Screening by composite charged particles: the case of quantum well trions

We investigate the screening properties of a two-dimensional gas of charged excitons (trions). In a first approach to this complex problem, we determine the Hartree response of these composite charged particles within a random phase approximation, showing the effect of the trion internal structure. Only in the long wave-length limit, trions behave as point charges with mass equal to the sum of the three particle components. For finite wave-vectors, the trion screening strongly deviate from the point charge behavior and can even vanish completely at a nodal wave-vector, due to a compensation between the contribution of the two electrons and the hole within a trion. Predictions are presented for the screening of a Coulomb potential, the scattering by charged impurities and the properties of trionic plasmons.