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

There is considerable evidence that the superconducting state of Sr 2 RuO 4 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 x z ± i d y z 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 ± i d and d ± i g 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 Sr 2 RuO 4 under hydrostatic pressure, and Sr 1.98 La 0.02 RuO 4 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 x z ± i d y z order. Two possible scenarios of the superconducting order parameter in Sr 2 RuO 4 remain difficult to distinguish. Here, the authors observe that the onset temperature of time reversal symmetry breaking tracks the superconducting transition temperature in Sr 2 RuO 4 , supporting a d x z ± i d y z order parameter.