405 research outputs found
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
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
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
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
Different effects of Ni and Co substitution on the transport properties of BaFe2As2
We report resistivity and Hall effect results on Ba(Fe1-xNix)2As2 and compare
them with those in Ba(Fe1-xCox)2As2. The Hall number RH is negative for all x
values from 0.01 to 0.14, which indicates that electron carriers dominate the
transport both in the magnetic and paramagnetic regime. We analyse the data in
the framework of a two-band model. Without any assumption on the number of
carriers, we show that the electron resistivity can be estimated with good
accuracy in the low temperature paramagnetic range. Although the phase diagrams
of the two families are very similar with respect to the extra electrons added
in the system, we find that the transport properties differ in several aspects.
First, we evidence that the contribution of holes to the transport is more
important for Ni doping than for Co doping. Secondly, Ni behaves as a stronger
scatterer for the electrons, as the increase of the residual electron
resistivity rho/x is about four times larger for Ni than for Co in the most
doped samples.Comment: 8 pages, 8 figure
High Field Studies of Superconducting Fluctuations in High-T_c Cuprates: Evidence for a Small Gap distinct from the Large Pseudogap
We have used pulsed magnetic fields up to 60Tesla to suppress the
contribution of superconducting fluctuations(SCF)to the conductivity above Tc
in a series of YBa2Cu3O6+x from the deep pseudogapped state to slight
overdoping. Accurate determinations of the SCF conductivity versus temperature
and magnetic field have been achieved. Their joint quantitative analyses with
respect to Nernst data allow us to establish that thermal fluctuations
following the Ginzburg-Landau(GL) scheme are dominant for nearly optimally
doped samples. The deduced coherence length xi(T) is in perfect agreement with
a gaussian (Aslamazov-Larkin) contribution for 1.01Tc<T<1.2Tc. 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. For all dopings we evidence that the fluctuations are highly
damped when increasing T or H. The data permits us to define a field Hc^prime
and a temperature Tc^prime above which the SCF are fully suppressed. The
analysis of the fluctuation magnetoconductance in the GL approach allows us to
determine the critical field Hc2(0). The actual values of Hc^prime(0) and
Hc2(0) are found quite similar and both increase with hole doping. These
depairing fields, which are directly connected to the magnitude of the SC gap,
do therefore follow the Tc variation which is at odds with the sharp decrease
of the pseudogap T* with increasing hole doping. This is on line with our
previous evidence that T* is not the onset of pairing. We finally propose a
three dimensional phase diagram including a disorder axis, which allows to
explain most peculiar observations done so far on the diverse cuprate families.Comment: revised version, to be published in Physical Review B. Small
modifications have been done in paragraphs VI.A and VI
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
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