Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals

International audienceWe report on the Nernst effect, specific heat and transportmeasurements performed in high quality (K,Ba)BiO3 single crystals close to optimal doping (Tc ∼ 31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than kBT . Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates

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

Lower Critical Field Hc1(T) and Pairing Symmetry Based on Eilenberger Theory

We quantitatively estimate different T-dependences of Hc1 between s wave and d wave pairings by Eilenberger theory. The T-dependences of Hc1(T) show quantitative deviation from those in London theory. We also study differences of Hc1(T) between p+ and p- wave pairing in chiral p wave superconductors. There, Hc1(T) is lower in p- wave pairing, and shows the same T-dependence as in s wave pairing.Comment: 2 pages, 1 figur

Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements

Specific heat has been measured down to 600 mK and up to 8 Tesla by the highly sensitive AC microcalorimetry on the MgCNi3 single crystals with Tc ~ 7 K. Exponential decay of the electronic specific heat at low temperatures proved that a superconducting energy gap is fully open on the whole Fermi surface, in agreement with our previous magnetic penetration depth measurements on the same crystals. The specific-heat data analysis shows consistently the strong coupling strength 2D/kTc ~ 4. This scenario is supported by the direct gap measurements via the point-contact spectroscopy. Moreover, the spectroscopy measurements show a decrease in the critical temperature at the sample surface accounting for the observed differences of the superfluid density deduced from the measurements by different techniques

Field and angular dependence of the Sommerfeld coefficient in Al-doped MgB2 single crystals

International audienceThe angular and field dependence of the Sommerfeld coefficient = lim Cel /T T→0 Cel being the electronic contribution to the specific heat has been measured in Al-doped MgB2 single crystals for x=0, x=0.1, and x 0.2 . We show that the decomposition previously introduced to describe H, where is the angle between the applied field and the c axis in pure samples Phys. Rev. Lett. 98, 137001 2007 is well adapted to doped samples: e.g., the contribution of the band to the specific heat is proportional to B/Bc2 whereas the contribution of the band is isotropic but highly nonlinear in field. We hence present the evolution of the coherence lengths of the two bands and corresponding Fermi velocities with doping

Upper and lower critical magnetic fields of superconducting NdFeAsO1−xFx single crystals studied by Hall-probe magnetization and specific heat

International audienceThe upper and lower critical fields have been deduced from specific heat and Hall-probe magnetization measurements in nonoptimally doped Nd O,F FeAs single crystals Tc 32-35 K . The anisotropy of the penetration depth is temperature independent and on the order of 4.0 1.5. Similarly specific-heat data lead to an anisotropy of the coherence length 5.5 1.5 close to Tc. Our results suggest the presence of rather large thermal fluctuations and the existence of a vortex liquid phase over a broad temperature range 5 K large at 2 T

AC Microcalorimetry of Superconducting MgCNi3 Single Crystals

Proceedings of the CSMAG'07 Conference, Kosice, July 9-12, 2007International audienceThe low-temperature speci¯c heat of single-crystal samples of super- conducting MgCNi3 with typical dimensions 200 ¹m were measured for the ¯rst time. A computer controlled ac microcalorimeter using an optical ¯ber and an infrared light-emitting diode as the heat source was used down to 2 K at magnetic ¯elds up to 8 T. The speci¯c heat data suggest a moderate coupling in MgCNi3