Type II superconductivity in SrPd2Ge2

Previous investigations have shown that SrPd2Ge2, a compound isostructural with "122" iron pnictides but iron- and pnictogen-free, is a conventional superconductor with a single s-wave energy gap and a strongly three-dimensional electronic structure. In this work we reveal the Abrikosov vortex lattice formed in SrPd2Ge2 when exposed to magnetic field by means of scanning tunneling microscopy and spectroscopy. Moreover, by examining the differential conductance spectra across a vortex and estimating the upper and lower critical magnetic fields by tunneling spectroscopy and local magnetization measurements, we show that SrPd2Ge2 is a strong type II superconductor with \kappa >> sqrt(2). Also, we compare the differential conductance spectra in various magnetic fields to the pair breaking model of Maki - de Gennes for dirty limit type II superconductor in the gapless region. This way we demonstrate that the type II superconductivity is induced by the sample being in the dirty limit, while in the clean limit it would be a type I superconductor with \kappa\ << sqrt(2), in concordance with our previous study (T. Kim et al., Phys. Rev. B 85, (2012)).Comment: 9 pages, 4 figure

Magnetotransport and the upper critical magnetic field in MgB2

Magnetotransport measurements are presented on polycrystalline MgB2 samples. The resistive upper critical magnetic field reveals a temperature dependence with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper critical field is much higher than what is known so far on polycrystals of MgB2 but it is in agreement with recent data obtained on epitaxial MgB2 films. The deviation of Bc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.Comment: 6 pages, incl. 3 figure

Upper critical field in {Ba1−x_{1-x}Kx_xBiO3_3}: magnetotransport versus magnetotunneling

Elastic tunneling is used as a powerful direct tool to determine the upper critical field $H_{c2}(T)$ in the high-$T_c$ oxide Ba$_{1-x}$K$_x$BiO$_3$. The temperature dependence of $H_{c2}$ inferred from the tunneling follows the Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A comparison will be made with resistively determined critical field data.Comment: 4 pages incl. 5 figure

Evidence for conventional superconductivity in SrPd2Ge2

Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band-structure calculations within the local-density approximation (LDA). The STS measurements show single s-wave superconducting energy gap \Delta(0) = 0.5 meV. Photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the "122" family.Comment: submitted to PR

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