  We address the problem of free carrier screening of exciton states in
two-dimensional monolayer semiconductors. Basic theoretical considerations are
presented concerning the applicability of the commonly used static
approximation of the screening effect and the implications are discussed. We
show that the low-frequency models lead to a major overestimation of the free
carrier response and are inadequate to describe the screening of strongly bound
excitons in monolayer materials. The presented arguments are consistent with
existing high-level many-body theories and transparently illustrate the
underlying physics.
