We report a detailed study of specific heat, electrical resistivity and thermal expansion in combination with inelastic neutron and inelastic X-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors
Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 K and 1.5 K respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function and phonon anharmonicity from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds.
We investigated the superconducting state of the noncentrosymmetric superconductors Li$_2$Pd$_x$Pt$_{3-x}$B with superconducting transition temperature $T_c$= 5.16(8) K ($x$=2.25), 3.56(8) K ($x=1.5$) and 2.60 K ($x=0$) by means of muon-spin rotation
($mu$SR) and specific heat experiments. The $mu$SR relaxation rate $sigma_{sc}$ was found to be constant at low temperatures for all the compounds. Data taken at different magnetic fields show that the magnetic penetration depth $lambda$ is field-independent for Li$_2$Pd$_{2.25}$Pt$_{0.75}$B and Li$_2$Pt$_{3}$B. The electronic contribution to the specific heat measured in Li$_2$Pd$_{1.5}$Pt$_{1.5}$B and Li$_2$Pt$_{3}$B increases exponentially at the lowest temperatures. These features suggest that the {it whole family} of Li$_2$Pd$_x$Pt$_{3-x}$B are single-gap s-wave superconductors across the entire doping regime.
