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With T_c’s below 40 K and evidence of a quantum critical point [1], the iron-based high-temperature superconductor BaFe₂(As_1-xP_x)₂ is an appealing system for investigating the behavior underlying superconductivity in high-T_c superconductors. By applying magnetic fields up to 35 T, we are able to suppress superconductivity and reveal the normal state of overdoped BaFe₂(As_1-xP_x)₂. We observe √H behavior indicating a nodal superconducting gap, saturation of the heat capacity at a magnetic field corresponding to the onset of the normal state, and enhancement of the quasiparticle mass sum as calculated from electronic specific heat coefficient as optimal doping is approached [1]. Our comparison of specific heat as a function of magnetic field to specific heat as a function of temperature, as well as other measurements, forms a consistent treatment of specific heat measurements in high-temperature superconductors.

[1] K. Hashimoto et al. A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe₂(As_1–xP_x)₂ Science 336, 1554-1557 (2012)
[2] C. M. Moir et al. Multi-band mass enhancement towards critical doping in a pnictide superconductor npj Quantum Materials 4, 8 (2019)