Near critical doping, high-temperature superconductors exhibit multiple anomalies associated with enhanced electronic correlations and quantum criticality. Quasiparticle mass enhancement approaching optimal doping has been reported in quantum oscillation measurements in both cuprate and pnictide superconductors. Although the data are suggestive of enhanced interactions, the microscopic theory of quantum oscillation measurements near a quantum critical point is not yet firmly established. It is therefore desirable to have a direct thermodynamic measurement of quasiparticle mass. Here we report high-magnetic field measurements of heat capacity in the doped pnictide superconductor BaFe₂(As_1−xP_x)₂. We observe saturation of the specific heat at high magnetic field in a broad doping range above optimal doping which enables a direct determination of the electronic density of states recovered when superconductivity is suppressed. Our measurements find a strong total mass enhancement in the Fermi pockets that superconduct. This mass enhancement extrapolates to a mass divergence at a critical doping of x = 0.28.

在接近临界掺杂的情况下，高温超导体表现出与增强的电子相关性和量子临界性相关的多个异常现象。在铜酸盐和磷化物超导体的量子振荡测量中已经报道了接近最佳掺杂的准粒子质量增强。尽管数据提示相互作用增强，但尚未牢固建立量子临界点附近的量子振荡测量的微观理论。因此，需要对准粒子质量进行直接的热力学测量。在这里，我们报告了在掺杂的肽超导体BaFe ₂（As _{1- x} P _x）_2中对热容量的高磁场测量。我们在最佳掺杂之上的宽掺杂范围内观察到高磁场下比热的饱和度，这可以直接确定超导性被抑制时恢复的态的电子密度。我们的测量结果发现超导的费米腔中的总质量有了很大的提高。该质量增强推断出在临界掺杂x  = 0.28时的质量散度。