Comparison of response of superconductivity to impurity concentration in UBe13 and LuBe13

Uhlirova et al. reported superconductivity in LuBe13 and assigned it tentatively to be BCS-type, with specific heat measurements down to 0.45 K showing the partial onset of a bulk transition. In order to check LuBe13 for unconventional behavior, as well as to further characterize the unconventional superconductivity in UBe13, we have prepared and characterized down to 0.40 K three arc-melted samples each of MBe13, M = Lu, U, using three different purities (99.999%, 99.96%, and 99.8%) of Be but with the same high purity M (Lu or U) for all three. The measurements down to 0.40 K allow the detection of the maximum in the specific heat in all three LuBe13 samples. The resulting superconducting properties strongly depend on impurity level in UBe13 (40% decrease in ΔC/Tc, 15% decrease in Tcmid) while the three LuBe13 samples exhibit significantly smaller changes (10% and 5% respectively) with purity. The comparison of properties at the first two levels of purity (99.999% vs 99.96%) is even more disparate: 12% decrease in Tcmid in UBe13 vs no change in LuBe13. These results are consistent with previous results that argue for unconventional superconductivity in UBe13, and are consistent with Uhlirova et al.‘s assignment of LuBe13 as a conventional, BCS superconductor. More generally, this example of comparing superconducting properties vs controlled gradations in impurity levels with two compositionally and structurally “matched” superconducting compounds (one conventional and one of to-be-determined coupling behavior) offers a new method for determining unconventional behavior.