Magnetic relaxation and the flux diffusion barrier for TlSr2Ca2Cu3Oy doped with Pb and Ba determined by complex ac susceptibility measurements

Transport and Point Contact Measurements on Pr1-xCexPt4Ge12 Superconducting Polycrystals

We performed a detailed investigation of the superconducting properties of polycrystalline Pr1-xCexPt4Ge12 pellets. We report the effect of Ce substitution, for x = 0.07, on magnetic field phase diagram H-T. We demonstrate that the upper critical field is well described by the Ginzburg-Landau model and that the irreversibility field line has a scaling behaviour similar to cuprates. We also show that for magnetic fields lower than 0.4 T, the activation energy follows a power law of the type ?-1/2, suggesting a collective pinning regime with a quasi-2D character for the Ce-doped compound with x = 0.07. Furthermore, by means of a point contact Andreev reflection spectroscopy setup, we formed metal/superconductor nano-junctions as small as tens of nanometers on the PrPt4Ge12 parent compound (x = 0). Experimental results showed a wide variety of conductance features appearing in the dI/dV vs. V spectra, all explained in terms of a modified Blonder-Tinkham-Klapwijk model considering a superconducting order parameter with nodal directions as well as sign change in the momentum space for the sample with x = 0. The numerical simulations of the conductance spectra also demonstrate that s-wave pairing and anisotropic s-waves are unsuitable for reproducing experimental data obtained at low temperature on the un-doped compound. Interestingly, we show that the polycrystalline nature of the superconducting PrPt4Ge12 sample can favour the formation of an inter-grain Josephson junction in series with the point contact junction in this kind of experiments.

Vortex creep activation energies and depinning currents in CaKFe4As4and Ba0.6K0.4Fe2As2revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4and Ba0.6K0.4Fe2As2single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4and Ba0.6K0.4Fe2As2superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4is slightly smaller than that in Ba0.6K0.4Fe2As2,which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values ofJc0(0) ∼ 1.0 × 108and 2.2 × 108A cm-2for CaKFe4As4and Ba0.6K0.4Fe2As2superconductors, respectively, which suggest potential applications.