885 research outputs found
High Field determination of superconducting fluctuations in high-Tc cuprates
Large pulsed magnetic fields up to 60 Tesla are used to suppress the
contribution of superconducting fluctuations (SCF) to the ab-plane conductivity
above Tc in a series of YBa2Cu3O6+x single crystals. The fluctuation
conductivity is found to vanish nearly exponentially with temperature, allowing
us to determine precisely the field H'c(T) and the temperature T'c above which
the SCFs are fully suppressed. T'c is always found much smaller than the
pseudogap temperature. A careful investigation near optimal doping shows that
T'c is higher than the pseudogap T*, which indicates that the pseudogap cannot
be assigned to preformed pairs. For nearly optimally doped samples, the
fluctuation conductivity can be accounted for by gaussian fluctuations
following the Ginzburg-Landau scheme. A phase fluctuation contribution might be
invoked for the most underdoped samples in a T range which increases when
controlled disorder is introduced by electron irradiation. Quantitative
analysis of the fluctuating magnetoconductance allows us to determine the
critical field Hc2(0) which is found to be quite similar to H'c(0) and to
increase with hole doping. Studies of the incidence of disorder on both T'c and
T* enable us to propose a three dimensional phase diagram including a disorder
axis, which allows to explain most observations done in other cuprate families.Comment: 10 pages, 10 figures, invited paper at the M2SHTSC Conference
Washington (2012
Superconducting Fluctuations, Pseudogap and Phase Diagram in Cuprates
We report transport measurements using pulsed magnetic fields to suppress the
superconducting fluctuations (SCF) conductivity in a series of YBa_2Cu_3O_(6+x)
samples. These experiments allow us altogether to measure the temperature T'c
at which SCF disappear, and the pseudogap temperature T*. While the latter are
consistent with previous determinations of T*, we find that T'c is slightly
larger than similar data taken by Nernst measurements. A careful investigation
near optimal doping shows that T* becomes smaller than T'c, which is an
unambiguous evidence that the pseudogap cannot be assigned to preformed pairs.
Studies of the incidence of disorder on both T'c and T* allow us to propose a
phase diagram including disorder which explains most observations done in other
cuprate families, and to discuss the available knowledge on the pseudogap line
in the phase diagram.Comment: New version with minor correction
Total suppression of superconductivity by high magnetic fields in YBa2 Cu3O6.6
We have studied in fields up to 60T the variation of the transverse
magnetoresistance (MR) of underdoped YBCO6.6 crystals either pure or with Tc
reduced down to 3.5K by electron irradiation. We evidence that the normal state
MR is restored above a threshold field H'c(T), which is found to vanish at
T'c>>Tc. In the pure YBCO6.6 sample a 50 Tesla field is already required to
completely suppress the superconducting fluctuations at Tc. While disorder does
not depress the pseudogap temperature, it reduces drastically the phase
coherence established at Tc and weakly H'c(0), T'c and the onset Tnu of the
Nernst signal which are more characteristic of the 2D local pairing.Comment: 4 pages, 4 figure
Nernst effect and disorder in the normal state of high-T_{c} cuprates
We have studied the influence of disorder induced by electron irradiation on
the Nernst effect in optimally and underdoped YBa2Cu3O(7-d) single crystals.
The fluctuation regime above T_{c} expands significantly with disorder,
indicating that the T_{c} decrease is partly due to the induced loss of phase
coherence. In pure crystals the temperature extension of the Nernst signal is
found to be narrow whatever the hole doping, contrary to data reported in the
low-T_{c} cuprates families. Our results show that the presence of "intrinsic"
disorder can explain the enhanced range of Nernst signal found in the pseudogap
phase of the latter compounds.Comment: revised version. to be published in Physical Review Letter
Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs
The resistivity, Hall effect and transverse magnetoresistance (MR) have been
measured in low residual resistivity single crystals of LiFeAs. A comparison
with angle resolved photoemission spectroscopy and quantum oscillation data
implies that four carrier bands unevenly contribute to the transport. However
the scattering rates of the carriers all display the T^2 behavior expected for
a Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2
variation permit us to extract the superconducting fluctuation contribution to
the conductivity. Though below Tc the anisotropy of superconductivity is rather
small, the superconducting fluctuations display a quasi ideal two-dimensional
behavior which persists up to 1.4 Tc. These results call for a refined
theoretical understanding of the multiband behavior of superconductivity in
this pnictide.Comment: 8pages with supplementary material, 6 figure
Photon emission by an ultra-relativistic particle channeling in a periodically bent crystal
This paper is devoted to a detailed analysis of the new type of the undulator
radiation generated by an ultra-relativistic charged particle channeling along
a crystal plane, which is periodically bent by a transverse acoustic wave, as
well as to the conditions limiting the observation of this phenomenon. This
mechanism makes feasible the generation of electromagnetic radiation, both
spontaneous and stimulated, emitted in a wide range of the photon energies,
from X- up to gamma-rays
Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal
for x=6% for various field H values and orientations. The sharpness of the
superconducting and magnetic transitions demonstrates a homogeneity of the Co
doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of
the NMR spectra is found very anisotropic and very narrow for H//ab which
allows to rule out the interpretation of Ref.[6] in terms of strong Co induced
electronic inhomogeneities. We propose that a distribution of hyperfine
couplings and chemical shifts due to the Co effect on its nearest As explains
the observed linewidths and relaxations. All these measurements show that Co
substitution induces a very homogeneous electronic doping in BaFe2As2, from
nano to micrometer lengthscales, on the contrary to the K doping.Comment: 6 pages, 4 figure
Hall effect and resistivity study of the magnetic transition, carrier content and Fermi liquid behavior in Ba(Fe(1-x) Cox)2As2
The negative Hall constant R_H measured all over the phase diagram of
Ba(Fe(1-x)Cox)2As2 allows us to show that electron carriers always dominate the
transport properties. The evolution of R_H with x at low doping (x<2%)
indicates that important band structure changes happen for x<2% prior to the
emergence of superconductivity. For higher x, an change of the electron
concentration with T is required to explain the low T variations of R_H, while
the electron scattering rate displays the T^2 law expected for a Fermi liquid.
The T=0 residual scattering is affected by Co disorder in the magnetic phase,
but is rather dominated by incipient disorder in the paramagnetic state.Comment: 5 pages, 4 figures, published versio
Strong pinning and vortex energy distributions in single-crystalline Ba(Fe1−xCox)2As2
International audienceThe interrelation between heterogeneity and flux pinning is studied in Ba(Fe1−xCox)2As2 single crystals with widely varying Co content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x=0.075 and x=0.1 reveals highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density yields a typical length scale of the relevant disorder of 40-60 nm
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