398 research outputs found
Influence of carbon substitution on the heat transport in single crystalline MgB2
We report data on the thermal conductivity \kappa(T,H) in the basal plane of
hexagonal single-crystalline and superconducting Mg(B_{1-x}C_x)_2 (x= 0.03,
0.06) at temperatures between 0.5 and 50 K, and in external magnetic fields H
between 0 and 50 kOe. The substitution of carbon for boron leads to a
considerable reduction of the electronic heat transport, while the phonon
thermal conductivity seems to be much less sensitive to impurities. The
introduction of carbon enhances mostly the intraband scattering in the
\sigma-band. In contrast to the previously observed anomalous behavior of pure
MgB, the Wiedemann-Franz law is valid for Mg(B_0.94 C_0.06)_2 at low
temperatures.Comment: 4 pages, 4 figures. Final version to appear in Phys. Rev.
Disorder effects on the superconducting properties of BaFeCoAs single crystals
Single crystals of superconducting BaFeCoAs were exposed
to neutron irradiation in a fission reactor. The introduced defects decrease
the superconducting transition temperature (by about 0.3 K) and the upper
critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the
critical current densities by a factor of up to about 3. These changes are
discussed in the context of similar experiments on other superconducting
materials
Experimental confirmation of the low B isotope coefficient in MgB2
Recent investigations have shown that the first proposed explanations of the
disagreement between experimental and theoretical value of isotope coefficient
in MgB2 need to be reconsidered. Considering that in samples with residual
resistivity of few mu-Ohm cm critical temperature variations produced by
disorder effects can be comparable with variations due to the isotopic effect,
we adopt a procedure in evaluating the B isotope coefficient which take account
of these effects, obtaining a value which is in agreement with previous results
and then confirming that there is something still unclear in the physics of
MgB2.Comment: 8 pages, 3 figures Title has been changed A statement has been added
in page 7 of the pdf file "Finally we would..." Reference 21 has been added
Figure 1 anf Figure 2 have been change
Anisotropic critical currents in FeSe0.5Te0.5 films and the influence of neutron irradiation
We report on measurements of the superconducting properties of FeSe05Te05
thin films grown on lanthanum aluminate. The films have high transition
temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T.
The temperature dependence of the upper critical field and the irreversibility
lines are steep and anisotropic, as recently reported for single crystals. The
critical current densities, assessed by magnetization measurements in a vector
VSM, were found to be well above 10^9 Am-2 at low temperatures. In all samples,
the critical current as a function of field orientation has a maximum, when the
field is oriented parallel to the film surface. The maximum indicates the
presence of correlated pinning centers. A minimum occurs in three films, when
the field is applied perpendicular to the film plane. In the forth film,
instead, a local maximum caused by c-axis correlated pinning centers was found
at this orientation. The irradiation of two films with fast neutrons did not
change the properties drastically, where a maximum enhancement of the critical
current by a factor of two was found
Neutron Irradiation of Sm-1111
SmFeAsOF was irradiated in a fission reactor to a fast (E > 0.1
MeV) neutron fluence of 4x10^ m. The introduced defects increase
the normal state resistivity due to a reduction in the mean free path of the
charge carriers. This leads to an enhancement of the upper critical field at
low temperatures. The critical current density within the grains, Jc, increases
upon irradiation. The second maximum in the field dependence of Jc disappears
and the critical current density becomes a monotonically decreasing function of
the applied magnetic field
Effect of two bands on critical fields in MgB2 thin films with various resistivity values
Upper critical fields of four MgB2 thin films were measured up to 28 Tesla at
Grenoble High Magnetic Field Laboratory. The films were grown by Pulsed Laser
Deposition and showed critical temperatures ranging between 29.5 and 38.8 K and
resistivities at 40 K varying from 5 to 50 mWcm. The critical fields in the
perpendicular direction turned out to be in the 13-24 T range while they were
estimated to be in 42-57 T the range in ab-planes. In contrast to the
prediction of the BCS theory, we did not observe any saturation at low
temperatures: a linear temperature dependence is exhibited even at lowest
temperatures at which we made the measurements. Moreover, the critical field
values seemed not to depend on the normal state resistivity value. In this
paper, we analyze these data considering the multiband nature of
superconductivity in MgB2 We will show how the scattering mechanisms that
determine critical fields and resistivity can be different.Comment: 17 pages, 3 figure
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
Microstructural evolution throughout the structural transition in 1111 oxy-pnictides
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
Slow magnetic fluctuations and superconductivity in fluorine-doped NdFeAsO
Among the widely studied superconducting iron-pnictide compounds belonging to
the Ln1111 family (with Ln a lanthanide), a systematic investigation of the
crossover region between the superconducting and the antiferromagnetic phase
for the Ln = Nd case has been missing. We fill this gap by focusing on the
intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and
muon-spin spectroscopy measurements. The long-range order we detect at low
fluorine doping is replaced by short-range magnetic interactions at x = 0.08,
where also superconductivity appears. In this case, longitudinal-field
muon-spin spectroscopy experiments show clear evidence of slow magnetic
fluctuations that disappear at low temperatures. This fluctuating component is
ascribed to the glassy-like character of the magnetically ordered phase of
NdFeAsO at intermediate fluorine doping
Two-band effects in transport properties of MgB2
We present resistivity and thermal conductivity measurements on bulk samples,
prepared either by a standard method or by a one-step technique. The latter
samples, due to their high density and purity, show residual resistivity values
as low as 0.5 mW cm and thermal conductivity values as high as 215 W/mK, higher
than the single crystal ones. Thermal and electrical data of all the samples
are analysed in the framework of the Bloch-Gruneisen equation giving reliable
parameter values. In particular the temperature resitivity coefficient,
obtained both from resistivity and thermal conductivity, in the dirty sample
comes out ten time larger than in the clean ones. This result supports the
hypothesis of ref. [1] that p and s bands conduct in parallel, prevailing p
conduction in clean samples and s conduction in dirty samples .Comment: 8 pages, 5 figures, Presented at the BOROMAG workshop, June 17-19
2002, Genoa, Ital
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