Unsplit superconducting and time reversal symmetry breaking transitions in Sr(2)RuO(4) 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(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 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(xz) ± id(yz) order.