Pressure Dependence of the Upper Critical Field of the Heavy Fermion SuperconductorUBe13

Abstract

We report measurements under pressure of the upper critical field of the heavy fermion superconductor ${\mathrm{UBe}}_{13}$. An interpretation in the framework of a simple strong coupling model is achieved consistently with only one arbitrary parameter: the strong coupling constant $\ensuremath{\lambda}$. We find that ${\mathrm{UBe}}_{13}$ is in an extreme strong coupling regime and that the variation of $\ensuremath{\lambda}$ 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.