Unsplit superconducting and time reversal symmetry breaking transitions in Sr₂RuO₄ under hydrostatic pressure and disorder

There is considerable evidence that the superconducting state of Sr₂RuO₄ breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature, T_TRSB, is generally found to match the critical temperature, T_c, within resolution. In combination with evidence for even parity, this result has led to consideration of a d_xz± id_yz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding T_TRSB = T_c, but it has a hard-to-explain horizontal line node at k_z = 0. Therefore, s ± id and d ± ig order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain T_TRSB≈ T_c. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr₂RuO₄ under hydrostatic pressure, and Sr_1.98La_0.02RuO₄ at zero pressure. Both hydrostatic pressure and La substitution alter T_c without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected, T_TRSB should track changes in T_c, while if it is accidental, these transition temperatures should generally separate. We observe T_TRSB to track T_c, supporting the hypothesis of d_xz ± id_yz order.