378 research outputs found
Disorder, Metal-Insulator crossover and Phase diagram in high-Tc cuprates
We have studied the influence of disorder induced by electron irradiation on
the normal state resistivities of optimally and underdoped YBa2CuOx
single crystals, using pulsed magnetic fields up to 60T to completely restore
the normal state. We evidence that point defect disorder induces low T upturns
of rho(T) which saturate in some cases at low T in large applied fields as
would be expected for a Kondo-like magnetic response. Moreover the magnitude of
the upturns is related to the residual resistivity, that is to the
concentration of defects and/or their nanoscale morphology. These upturns are
found quantitatively identical to those reported in lower Tc cuprates, which
establishes the importance of disorder in these supposedly pure compounds. We
therefore propose a realistic phase diagram of the cuprates, including
disorder, in which the superconducting state might reach the antiferromagnetic
phase in the clean limit.Comment: version 2 with minor change
Superconducting fluctuations and pseudogap 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 YBa2Cu3O(6+x). These experiments allow us to determine
the field H'c(T) and the temperature T'c above which the SCFs are fully
suppressed. A careful investigation near optimal doping shows that T'c is
higher than the pseudogap temperature T*, which is an unambiguous evidence that
the pseudogap cannot be assigned to preformed pairs. Accurate determinations of
the SCF contribution to the conductivity versus temperature and magnetic field
have been achieved. They can be accounted for by thermal fluctuations following
the Ginzburg-Landau scheme for nearly optimally doped samples. 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 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* allow us to propose a three dimensional phase
diagram including a disorder axis, which allows to explain most observations
done in other cuprate families.Comment: Paper presented at the "Eurasia-Pacific Summer School & Conference on
Correlated Electrons", Turunc-Marmaris, Turkey, July 4-14, 201
Respective influences of pair breaking and phase fluctuations in disordered high Tc superconductors
Electron irradiation has been used to introduce point defects in a controlled
way in the CuO2 planes of underdoped and optimally doped YBCO. This technique
allows us to perform very accurate measurements of Tc and of the residual
resistivity in a wide range of defect contents xd down to Tc=0. The Tc decrease
does not follow the variation expected from pair breaking theories. The
evolutions of Tc and of the transition width with xd emphasize the importance
of phase fluctuations, at least for the highly damaged regime. These results
open new questions about the evolution of the defect induced Tc depression over
the phase diagram of the cupratesComment: 5 pages, 4 figure
Reply to Comment on "High-field studies of superconducting fluctuations in high-Tc cuprates: Evidence for a small gap distinct from the large pseudogap" by M.V. Ramallo et al
The experimental investigations done in our paper Phys.Rev.B84,014522(2011)
allowed us to establish that the superconducting fluctuations (SCF) always die
out sharply with increasing T. But contrary to the claim done in the comment of
Ramallo et al., this sharp cutoff of SCF measured in YBa2Cu3O{6+x} depends on
hole doping and/or disorder. So our data cannot be used to claim for a
universality of the extended gaussian Ginzburg Landau theory proposed by the
authors of the comment. Furthermore, to explain quantitatively our data near
optimal doping using this model they need to consider that fluctuations in the
two CuO2 planes of a bilayer are totally decoupled, which is not physically
well justified. On the contrary a consistent interpretation of all our data
(paraconductivity, Nernst effect and magnetoresistance) has been done by
considering that the coupling between the two layers of the unit cell is
dominant at least up to 1.1Tc.Comment: Reply to the comment published in Phys. Rev. B 85,106501 (2012
Longitudinal magnetoresistance in Co-doped BaFe2As2 and LiFeAs single crystals: Interplay between spin fluctuations and charge transport in iron-pnictides
The longitudinal in-plane magnetoresistance (LMR) has been measured in
different Ba(Fe_(1-x)Co_x)2As2 single crystals and in LiFeAs. For all these
compounds, we find a negative LMR in the paramagnetic phase whose magnitude
increases as H^2. We show that this negative LMR can be readily explained in
terms of suppression of the spin fluctuations by the magnetic field. In the
Co-doped samples, the absolute value of the LMR coefficient is found to
decrease with doping content in the paramagnetic phase. The analysis of its T
dependence in an itinerant nearly antiferromagnetic Fermi liquid model
evidences that the LMR displays a qualitative change of T variation with
increasing Co content. The latter occurs at optimal doping for which the
antiferromagnetic ground state is suppressed. The same type of analysis for the
negative LMR measured in LiFeAs suggests that this compound is on the verge of
magnetism.Comment: 6 pages, 6 figure
Synthesis of sodium cobaltate NaCoO single crystals with controlled Na ordering
In this study, we synthesized single crystals of NaCoO with
using the optical floating zone technique. A thorough
electrochemical treatment of the samples permitted us to control the
de-intercalation of Na to obtain single crystal samples of stable Na ordered
phases with . Comparisons of the bulk magnetic properties with those
observed in the Na ordered powder samples confirmed the high quality of these
single crystal phases. The ab plane resistivity was measured for the Na ordered
samples and it was quite reproducible for different sample batches. The data
were analogous to those found in previous initial experimental studies on
single crystals, but the lower residual resistivity and sharper
anti-ferromagnetic transitions determined for our samples confirmed their
higher quality.Comment: 15 pages, 7 figure
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
Incommensurate spin density wave in Co-doped BaFe2As2
57Fe Mossbauer spectroscopy measurements are presented in the underdoped
Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~
2 and 12K respectively). The spectral shapes in the so-called spin-density wave
(SDW) phase are interpreted in terms of incommensurate modulation of the
magnetic structure, and allow the shape of the modulation to be determined. In
undoped BaFe2As2, the magnetic structure is commensurate, and we find that
incommensurability is present at the lowest doping level (x=0.014). As Co
doping increases, the low temperature modulation progressively loses its
"squaredness" and tends to a sine-wave. The same trend occurs for a given
doping level, as temperature increases. We find that a magnetic hyperfine
component persists far above the SDW transition, its intensity being
progressively tranferred to a paramagnetic component on heating.Comment: 7 pages, 8 figures, published in EP
Atomic coexistence of superconductivity and incommensurate magnetic order in the Ba(Fe1-xCox)2As2 pnictide
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal
for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow
to show that all Fe sites experience an incommensurate magnetic ordering below
T=31K. Comparison with undoped compound allows to estimate a typical moment of
0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of
incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of
0.04. Below TC=21.8K, a full volume superconductivity develops as shown by
susceptibility and relaxation rate, and magnetic order remains unaffected,
demonstrating coexistence of both states on each Fe site.Comment: 4 pages, 4 figure
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