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M. Tropeano,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura, A. Martinelli,
A. Palenzona,
I. Pallecchi,
A. Sala,
I. Sheikin,
F. Bernardini,
M. Monni,
S. Massidda,
M. Putti
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ABSTRACT: 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 recently 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.
Physical Review B 04/2013; 81(18):184504. · 3.69 Impact Factor
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ABSTRACT: The crystal and magnetic structures of 10 and 20 nm sized (La1-xCax)MnO3 (x = 0.37, 0.50, 0.75) have been investigated between 5 and 300 K by means of Rietveld refinement of neutron powder diffraction data, coupled with transmission electron microscope observation and magnetization measurements. TEM observation reveals that nanoparticles are strongly affected by strain fields, probably originating from surface pressure. Irrespective of the composition, charge and orbital orderings are suppressed and Fz and Cy spin orderings coexist at low temperature; Cy and Fz orderings likely occur within the strained regions of the nanoparticles and in the matrix respectively. Moreover Gz and Az orderings are sometimes observed, and are likely to be taking place at the border of the strained regions.
Journal of Physics Condensed Matter 04/2013; 25(17):176003. · 2.55 Impact Factor
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ABSTRACT: The effect of Ru substitution on the local structure of layered
SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ superconductor has been studied by As
$K$- and Sm $L_3$ - edges x-ray-absorption spectroscopy. The extended
x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As
bondlengths in the Ru substituted samples with the latter being $\sim$0.03 \AA\
longer. Local disorder induced by the Ru substitution is mainly confined to the
FeAs layer while the SmO spacer layer sustains a relative order, consistent
with the x-ray-absorption near-edge structure spectra. The results suggest
that, in addition to the order/disorder in the active active iron-arsenide
layer, its coupling to the rare-earth\textminus oxygen spacer layer needs to be
considered for describing the electronic properties of these layered
superconductors.
06/2012;
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ABSTRACT: The effect of Ru substitution on the local structure of layered SmFe1−xRuxAsO0.85F0.15 superconductor has been studied by As K- and Sm L3-edge x-ray-absorption spectroscopy. The extended x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As bond lengths in the Ru substituted samples with the latter being ∼0.03 Å longer. Local disorder induced by the Ru substitution is mainly confined to the FeAs layer while the SmO spacer layer sustains a relative order, consistent with the x-ray-absorption near-edge structure spectra. The results suggest that, in addition to the order/disorder in the active FeAs layer, its coupling to the rare-earth–oxygen spacer layer needs to be considered for describing the electronic properties of these layered superconductors.
Phys. Rev. B. 06/2012; 85(21).
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A. Palenzona,
A. Sala,
C. Bernini,
V. Braccini,
M. R. Cimberle,
C. Ferdeghini,
G. Lamura, A. Martinelli,
I. Pallecchi,
G. Romano,
M. Tropeano,
R. Fittipaldi,
A. Vecchione,
A. Polyanskii,
F. Kametani,
M. Putti
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ABSTRACT: A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on
a melting process and a subsequent annealing treatment. With respect to the
standard sintering technique, it produces much more homogeneous and denser
samples, characterized by large and well interconnected grains. The resulting
samples exhibit optimal critical temperature values, sharp resistive and
magnetic transitions, large magnetic hysteresis loops and high upper critical
fields are observed. Interestingly, the global critical current density is much
enhanced as compared to the values reported in literature for bulk samples of
the same 11 family, reaching about 103 A/cm2 at zero field at 4.2 K as assessed
by magnetic, transport and magneto-optical techniques. Even more importantly,
its field dependence turns out to be very weak, such that at \mu_{0}H = 7 T it
is suppressed only by a factor \sim2.
05/2012;
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ABSTRACT: We report extended X-ray absorption fine structure measurements at the Mn K edge on La0.25Ca0.75Mn0.92Cr0.08O3 samples. Macroscopically, the Mn substitution induces a short-range charge-ordered state in spite of the long-range one present
in the parent manganite La0.25Ca0.75MnO3. We show how on a local scale the structural EXAFS parameters still confirm the presence, with only a slight reduction, of
the features characteristic of the charge ordered un-substituted compounds.
KEY WORDS:manganites–EXAFS–phase transitions.
Journal of Superconductivity 04/2012; 18(5):643-647.
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ABSTRACT: In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The
Fe(Te, Se) samples exhibit s-shaped resistivity ρ(T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic
temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior
of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density
of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures
suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity
curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature
much more strongly than the superconducting T
c
. Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high-T
c
cuprates.
KeywordsPseudogap–Pnictides and chalcogenides–Transport properties
Journal of Superconductivity and Novel Magnetism 04/2012; 24(5):1751-1760. · 0.65 Impact Factor
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ABSTRACT: We report μSR measurements on SmFeAsO1−x
F
x
which helped us to identify the signature of diluted ferromagnetic inclusions, ubiquitous in the iron pnictides. These impurities
are characterized by a Curie temperature close to room temperature and they seem responsible for a non negligible magnetic
relaxation of the implanted muons, which should not be confused with intrinsic pnictide properties.
Journal of Superconductivity and Novel Magnetism 04/2012; 22(6):585-588. · 0.65 Impact Factor
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ABSTRACT: The microstructural evolution throughout the first order tetragonal to
orthorhombic structural transition is analyzed by powder diffraction analysis
for two different systems belonging to the class of compounds referred to as
1111 oxy-pnictides: (La1-yYy)FeAsO and SmFeAs(O1-xFx). Both systems are
characterized by a similar behaviour: on cooling microstrain along the
tetragonal hh0 direction takes place and increases as the temperature is
decreased. Just above the structural transition microstrain reaches its maximum
value and then is abruptly suppressed by symmetry breaking. No volume
discontinuity throughout the first order transition is observed and a
groupsubgroup relationship holds between the tetragonal and the orthorhombic
structures, thus suggesting that orbital ordering drives symmetry breaking.
Microstrain reflects a distribution of lattice parameters in the tetragonal
phase and explains the occurrence of anisotropic properties commonly attributed
to nematic correlations; in this scenario the nematic behaviour is induced by
the tendency towards ordering of Fe orbitals.
04/2012;
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Physical Review B 01/2012; 85:019902-1. · 3.69 Impact Factor
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ABSTRACT: We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold x(c).
Physical Review Letters 11/2011; 107(22):227003. · 7.37 Impact Factor
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T Shiroka,
G Lamura,
S Sanna,
G Prando,
R De Renzi,
M Tropeano,
M R Cimberle, A Martinelli,
C Bernini,
A Palenzona,
R Fittipaldi,
A Vecchione,
P Carretta,
A S Siri,
C Ferdeghini,
M Putti
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ABSTRACT: The evolution of the antiferromagnetic order parameter in CeFeAsO(1-x)F(x) as a function of the fluorine content x was investigated primarily via zero-field muon-spin spectroscopy. The long-range magnetic order observed in the undoped compound gradually turns into a short-range order at x = 0.04, seemingly accompanied or induced by a drastic reduction of the magnetic moment of the iron ions. Superconductivity appears upon a further increase in doping (x > 0.04) when, unlike in the cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidences the presence of a crossover region, where the superconducting and the magnetic order parameters coexist on a nanoscopic range.
Physical Review B 11/2011; 84:195123. · 3.69 Impact Factor
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ABSTRACT: We report on the recovery of the short-range static magnetic order and on the
concomitant degradation of the superconducting state in optimally F-doped
SmFe_(1-x)Ru_(x)AsO_0.85F_0.15 for 0.1< x<0.6. The two reduced order parameters
coexist within nanometer-size domains in the FeAs layers and finally disappear
around a common critical threshold x_c=0.6. Superconductivity and magnetism are
shown to be closely related to two distinct well-defined local electronic
environments of the FeAs layers. The two transition temperatures, controlled by
the isoelectronic and diamagnetic Ru substitution, scale with the volume
fraction of the corresponding environments. This fact indicates that
superconductivity is assisted by magnetic fluctuations, which are frozen
whenever a short-range static order appears, and totally vanish above the
magnetic dilution threshold x_c.
Physical Review Letters 11/2011; 107:227003. · 7.37 Impact Factor
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ABSTRACT: In this work we investigate the Ru substituted LaFeAsO compound, by studying
the magnetotransport behaviour and its relationship with the band structure, in
different regimes of temperature, magnetic field and Ru content. In particular
we analyse the magnetoresistance of LaFe1-xRuxAsO (0 <= x <= 0.6) samples with
the support of ab initio calculations and we find out that in the whole series:
(i) the transport is dominated by electron bands only; (ii) the
magnetoresistance exhibits distinctive features related to the presence of
Dirac cones; indeed, ab initio calculations confirm the presence of anisotropic
Dirac cones in the band structure; (iii) the low temperature mobility is
exceptionally high and reaches 18.6 m2/(Vs) in the Ru-free sample at T=2K, in
the extreme limit of a single Landau level occupied in the Dirac cones; (iv)
the mobility drops abruptly above 10K-15K; (v) the disorder has a very weak
effect on the band mobilities and on the transport properties; (vi) there
exists a correlation between the temperature ranges of Dirac cones and SDW
carrier condensation. These findings may be of crucial importance in the
investigation of the pairing mechanism in the F-doped superconducting
La(Fe,Ru)As(O,F) compounds related to this series of parent compounds.
Physical Review B 10/2011; 84(13):134524. · 3.69 Impact Factor
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ABSTRACT: The crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O(3) (x = 0.00, 0.01, 0.02, 0.03) has been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling an orthorhombic to monoclinic phase transition occurs on account of the charge and orbital ordering taking place in the Mn sub-lattice; at low temperature phase separation takes place and the main monoclinic phase coexists with a secondary orthorhombic phase, whose amount slightly increases with the increase of Cr content. Cr(3+) is not involved in orbital ordering or superexchange interactions. The charge and magnetic ordering are decoupled: the Mn moments order according to a CE-type structure in all samples.
Journal of Physics Condensed Matter 09/2011; 23(41):416005. · 2.55 Impact Factor
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ABSTRACT: In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.
Physical Review Letters 06/2011; 106(22):227001. · 7.37 Impact Factor
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ABSTRACT: In the present paper we report an in situ high-energy x-ray diffraction analysis of MgB2 tapes
during the preparation process. The experiment was performed in a specifically designed
furnace working in a reducing atmosphere, compatible with the Laue diffraction condition.
The MgB2 synthesis was realized starting from MgH2 and amorphous B in powder form as
precursors, varying reaction temperature and testing different cooling processes. We analysed
both the MgB2 synthesis and the sintering process of tapes prepared with these powders. Phase
evolution, micro- and crystallographic structure were monitored during the different thermal
treatments. Among the main results we observed the formation of MgB2 at an extraordinary
low temperature (300 ◦C), probably as a result of a solid-state reaction between MgH2 and B.
Furthermore, we studied the dependence of the microstructure upon the thermal treatment and
its effect on the critical current performance of the superconducting tapes.
Superconductor Science and Technology 01/2011; 24:065014 (7pp). · 2.66 Impact Factor
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ABSTRACT: We prepared polycrystalline SmFeAsO1-xFx (Sm1111) bulk samples by sintering
and hot isostatic pressing (HIP) in order to study the effects of phase purity
and relative density on the intergranular current density. Sintered and HIPped
Sm1111 samples are denser with fewer impurity phases, such as SmOF and the
grain boundary wetting phase, FeAs. We found quite complex magnetization
behavior due to variations of both the inter and intragranular current
densities. Removing porosity and reducing second phase content enhanced the
intergranular current density, but HIPping reduced Tc and the intragranular
current density, due to loss of fluorine and reduction of Tc. We believe that
the HIPped samples are amongst the purest polycrystalline 1111 samples yet
made. However, their intergranular current densities are still small, providing
further evidence that polycrystalline pnictides, like polycrystalline cuprates,
are intrinsically granular.
11/2010;
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S. Sanna,
R De Renzi,
T Shiroka,
G. Lamura,
G. Prando,
P. Carretta,
M. Putti, A. Martinelli,
M. R. Cimberle,
M. Tropeano,
A. Palenzona
[show abstract]
[hide abstract]
ABSTRACT: We report on the coexistence of magnetic and superconducting states in CeFeAsO1-xFx for x=0.06(2), characterized by transition temperatures T_m=30 K and T_c=18 K, respectively. Zero and transverse field muon-spin relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the CeFeAsO1-xFx phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies. Comment: 4 pages, 3 figs, accepted for publication as PRB Rapid comm
Physical Review B 08/2010; 82:060508. · 3.69 Impact Factor
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ABSTRACT: In this paper we carry out a direct comparison between 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: SmFeAsO1 − xFx with the largest Tc and the largest anisotropy and Fe1 + yTe1 − xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1 − xFx series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for 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: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1 + yTe1 − xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1 − 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.
Superconductor Science and Technology 04/2010; 23(5):054001. · 2.66 Impact Factor
