780 research outputs found
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
Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxy-pnictides
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.Comment: Approved for publication in Phys. Rev. Letter
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
Nanoscopic coexistence of magnetic and superconducting states within the FeAs layers of CeFeAsO1-xFx
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 com
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
Crossover between magnetism and superconductivity in low H-doped LaFeAsO
By a systematic study of the hydrogen-doped LaFeAsO system by means of dc
resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the
question of universality of the phase diagram of rare-earth-1111 pnictides. In
many respects, the behaviour of LaFeAsO_(1-x)H_(x) resembles that of its widely
studied F-doped counterpart, with H^- realizing a similar (or better)
electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range
SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K
and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk
superconductivity. Unlike the abrupt M-SC transition found in the parent
La-1111 compound, the presence a crossover region makes the H-doped system
qualitatively similar to other Sm-, Ce-, or Nd-1111 families.Comment: to appear in Journal of Physics: Condensed Matte
Analysis of charge transfer mechanism on (Ba1-xNdxCuO2+d)2/(CaCuO2)n superconducting superlattices by thermoelectric power measurements
We have investigated the charge transfer mechanism in artificial
superlattices by Seebeck effect measurements. Such a technique allows a precise
determination of the amount of charge transferred on each CuO2 plane. A
systematic characterization of thermoelectric power in (BaCuO2+d)2/(CaCuO2)n
and (Ba0.9Nd0.1CuO2+d)2/(CaCuO2)n superlattices demonstrates that electrical
charge distributes uniformly among the CuO2 planes in the Ca-block. The
differences observed in the Seebeck effect behavior between the Nd-doped and
undoped superlattices are ascribed to the different metallic character of the
Ba-block in the two cases. Finally, the special role of structural disorder in
superlattices with n=1 is pointed out by such analysis.Comment: subitted to PRB, 15 pages, 3 figure
Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex-situ tapes
Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders
showed a strong enhancement of the irreversibility field H*, the upper critical
field Hc2 and the critical current density Jc(H) together with the suppression
of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and
10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at
25 K, high values for not-doped MgB2. The enhanced Jc and H* values are
associated with significant grain refinement produced by milling of the MgB2
powder, which enhances grain boundary pinning, although at the same time also
reducing the connectivity from about 12% to 8%. Although enhanced pinning and
diminished connectivity are in opposition, the overall influence of ball
milling on Jc is positive because the increased density of grains with a size
comparable with the mean free path produces strong electron scattering that
substantially increases Hc2, especially Hc2 perpendicular to the Mg and B
planes.Comment: 26 pages, 9 figures, submitted to J. Appl. Phy
Magneto Seebeck effect in REFeAsO (RE=rare earth) compounds: probing the magnon drag scenario
We investigate Seebeck effect in REFeAsO (RE=rare earth)compounds as a
function of temperature and magnetic field up to 30T. The Seebeck curves are
characterized by a broad negative bump around 50K, which is sample dependent
and strongly enhanced by the application of a magnetic field. A model for the
temperature and field dependence of the magnon drag contribution to the Seebeck
effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts
for the magnitude and scaling properties of such bump feature in our
experimental data. This analysis allows to extract precious information on the
coupling between electrons and AFM spin fluctuations in these parent compound
systems, with implications on the pairing mechanism of the related
superconducting compounds
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