Upper critical magnetic field of the heavy-electron superconductors ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$ (x=0 and 2.9%) doped with paramagnetic Gd and other rare-earth ions

The temperature T dependence of the upper critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ of the heavy-electron superconductors ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Th}}_{\mathit{x}}$ ${\mathrm{Be}}_{13}$ (x=0 and 2.9%) doped with various concentrations of Gd has been determined from low-frequency ac magnetic susceptibility measurements in magnetic fields up to 60 kOe. The ${\mathit{H}}_{\mathit{c}2}$(T) curves for ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Gd}}_{\mathit{x}}$ ${\mathrm{Be}}_{13}$ samples deviate from the ${\mathit{H}}_{\mathit{c}2}$(T) curves of ${\mathrm{UBe}}_{13}$ near ${\mathit{T}}_{\mathit{c}}$, which is consistant with the depairing of superconducting electrons via the Zeeman interaction between the spins of the superconducting electrons and the exchange field associated with the Gd spins. This suggests that ${\mathrm{UBe}}_{13}$ exhibits singlet superconductivity. In contrast, the ${\mathit{H}}_{\mathit{c}2}$(T) curves for (${\mathrm{U}}_{0.97}$ ${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ doped with Gd scale with ${\mathit{H}}_{\mathit{c}2}$(T) of pure (${\mathrm{U}}_{0.97}$ ${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$ and do not reflect superconducting depairing by the Gd ions. These results are consistent with either strong spin-orbit scattering due to the presence of Th in ${\mathrm{UBe}}_{13}$, or to a qualitatively different type of superconductivity involving triplet spin pairing in (${\mathrm{U}}_{0.97}$ ${\mathrm{Th}}_{0.03}$)${\mathrm{Be}}_{13}$. Measurements of the temperature dependence of ${\mathit{H}}_{\mathit{c}2}$ for ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{R}}_{\mathit{x}}$ ${\mathrm{Be}}_{13}$ compounds where R=La, Lu, and Ce for various compositions x as well as ${\mathrm{U}}_{0.985}$ ${\mathit{R}}_{0.015}$ ${\mathrm{Be}}_{13}$ compounds for R=Tb, Dy, Ho, and Er are also presented and compared with the Gd-doped ${\mathrm{UBe}}_{13}$ system. The results of low-temperature specific-heat measurements of ${\mathrm{UBe}}_{13}$ doped with various concentrations of Gd are also discussed.