Upper critical magnetic fields of pure and thoriated UBe13

We present measurements of the upper critical magnetic field $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ determined from the dc magnetization of polycrystalline samples of ${\mathrm{U}}_{\mathrm{}(1-x)\mathrm{}}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$ with $x=0\mathrm{}$ and $x=0.0331\mathrm{}$ and a single crystal of pure U${\mathrm{Be}}_{13}$. We find changes in slope in the $H_{c2\mathrm{}}$ vs $T$ phase diagrams of both polycrystalline samples which suggests entrance into a second superconducting phase at about $\frac{T_c}{2}$. A degradation of the superconducting critical field is observed in the single crystal which may be related to the absence of second-phase behavior in this sample. Our results are discussed in light of recent experimental and theoretical treatments of second-phase behavior in heavy-fermion superconductors.