and the largest anisotropy and Fe1+y(Te1-x,Sex), with the largest Hc2, the
lowest Tc and the lowest anisotropy. In the case of the SmFeAs(O1-xFx) series,
we find that a single band description allows to extract an approximated
estimation of band parameters such as carrier density and mobility from
experimental data, although the behaviour of Seebeck effect as a function of
doping demonstrates that a multiband description would be more appropriate. On
the contrary, experimental data of the Fe1+y(Te1-x,Sex) series exhibit a
strongly compensated behaviour, which can be described only within a multiband
model. In the Fe1+y(Te1-x,Sex) series, the role of the excess Fe, tuned by Se
stoichiometry, is found to be twofold: it dopes electrons in the system and it
introduces localized magnetic moments, responsible for Kondo like scattering
and likely pair-breaking of Cooper pairs. Hence, excess Fe plays a crucial role
also in determining superconducting properties such as the Tc and the upper
critical field Bc2. The huge Bc2 values of the Fe1+y(Te1-x,Sex) samples are
described by a dirty limit law, opposed to the clean limit behaviour of the
SmFeAs(O1-xFx) samples. Hence, magnetic scattering by excess Fe seems to drive
the system in the dirty regime, but its detrimental pairbreaking role seems not
to be as severe as predicted by theory. This issue has yet to be clarified,
addressing the more fundamental issue of the interplay between magnetism and
superconductivity
Evidence for polarons in iron pnictides of the Ln-1111 and AE-122 families
Examination of the electrical resistivities of iron pnictides shows that they
can be accounted by conduction by polarons. Their activation energies show a
linear behaviour with the critical temperatures of the spin density waves
(SDW), T*, as both vary with pressure. The slope matches the ratio SDW gap to
T*, while the intercept can be related to the transition temperature of the
lattice distortion, T0. An adapted Landau free energy predicts the observed
order of the transitions, according to which is higher, T* or T0. Simple
arguments favour combined Jahn-Teller antiferromagnetic bipolarons.Comment: 14 pages with 4 Figure
Magneto-transport and magnetic susceptibility of SmFeAsO1-xFx (x = 0.0 and 0.20)
Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting
SmFeAsO0.80F0.20 are explored through resistivity with temperature under
magnetic field {\rho}(T, H), AC and DC magnetization (M-T), and Specific heat
(Cp) measurements. The Resistivity measurement shows superconductivity for x =
0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is
estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of
superconducting transition in magneto transport is studied through thermally
activated flux flow in applied field up to 130 kOe. The flux flow activation
energy (U/kB) is estimated ~1215K for 1kOe field. Magnetic measurements
exhibited bulk superconductivity with lower critical field (Hc1) of ~1.2kOe at
2K. In normal state, the paramagnetic nature of compound confirms no trace of
magnetic impurity which orders ferromagnetically. AC susceptibility
measurements have been carried out for SmFeAsO0.80F0.20 sample at various
amplitude and frequencies of applied AC drive field. The inter-granular
critical current density (Jc) is estimated. Specific heat [Cp(T)] measurement
showed an anomaly at around 140K due to the SDW ordering of Fe, followed by
another peak at 5K corresponding to the antiferromagnetic (AFM) ordering of
Sm+3 ions in SmFeAsO compound. Interestingly the change in entropy (marked by
the Cp transition height) at 5K for Sm+3 AFM ordering is heavily reduced in
case of superconducting SmFeAsO0.80F0.20 sample.Comment: 18 pages text + Figs: comments/suggestions welcome
([email protected]
Effect of the chemical pressure on superconductivity and SDW in undoped and 15%F doped La1-yYyFeAsO compounds
We present a study concerning the partial substitution of yttrium at the
lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15
compounds. We prepared samples with a nominal yttrium content up to 70%
producing simultaneous shrinkage of both the a- and c-lattice parameters by
1.8% and 1.7%, respectively. The chemical pressure provided by the partial
substitution with this smaller ion size causes a lowering of the spin density
wave temperature in the undoped compounds, as well as an increase of the
superconducting transition temperatures in the doped ones. The 15%
fluorine-doped samples reach a maximum critical temperature of 40.2 K for the
50% yttrium substitution. Comparison with literature data indicates that
chemical pressure cannot be the only mechanism which tunes drastically both
TSDW and Tc in 1111 compounds. Our data suggest that structural disorder
induced by the partial substitution in the La site or by doping could play an
important role as well
New Fe-based superconductors: properties relevant for applications
Less than two years after the discovery of high temperature superconductivity
in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe
layers (1111, 122, 11, 111) are available. They share several characteristics
with cuprate superconductors that compromise easy applications, such as the
layered structure, the small coherence length, and unconventional pairing, On
the other hand the Fe-based superconductors have metallic parent compounds, and
their electronic anisotropy is generally smaller and does not strongly depend
on the level of doping, the supposed order parameter symmetry is s wave, thus
in principle not so detrimental to current transmission across grain
boundaries. From the application point of view, the main efforts are still
devoted to investigate the superconducting properties, to distinguish intrinsic
from extrinsic behaviours and to compare the different families in order to
identify which one is the fittest for the quest for better and more practical
superconductors. The 1111 family shows the highest Tc, huge but also the most
anisotropic upper critical field and in-field, fan-shaped resistive transitions
reminiscent of those of cuprates, while the 122 family is much less anisotropic
with sharper resistive transitions as in low temperature superconductors, but
with about half the Tc of the 1111 compounds. An overview of the main
superconducting properties relevant to applications will be presented. Upper
critical field, electronic anisotropy parameter, intragranular and
intergranular critical current density will be discussed and compared, where
possible, across the Fe-based superconductor families
Isoelectronic Ru substitution at Fe-site in Sm(Fe1-xRux)As(O0.85F0.15) compound and its effects on structural, superconducting and normal state properties
In this work we present a systematic experimental and theoretical study of
the structural, transport and superconducting properties of
Sm(Fe1-xRux)As(O0.85F0.15) polycrystalline samples as a function of Ru content
(x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe
site of F-doped compounds allows to better clarify the role of structural
disorder in modifying the normal and superconducting properties of these newly
discovered multiband superconductors. Two different regions are identified: the
Fe-rich phase (x<0.5) where superconducting and normal state properties are
strongly affected by disorder induced by Ru substitution; the Ru-rich phase
(x>0.5) where the system is metallic and strongly compensated and the presence
of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic
features and related spin fluctuations may be the cause for the suppression of
superconductivity.Comment: Accepted on Physical Review
Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films
Point-contact (PC) Andreev-reflection (AR) measurements of the
superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have
been carried out. The value of the gap is distributed in the range 2\Delta
\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV.
Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve
giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical
temperature from the BCS fit). At the same time, an expected second larger gap
feature was difficult to resolve distinctly on the AR spectra making
determination reliability of the second gap detection questionable. Possible
reasons for this and the origin of other features like clear-cut asymmetry in
the AR spectra and current regime in PCs are discussed.Comment: 6 two-column pages, 6 figs., 26 Refs., to be published in
Superconductor Science and Technolog
Specific-heat study of superconducting and normal states in FeSe1-xTex (0.6<=x<=1) single crystals: Strong-coupling superconductivity, strong electron-correlation, and inhomogeneity
The electronic specific heat of as-grown and annealed single-crystals of
FeSe1-xTex (0.6<=x<=1) has been investigated. It has been found that annealed
single-crystals with x=0.6-0.9 exhibit bulk superconductivity with a clear
specific-heat jump at the superconducting (SC) transition temperature, Tc. Both
2Delta_0/kBTc [Delta_0: the SC gap at 0 K estimated using the single-band BCS
s-wave model] and Delta C/(gamma_n-gamma_0)Tc [Delta C$: the specific-heat jump
at Tc, gamma_n: the electronic specific-heat coefficient in the normal state,
gamma_0: the residual electronic specific-heat coefficient at 0 K in the SC
state] are largest in the well-annealed single-crystal with x=0.7, i.e., 4.29
and 2.76, respectively, indicating that the superconductivity is of the strong
coupling. The thermodynamic critical field has also been estimated. gamma_n has
been found to be one order of magnitude larger than those estimated from the
band calculations and increases with increasing x at x=0.6-0.9, which is
surmised to be due to the increase in the electronic effective mass, namely,
the enhancement of the electron correlation. It has been found that there
remains a finite value of gamma_0 in the SC state even in the well-annealed
single-crystals with x=0.8-0.9, suggesting an inhomogeneous electronic state in
real space and/or momentum space.Comment: 22 pages, 1 table, 6 figures, Version 2 has been accepted for
publication in J. Phys. Soc. Jp
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