Zero-field muon-spin-resonance and lower-critical-field data are presented for a wide range of Th concentrations in ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Th}}_{\mathit{x}}$${\mathrm{Be}}_{13}$, spanning the region where both a superconducting and a second, lower-temperature phase transition are observed. Overall T-x phase boundaries are assigned and discussed according to the nature of the lower phase transition. Arguments for associating the lower phase with a possible magnetic (time-reversal–violating) superconducting state are given.

• Received 18 May 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2816