1,014 research outputs found
Thermal properties of MgB2: the effect of disorder on gap amplitudes and relaxation times of p and s bands
We present thermal conductivity and specific heat measurements on MgB2 and
Mg-AlB2 samples. Thermal properties have been analysed by using a two-gap model
in order to estimate the gap amplitudes, D(0)p and D(0)s and the intra-band
scattering rates, Gss and Gpp. As a function of Al doping and disorder D(0)s
rapidly decreases, while D(0)p is rather constant. Gss and Gpp are increased by
the disorder, being Gpp more affected than Gss.Comment: 2 pages, 3 figures, presented to the conference M2S-HTSC, 25-30 May
2003, Rio de Janeir
Tetragonal to orthorhombic phase transition in SmFeAsO: a synchrotron powder diffraction investigation
The crystal structure of SmFeAsO has been investigated by means of Rietveld
refinement of high resolution synchrotron powder diffraction data collected at
300 K and 100 K. The compound crystallizes in the tetragonal P4/nmm space group
at 300 K and in the orthorhombic Cmma space group at 100 K; attempts to refine
the low temperature data in the monoclinic P112/n space group diverged. On the
basis of both resistive and magnetic analyses the tetragonal to orthorhombic
phase transition can be located at T about 140 K.Comment: Submitted to: Superconductor Science and Technology PACS: 61.05.cp,
61.66.Fn, 74.10.+v, 74.62.Dh, 74.70.D
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
Coupling between 4f and itinerant electrons in SmFeAsO1-xFx (0.15 < x < 0.2) superconductors: an NMR study
F NMR measurements in SmFeAsOF, for ,
are presented. The nuclear spin-lattice relaxation rate increases upon
cooling with a trend analogous to the one already observed in
CeCuAu, a quasi two-dimensional heavy-fermion intermetallic
compound with an antiferromagnetic ground-state. In particular, the behaviour
of the relaxation rate either in SmFeAsOF or in
CeCuAu can be described in the framework of the self-consistent
renormalization theory for weakly itinerant electron systems. Remarkably, no
effect of the superconducting transition on F is detected, a
phenomenon which can hardly be explained within a single band model.Comment: 4 figure
Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides
In this work we study the effect of the rare earth element in iron
oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out
Density Functional Theory calculations of the band structure, which evidence
the multiband character of these compounds and the presence of Dirac cones
along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other
hand, we explore transport behavior by means of resistivity, Hall resistance
and magnetoresistance measurements, which confirm the dominant role of Dirac
cones. By combining our theoretical and experimental approaches, we extract
information on effective masses, scattering rates and Fermi velocities for
different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of
Physics
Superconducting phase fluctuations in SmFeAsOF from diamagnetism at low magnetic field above
Superconducting fluctuations (SF) in SmFeAsOF (characterized
by superconducting transition temperature K) are
investigated by means of isothermal high-resolution dc magnetization
measurements. The diamagnetic response to magnetic fields up to 1 T above
is similar to what previously reported for underdoped cuprate
superconductors and it can be justified in terms of metastable superconducting
islands at non-zero order parameter lacking of long-range coherence because of
strong phase fluctuations. In the high-field regime ( T) scaling
arguments predicted on the basis of the Ginzburg-Landau theory of conventional
SF are found to be applicable, at variance with what observed in the low-field
regime. This fact enlightens that two different phenomena are simultaneously
present in the fluctuating diamagnetism, namely the phase SF of novel character
and the conventional SF. High magnetic fields (1.5 T )
are found to suppress the former while leaving unaltered the latter one.Comment: 7 pages, 5 figure
Effect of Ru susbstitution on atomic displacements in the layered SmFe_{1-x}Ru_xAsO_{0.85}F_{0.15} superconductor
The effect of Ru substitution on the local structure of layered
SmFeRuAsOF superconductor has been studied by As
- and Sm - 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 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
Tuning topological disorder in MgB
We carried out Raman measurements on neutron-irradiated and Al-doped MgB
samples. The irradiation-induced topological disorder causes an unexpected
appearance of high frequency spectral structures, similar to those observed in
lightly Al-doped samples. Our results show that disorder-induced violations of
the selection rules are responsible for the modification of the Raman spectrum
in both irradiated and Al-doped samples. Theoretical calculations of the phonon
density of states support this hypothesis, and demonstrate that the high
frequency structures arise mostly from contributions at of the
E phonon mode.Comment: 4 pages, 4 figure
- …