Plasmons and screening in monolayer and multilayer black phosphorus
  Black phosphorus exhibits a high degree of band anisotropy. However, we found that its in-plane static screening remains relatively isotropic for momenta relevant to elastic long-range scattering processes. On the other hand, the collective electronic excitations in the system exhibit a strong anisotropy. Band non-parabolicity leads to a plasmon frequency which scales as $n^{\beta}$, where $n$ is the carrier concentration, and $\beta<\tfrac{1}{2}$. Screening and charge distribution in the out-of-plane direction are also studied using a non-linear Thomas-Fermi model. 
Tony Low, Rafael Rold\'an, Han Wang, Fengnian Xia, Phaedon Avouris,   Luis Mart\'in Moreno and Francisco Guinea
cond-mat.mtrl-sci cond-mat.mes-hall