POSITIVE CURVATURE IN THE UPPER CRITICAL-FIELD OF HEAVY FERMION SUPERCONDUCTORS

New data on the upper critical magnetic field of single crystals of URu2Si2 show distinct upward curvature for the field H in the tetragonal basal plane, but not for fields along the tetragonal axis. The data for H<5 T can be explained as a result of the coupling of a multi-component superconducting order parameter with the magnetization of a spin-density wave. Alternatively, an empirical model succesfully applied to UPt3 and U6Fe provides an excellent fit of Hc2(T) for URu2Si2 over the entire experimental temperature (0.4 K ≤T≤1.5 K) and field (0≤H≤ 8 T) ranges. Our results imply that unconventional superconductivity may occur over a wide interval of electronic effective mass ratio 20 < m*/me < 200. © 1990.