Anisotropic transient reflectivity across optimal doping in the isovalent-doped superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$

The isovalent-doped high-$T_c$ superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is characterized by a rich temperature-doping phase diagram, which includes structural, antiferromagnetic, electron nematic, and superconducting phase transitions. Of particular note is the proposed existence of a quantum critical point at optimal doping. In this work, we use 1.5 eV pump-probe reflectivity measurements to study the recombination dynamics of photoexcited quasiparticles as a function of temperature, doping, and polarization. We find that the low-temperature response is strongly anisotropic across a wide range of dopings, both above and below optimal. This indicates that the anisotropy arises independently of the orthorhombic-tetragonal and antiferromagnetic phase transitions, which occur only on the underdoped side of the phase diagram.