Anomalous upper critical field in the heavy-fermion superconductor UBe ₁₃ studied by DC magnetization measurements

We have studied anomalous upper critical field Bc2(T) of a single crystal of UBe13 by DC magnetization measurements down to 0.14 K and up to 8 T. We have observed a broad anomaly and a peak effect below Bc2 in magnetization curves. Although a possibility of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been proposed as an origin of the unusual upper critical field by previous works, our results have shown that thermal equilibrium magnetization curves reveal no step-like behavior, which could be observed at 1st-order phase transition from a superconducting mixed state to the FFLO state. We have also investigated anisotropy of Bc2 for B || <001> and B || <110>, and found that Bc2 for B || <001< is larger than Bc2 for B || <110> below ∼ 0.5 K. These results might indicate that the anomalous upward curvature of Bc2 in UBe13 is related to anisotropy of Bc2 rather than the FFLO state.