Strong coupling effects on the upper critical field of the heavy-fermion superconductor UBe13

The upper critical fieldH c2 (T) of a high quality single crystal of UBe13 is studied with very low noise resistivity measurements. It shows a large but finite slope of —45 T/K, an unusual temperature dependence with an inflexion point atT/T C ∼ 0.5 and a large saturated limit forT→ 0 ofH c2 (0) = 14 T. The complete temperature dependence ofH c2 (T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurence of a non-uniform superconducting state (FFLO state) at low temperature.