Critical magnetic fields of superconducting aluminum-substituted Ba8Si42Al4 clathrate

In recent years, efforts have been made to explore the superconductivity of clathrates containing crystalline frameworks of group-IV elements. The superconducting silicon clathrate is unusual in that the structure is dominated by strong sp(3) covalent bonds between silicon atoms, rather than the metallic bonding that is more typical of traditional superconductors. This paper reports on critical magnetic fields of superconducting Al-substituted silicon clathrates, which were investigated by transport, ac susceptibility, and dc magnetization measurements in magnetic fields up to 90 kOe. For the sample Ba8Si42Al4, the critical magnetic fields were measured to be H-C1 = 40.2 Oe and H-C2 = 66.4 kOe. The London penetration depth of 4360 angstrom and the coherence length 70 angstrom were obtained, whereas the estimated Ginzburg-Landau parameter of k = 62 revealed that Ba8Si42Al4 is a strong type-II superconductor. (C) 2015 AIP Publishing LLC.