First-principles description of tightly bound Fermi-polarons in monolayer black phosphorous beyond an interacting three-particle trion state

Monolayer black phosphorus, or phosphorene, is a quasi-two-dimensional material with a highly anisotropic structure and electronic band structure. Upon charge carrier doping or strong optical illumination, a finite anisotropic Fermi surface emerges along the zigzag direction. This unique property of phosphorene, along with the presence of an experimentally observed excited state with large binding energy below exciton, presents an interesting opportunity to study multiparticle excitations in low dimensionality. In this work, we use our recently developed ab initio formalism and code to study the electronic and optical properties of Fermi polarons in phosphorene, described within a 4-particle Suris tetron formalism, as a function of charge carrier concentration. Surprisingly, we find that, even for small carrier concentrations, the lowest-energy Fermi polaron cannot be accurately described by a 3-particle formalism typically employed to describe trions due to large exchange interactions between Fermi-surface holes and electrons and the large anisotropy in the Fermi surface. This work highlights how accurate first-principles calculations can address complex multi-particle excitations in materials, for which there is often a delicate balance between screening, Coulomb interaction, and spin orders.