351 research outputs found
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
High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc
The advent of high transverse-field muon spin rotation (TF-muSR) has led to
recent muSR investigations of the magnetic-field response of cuprates above the
superconducting transition temperature T_c. Here the results of such
experiments on hole-doped cuprates are reviewed. Although these investigations
are currently ongoing, it is clear that the effects of high field on the
internal magnetic field distribution of these materials is dependent upon a
competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the
response to the external field above Tc is dominated by heterogeneous spin
magnetism. However, the magnetism that dominates the observed inhomogeneous
line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely
different kind of magnetism in the heavily overdoped regime. The origin of the
magnetism above x ~ 0.19 is currently unknown, but its presence hints at a
competition between superconductivity and magnetism that is reminiscent of the
underdoped regime. In contrast, the width of the internal field distribution of
underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of
superconducting carriers. This observation suggests that the magnetic response
above Tc is not dominated by electronic moments, but rather inhomogeneous
fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference
Irradiation-induced confinement in a quasi-one-dimensional metal
The anisotropic resistivity of PrBaCuO has been measured as a
function of electron irradiation fluence. Localization effects are observed for
extremely small amounts of disorder corresponding to electron mean-free-paths
of order 100 unit cells. Estimates of the localization corrections suggest that
this anomalous localization threshold heralds a crossover to a ground state
with pronounced one-dimensional character in which conduction electrons become
confined to a small cluster of chains.Comment: 4 pages, 4 figure
Promjene spektralnih svojstava kuprata visokog Tc izazvane defektima
Superconductivity in high-Tc cuprates is particularly sensitive to disorder due to the unconventional d-wave pairing symmetry. We investigated effects of disorder on the spectral properties of Bi2Sr2CaCu2O8+x high-Tc superconductor. We found that already small defect densities suppress the characteristic spectral signature of the superconducting state. The spectral line shape clearly reflects new excitations within the gap, as expected for defect-induced pair breaking. At the lowest defect concentrations the normal state remains unaffected, while increased disorder leads to suppression of the normal quasiparticle peaks.Zbog nekonvencionalne d-valne simetrije, supravodljivost u visokotemperaturnim kupratima je posebno osjetljiva na neuređenost. Ispitivali smo utjecaj neuređenosti na spektralna svojstva Bi2Sr2CaCu2O8+x supravodiča primjenom ARPES metode. Već mala gustoća defekata smanjuje karakteristični spektralni odziv supravodljivog stanja. Oblik spektralne linije očito reflektira nova pobuđenja unutar zabranjene vrpce, kako se i očekuje u slučaju razbijanja Cooperovih parova izazvanog defektima. Za male koncentracije defekata, normalno stanje ostaje nepromijenjeno, dok viši stupanj neuređenosti smanjuje intenzitet spektralnih linija kvazičestica
Promjene spektralnih svojstava kuprata visokog Tc izazvane defektima
Superconductivity in high-Tc cuprates is particularly sensitive to disorder due to the unconventional d-wave pairing symmetry. We investigated effects of disorder on the spectral properties of Bi2Sr2CaCu2O8+x high-Tc superconductor. We found that already small defect densities suppress the characteristic spectral signature of the superconducting state. The spectral line shape clearly reflects new excitations within the gap, as expected for defect-induced pair breaking. At the lowest defect concentrations the normal state remains unaffected, while increased disorder leads to suppression of the normal quasiparticle peaks.Zbog nekonvencionalne d-valne simetrije, supravodljivost u visokotemperaturnim kupratima je posebno osjetljiva na neuređenost. Ispitivali smo utjecaj neuređenosti na spektralna svojstva Bi2Sr2CaCu2O8+x supravodiča primjenom ARPES metode. Već mala gustoća defekata smanjuje karakteristični spektralni odziv supravodljivog stanja. Oblik spektralne linije očito reflektira nova pobuđenja unutar zabranjene vrpce, kako se i očekuje u slučaju razbijanja Cooperovih parova izazvanog defektima. Za male koncentracije defekata, normalno stanje ostaje nepromijenjeno, dok viši stupanj neuređenosti smanjuje intenzitet spektralnih linija kvazičestica
Is there still a need for prophylactic intra-abdominal drainage in elective major gastro-intestinal surgery?
SummaryProphylactic drainage of the abdominal cavity after gastro-intestinal surgery is widely used. The rationale is that intra-abdominal drainage enhances early detection of complications (gastro-intestinal leakage, hemorrhage, bile leak), prevents collection of fluid or pus, reduces morbidity and mortality, and decreases the duration of hospital stay. However, dogmatic attitudes favoring systematic drain placement should be questioned. The aim of this review was to evaluate the evidence supporting systematic use of prophylactic abdominal drainage following gastrectomy, pancreatectomy, liver resection, and rectal resection. Based on this review of the literature: (i) there was no evidence in favor of intra-peritoneal drainage following total or sub-total gastrectomy with respect to morbidity-mortality, nor was it helpful in the diagnosis or management of leakage, however the level of evidence is low, (ii) following pancreatic resection, data are conflicting but, overall, suggest that the absence of drainage is prejudicial, and support the notion that short-term drainage is better than long-term drainage, (iii) after liver resection without hepatico-intestinal anastomosis, high level evidence supports that there is no need for abdominal drainage, and (iv) following rectal resection, data are insufficient to establish recommendations. However, results from the French multicenter randomized controlled trial GRECCAR5 (NCT01269567) should provide new evidence this coming year. Accumulating data support that systematic drainage of the abdominal cavity in digestive surgery is a non-beneficial and obsolete practice, except following pancreatectomy where the consensus appears to indicate the usefulness of short-term drainage. While the level of evidence is high for liver resections, new randomized controlled trials are awaited regarding gastric, pancreatic and rectal surgery
Reduction of Tc due to Impurities in Cuprate Superconductors
In order to explain how impurities affect the unconventional
superconductivity, we study non-magnetic impurity effect on the transition
temperature using on-site U Hubbard model within a fluctuation exchange (FLEX)
approximation. We find that in appearance, the reduction of Tc roughly
coincides with the well-known Abrikosov-Gor'kov formula. This coincidence
results from the cancellation between two effects; one is the reduction of
attractive force due to randomness, and another is the reduction of the damping
rate of quasi-particle arising from electron interaction. As another problem,
we also study impurity effect on underdoped cuprate as the system showing
pseudogap phenomena. To the aim, we adopt the pairing scenario for the
pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by
impurities. We find that 'pseudogap breaking' by impurities plays the essential
role in underdoped cuprate and suppresses the Tc reduction due to the
superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS
Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES
Despite many ARPES investigations of iron pnictides, the structure of the
electron pockets is still poorly understood. By combining ARPES measurements in
different experimental configurations, we clearly resolve their elliptic shape.
Comparison with band calculation identify a deep electron band with the dxy
orbital and a shallow electron band along the perpendicular ellipse axis with
the dxz/dyz orbitals. We find that, for both electron and hole bands, the
lifetimes associated with dxy are longer than for dxz/dyz. This suggests that
the two types of orbitals play different roles in the electronic properties and
that their relative weight is a key parameter to determine the ground state
Experimental study of a 28 GHz high-power long-pulse cyclotron autoresonance maser (CARM) oscillator
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