588 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
89Y NMR Probe of Zn Induced Local Magnetism in YBa2(Cu(1-y)Zn(y))3O(6+x)
We present detailed data and analysis of the effects of Zn substitution on
the planar Cu site in YBaCuO (YBCO) as evidenced from
our Y NMR measurements on oriented powders. For we find
additional NMR lines which are associated with the Zn substitution. From our
data on the intensities and temperature dependence of the shift, width, and
spin-lattice relaxation rate of these resonances, we conclude that the spinless
Zn 3 state induces local moments on the near-neighbour () Cu
atoms. Additionally, we conjecture that the local moments actually extend to
the farther Cu atoms with the magnetization alternating in sign at subsequent
sites. We show that this analysis is compatible with ESR data taken on
dilute Gd doped (on the Y site) and on neutron scattering data reported
recently on Zn substituted YBCO. For optimally doped compounds
Y resonances are not detected, but a large % -dependent
contribution to the Y NMR linewidth is evidenced and is also attributed
to the occurence of a weak induced local moment near the Zn. These results are
compatible with macroscopic magnetic measurements performed on YBCO
samples prepared specifically in order to minimize the content of impurity
phases. We find significant differences between the present results on the
underdoped YBCO samples and Al NMR data taken on Al
substituted on the Cu site in optimally doped LaCuO. Further
experimental work is needed to clarify the detailed evolution of the impurity
induced magnetism with hole content in the cuprates.Comment: To be published in EPJB 15 pages of text and figures in eps forma
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
Complex magnetic differentiation of cobalts in NaCoO with 22K N\'eel temperature
Single crystals of sodium cobaltates NaCoO with
were grown by the floating zone technique. Using electrochemical Na
de-intercalation method we reduced the sodium content in the as-grown crystals
down to pure phase with 22 K N\'eel temperature and . The
Co NMR study in the paramagnetic state of the K phase
permitted us to evidence that at least 6 Co sites are differentiated. They
could be separated by their magnetic behaviour into three types: a single site
with cobalt close to non-magnetic Co, two sites with the most magnetic
cobalts in the system, and the remaining three sites displaying an intermediate
behaviour. This unusual magnetic differentiation calls for more detailed NMR
experiments on our well characterized samples.Comment: 5 pages, 4 figures, submitted to JETP Let
59Co NMR study of the Co states in superconducting and anhydrous cobaltates
Co NMR spectra in oriented powders of NaCoO and in its
hydrated superconducting phase (HSC) NaCoO,1.3HO reveal a
single electronic Co state with identical independent NMR shift tensor.
These phases differ markedly from NaCoO, in which we resolve 3
types of Co sites. The large T variation of their spin susceptibilities and the anisotropy of the orbital susceptibility allow us
to conclude that charge disproportionation occurs, in a non magnetic Co
and two magnetic sites with about 0.3 and 0.7 holes in the multiplet.
The data are consistent with those for the single Co site in the anhydrous and
HSC phase assuming the expected Co charge.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
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
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