Symmetry-mixed bound-state order: extended degeneracy of (d + ig)-superconductivity in Sr2RuO4
Abstract
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 Sr2RuO4: Most noticeably, the derived strain-dependence is in excellent agreement with recent experiments [Hicks et al. Science (2014) and Grinenko et 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.