Elastocaloric evidence for symmetry breaking within the superconducting state in an optimally doped iron-based superconductor

The AC elastocaloric effect (ECE) is a highly sensitive, thermodynamic probe of phase transitions under uniaxial strain, directly probing the strain derivative of the entropy proximate to a transition. We report ECE measurements across optimal doping in the archetypal iron-based superconductor, Ba(Fe_1-xCo_x)₂As₂. The ECE signature at T_c for an overdoped sample is standard and quantitatively consistent with other thermodynamic probes. In contrast, for an optimally doped sample, ECE reveals a second thermodynamic transition close to T_c. No antiferromagnetic order is present at this doping level, and we further rule out re-entrant tetragonality as the origin of this transition using X-ray diffraction. Our observations strongly suggest a phase transition into a multicomponent superconducting state, thereby implying the presence of a sub-dominant pairing instability near optimal doping which exhibits a pronounced dependence on external strain. Our results thus motivate a re-examination of the pairing state and its relation to nematicity in this well-studied iron-based superconductor, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.