Elastocaloric measurements of BaFe₂(As_1-xP_x)₂

The isovalently substituted iron-based superconductor family BaFe₂(As_1-xP_x)₂ provides a clean system to investigate the interplay between magnetic and nematic quantum criticality and high-T_c superconductivity. Notably, although the AFM and structural phase transitions occur simultaneously in a first order transition across the doping phase diagram, inducing B_2g strain smears the structural transition into a crossover, while the AFM transition remains sharply identifiable.  We present measurements of this system via the AC elastocaloric effect, a recently developed thermodynamic probe which is sensitive to strain derivatives of the entropy. This technique, which can sensitively track the antiferromagnetic phase transition as strain is applied, establishes that 1/T_N^o (dT_N/dε_B2g) increases by nearly 6-fold as optimal doping is approached. These measurements also show that the thermodynamic nematic susceptibility decreases as optimal doping is approached, in contrast to simultaneous elastoresistivity measurements in which 1/ρ^o(dρ_B2g/dε_B2g) increases in magnitude. This suggests that the resistivity anisotropy/order parameter proportionality constant increases dramatically towards optimal T_c, which has been shown to be the case as well in the more disordered Co-doped Ba122 system.