The trion as an exciton interacting with a carrier

The X− trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton–electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X− trion creation operator in terms of excitons and electrons. We can also derive the X− trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.