Upper critical field peculiarities in the cubic superconductor LaRu4As12

Electrical resistivity of the filled skutterudite compound LaRu4As12 was investigated both as a function of temperature and magnetic field. Upon lowering temperature below around 0.85Tc ≈ 9K, the upper critical field increases quasilinearly with the slope (−dHc2/dT)=1.11 T/K, i.e., significantly faster than in the vicinity of Tc where (−dHc2/dT)Tc = 0.66T/K was found. As a consequence, a positive curvature emerges whose size is very similar to those observed for anisotropic superconductors. Anticipating a weakly anisotropic Fermi surface in the cubic compound LaRu4As12, we related the anomaly in Hc2(T) to multiband effects rather than to a strong anisotropy of a single energy gap.