Pressure dependence of the upper critical field of the heavy fermion superconductor UBe13

We report measurements under pressure of the upper critical field of the heavy fermion superconductor UBe13. An interpretation in the framework of a simple strong coupling model is achieved consistently with only one arbitrary parameter: the strong coupling constant lambda. We find that UBe13 is in an extreme strong coupling regime and that the variation of lambda with pressure explains the pressure dependence of the thermodynamic properties of both the normal and the superconducting phases. It reveals a strong interplay between the mass renormalization and the pairing mechanisms, yielding the first quantitative indication of a nonphonon mediated pairing in a superconductor. [S0031-9007(98)08084-3].