60 research outputs found
Comment on "Precision global measurements of London penetration depth in FeTe0.58Se0.42"
International audienceCho et al. [Phys. Rev. B 84, 174502 (2011)] have reported on the temperature dependence of the London penetration depth deduced from tunnel diode oscillator (TDO) measurements in optimally doped Fe(Se,Te) single crystals. According to their analysis, these measurements could suggest a nodeless two-gap pairing symmetry with strong pair-breaking effects. However, to reach this conclusion, the authors fit the temperature dependence of the superfluid density with a two band clean limit model, which is incompatible with the presence of strong pair-breaking effects, deduced from the T n temperature dependence of the London penetration depth below Tc/3. Moreover, they claim that their results are also ruling out the suggestion that surface conditions can significantly affect the TDO data, but this conclusion is based on one very specific damaging process and completely ignores the large dispersion in the previously published TDO data
Anharmonic suppression of Charge density wave in 2H-NbS
The temperature dependence of the phonon spectrum in the superconducting
transition metal dichalcogenide 2H-NbS is measured by diffuse and inelastic
x-ray scattering. A deep, wide and strongly temperature dependent softening, of
the two lowest energy longitudinal phonons bands, appears along the
symmetry line in reciprocal space. In sharp contrast to the
iso-electronic compounds 2H-NbSe, the soft phonons energies are finite,
even at very low temperature, and no charge density wave instability occurs, in
disagreement with harmonic ab-initio calculations. We show that 2H-NbS is
at the verge of the charge density wave transition and its occurrence is only
suppressed by the large anharmonic effects. Moreover, the anharmonicity and the
electron phonon coupling both show a strong in-plane anisotropy.Comment: 6 pages, 5 figures, accepted for publication in Physical Review
Thermodynamic phase diagram of Fe(Se0.5Te0.5) single crystals in fields up to 28 tesla
International audienceWe report on specific heat Cp , transport, Hall probe, and penetration depth measurements performed on Fe Se0.5Te0.5 single crystals Tc 14 K . The thermodynamic upper critical field Hc2 lines has been deduced from Cp measurements up to 28 T for both H c and H ab, and compared to the lines deduced from transport measurements up to 55 T in pulsed magnetic fields . We show that this thermodynamic Hc2 line presents a very strong downward curvature for T→Tc which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/Tc 0.99 for H ab which is the consequence of a very small value of the coherence length c 0 4 Å and ab 0 15 Å , confirming the strong renormalization of the effective mass as compared to DMFT calculations previously observed in ARPES measurements A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger, Phys. Rev. Lett. 104, 097002 2010 . Hc1 measurements lead to ab 0 =430 50 nm and c 0 =1600 200 nm and the corresponding anisotropy is approximatively temperature independent 4 , being close to the anisotropy of Hc2 for T→Tc. The temperature dependence of both T2 and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system
Charge density wave and superconductivity competition in LuIrSi : a proton irradiation study
Real-space modulated Charge Density Waves (CDW) are an ubiquituous feature in
many families of superconductors. In particular, how CDW relates to
superconductivity is an active and open question that has recently gathered
much interest since CDWs have been discovered in many cuprates superconductors.
Here we show that disorder induced by proton irradiation is a full-fledged
tuning parameter that can bring essential information to answer this question
as it affects CDW and superconductivity with different and unequivocal
mechanisms. Specifically, in the model CDW superconductor LuIrSi
that develops a 1D CDW below 77\,K and s-wave superconductivity below 4\,K, we
show that disorder enhances the superconducting critical temperature
and while it suppresses the CDW. Discussing how
disorder affects both superconductivity and the CDW, we make a compelling case
that superconductivity and CDW are competing for electronic density of states
at the Fermi level in LuIrSi, and we reconcile the results
obtained via the more common tuning parameters of pressure and doping. Owing to
its prototypical, 1D, Peierls type CDW and the s-wave, weak-coupling nature of
its superconductivity, this irradiation study of LuIrSi provides
the basis to understand and extend such studies to the more complex cases of
density waves and superconductivity coexistence in heavy fermions, Fe-based or
cuprates superconductors.Comment: 25 pages single column, 4 figures in main text + 3 figures in
appendi
Two-Gap Superconductivity in 2H-NbS2
International audienceWe performed specific heat measurements of the superconducting single crystal of 2H-NbS2 in the temperature range down to 0.6 K and magnetic fields up to 14 T. The temperature and magnetic field dependence of the electronic specific heat consistently indicate existence of two superconducting energy gaps in the system. The superconducting anisotropy depends on both temperature and magnetic field. Moreover, the angular dependence of the upper critical field deviates from the Ginzburg-Landau behavior and rather reminds that of MgB2. All these features point to a multigap superconductivity in 2H-NbS2. Our measurements are in a perfect agreement with the previous scanning tunneling spectroscopy of Guillamón et al
Lu5Ir4Si10 whiskers : morphologies, structure cristalline, mosaïcité, supraconductivité et onde de densité de charge
National audienceParmi la famille très riche de composés intermétalliques R5Ir4Si10 (R=terre-rare), la phase Lu5Ir4Si10 est supraconductrice en dessous de 4K et présente une transition liée à une onde de densité de charge à 80K [1]. Des cristaux en forme d'aiguilles ou whiskers ont été obtenus. Les conditions de synthèses sont présentées ainsi qu'une observation détaillée de leur taille et morphologie, obtenue au microscope électronique à balayage [2]. Des analyses par diffraction de rayons X ont confirmé que ces aiguilles cristallisaient sous la même structure quadratique P4/mbm que les cristaux massifs. Les paramètres de mailles a = 12.484(1) Å et c = 4.190(2) Å sont comparables à ceux de la littérature [3]. A l'aide d'un diffractomètre 4-cercles, nous avons trouvé que les aiguilles poussent selon l'axe c et que les faces latérales sont orientées selon la direction [110]. La mosaïcité de ces cristaux a été mesurée à l'ESRF, ligne ID27. Elle est inférieure à 0.15° le long de l'axe c. Enfin, des mesures de résistivité électriques sont détaillées. Elles confirment l'excellente qualité de ces cristaux
Disorder raises the critical temperature of a cuprate superconductor
With the discovery of charge density waves (CDW) in most members of the
cuprate high temperature superconductors, the interplay between
superconductivity and CDW has become a key point in the debate on the origin of
high temperature superconductivity. Some experiments in cuprates point toward a
CDW state competing with superconductivity, but others raise the possibility of
a CDW-superconductivity intertwined order, or more elusive pair-density wave
(PDW). Here we have used proton irradiation to induce disorder in crystals of
LaBaCuO and observed a striking 50% increase of
accompanied by a suppression of the CDW. This is in clear
contradiction with the behaviour expected of a d-wave superconductor for which
both magnetic and non-magnetic defects should suppress . Our
results thus make an unambiguous case for the strong detrimental effect of the
CDW on bulk superconductivity in LaBaCuO. Using tunnel
diode oscillator (TDO) measurements, we find evidence for dynamic layer
decoupling in PDW phase. Our results establish irradiation-induced disorder as
a particularly relevant tuning parameter for the many families of
superconductors with coexisting density waves, which we demonstrate on
superconductors such as the dichalcogenides and LuIrSi.Comment: 10 pages, 7 figure
De la libre circulation des travailleurs et de la libre prestation de services au cours d'une période transitoire d'adhésion
International audienceCJCE 27 mars 1990 Société Rush Portuguesa Lda et Office national d'immigration (aff. C 113/89
Du bon, du discutable et du mauvais, le dialogue franco-européen sur le droit social fondamental aux congés payés
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Linxin He, Droits sociaux fondamentaux et droit de l'Union européenne, thèse préparée sous la direction d'E. Pataut et soutenue à l'Université de Paris 1, le 18 septembre 2017
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