Symmetry-mixed bound-state order: extended degeneracy of $(d+ig)$-superconductivity in ${\mathrm{S}\mathrm{r}}_2\mathrm{R}\mathrm{u}{\mathrm{O}}_4$

We report a fluctuation-driven state of matter that develops near an accidental degeneracy point of two symmetry-distinct primary phases. Due to symmetry mixing, this bound-state order exhibits unique signatures, incompatible with either parent phase. Within a field-theoretical formalism, we derive the generic phase diagram for system with bound-state order, study its response to strain, and evaluate analytic expressions for a specific model. Our results support the $(d+ig)$-superconducting state as a candidate for ${\mathrm{S}\mathrm{r}}_2\mathrm{R}\mathrm{u}{\mathrm{O}}_4$: Most noticeably, the derived strain-dependence is in excellent agreement with recent experiments [Hicks $\mathit{\text{et}}\mathit{\text{al.}}$ Science (2014) and Grinenko $\mathit{\text{et}}\mathit{\text{al.}}$ arXiv (2020)]. The evolution above a non-vanishing strain from a joint onset of superconductivity and time-reversal symmetry-breaking to two split phase transitions provides a testable prediction for this scenario.