Muon spin relaxation measurements of magnetic-field penetration depth in Ba8Si46

Muon spin relaxation (μSR) and high-field magnetization measurements have been performed in Ba8Si46, a superconductor with Tc=8K having a silicon clathrate three-dimensional cage structure. The upper critical field Hc2=6.5T, determined from the magnetization, indicates a short coherence length ξ=72Å. The magnetic field penetration depth λ(T→0)=4000Å was obtained from the muon spin relaxation rate σ(T→0)∼0.46μs−1 observed in transverse fields of 0.1–1 T. These values of ξ and λ indicate that this system is a strongly type-II superconductor with a high value of κ=56. We deduce the effective Fermi temperature TF and compare the present system with other superconducting systems in a plot of Tc versus TF.