1,590 research outputs found
Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields
Electric polarization loops are measured at room temperature on highly pure
BiFeO3 single crystals synthesized by a flux growth method. Because the
crystals have a high electrical resistivity, the resulting low leakage currents
allow us to measure a large spontaneous polarization reaching 100
microC.cm^{-2}, a value never reported in the bulk. During electric cycling,
the slow degradation of the material leads to an evolution of the hysteresis
curves eventually preventing full saturation of the crystals.Comment: 8 pages, 3 figure
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
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
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
Evidence for two distinct energy scales in the Raman spectra of YBa2(CuNi)O6.95
We report low energy electronic Raman scattering from Ni-substituted
YBa2Cu3O6.95 single crystals with Tc ranging from 92.5 K to 78 K. The fully
symmetrical A1g channel and the B1g channel which is sensitive to the dx2-y2
gap maximum have been explored. The energy of the B1g pair-breaking peak
remains constant under Ni doping while the energy of the A1g peak scales with
Tc (EA1g/Tc=5). Our data show that the A1g peak tracks the magnetic resonance
peak observed in inelastic neutron scattering yielding a key explanation to the
long-standing problem of the origin the A1g peak.Comment: 10 pages, 4 figures and 1 tabl
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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