853 research outputs found
Short note on magnetic impurities in SmFeAsOF (x=0, 0.07) compounds revealed by zero-field As NMR
We have performed zero-field As nuclear magnetic resonance study of
SmFeAsOF (x=0, 0.07) polycrystals in a wide frequency range at
various temperatures. As resonance line was found at around 265 MHz
revealing the formation of the intermetallic FeAs clusters in the new layered
superconductors. We have also demonstrated that NMR is a sensitive tool for
probing the quality of these materials.Comment: Revised authorshi
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 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
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
Theoretical search for superconductivity in Sc3XB perovskites and weak ferromagnetism in Sc3X (X = Tl, In, Ga, Al)
A possibility for a new family of intermetallic perovskite superconductors
Sc3XB, with X = Tl, In, Ga and Al, is presented as a result of KKR electronic
structure and pseudopotential phonon calculations. The large values of computed
McMillan--Hopfield parameters on scandium suggest appearance of
superconductivity in Sc3XB compounds. On the other hand, the possibility of
weak itinerant ferromagnetic behavior of Sc3X systems is indicated by the small
magnetic moment on Sc atoms in two cases of X =~ l and In. Also the electronic
structure and resulting superconducting parameters for more realistic case of
boron--deficient systems Sc3XB_x are computed using KKR--CPA method, by
replacing boron atom with a vacancy. The comparison of the calculated
McMillan--Hopfield parameters of the Sc3XB series with corresponding values in
MgCNi3 and YRh3B superconductors is given, finding the favorable trends for
superconductivity.Comment: 13 pages, 13 figures. v3 - revise
Angular dependence of magnetoresistivity in c-oriented MgB2 thin film
The anisotropy of MgB2 is still under debate: its value, strongly dependent
on the sample and on the measuring method, ranges between 1.2 and 13. In this
work we present our results on a MgB2 c-oriented superconducting thin film. To
evaluate the anisotropy, we followed two different approaches. Firstly,
magnetoresistivity was measured as a function of temperature at selected
magnetic fields applied both parallel and perpendicular to the c-axis;
secondly, we measured magnetoresistivity at selected temperatures and magnetic
fields, varying the angle q between the magnetic field and the c-axis. The
anisotropy estimated from the ratio between the upper critical fields parallel
and perpendicular to the c-axis and the one obtained in the framework of the
scaling approach within the anisotropic Ginzburg-Landau theory are different
but show a similar trend in the temperature dependence. The obtained results
are compared and discussed in the light of the two-band nature of MgB2. A
comparison between critical fields in thin films and single crystal is also
performed.Comment: 13 pages, 4 figures, European Physical Journal B in pres
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
Critical Field of MGB2 : Crossover from Clean to Dirty Regimes
We have studied the upper critical field, Bc2, in poly-crystalline MgB2
samples in which disorder was varied in a controlled way to carry selectively p
and s bands from clean to dirty limit. We have found that the clean regime
survives when p bands are dirty and s bands are midway between clean and dirty.
In this framework we can explain the anomalous behaviour of Al doped samples,
in which Bc2 decreases as doping increases.Comment: 11 pages, 2 figure
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