The exciton eigenfunctions are denoted by \ phi _ \ lambda ( { \ mathbf { k } } ) where \ lambda labels the exciton state with eigenenergy E _ \ lambda and \ hbar { \ mathbf { k } } is the crystal momentum of charge carriers in the solid.
12.
When "'k "'relates to crystal momentum instead of true momentum, the concept of "'k "'- space is still meaningful and extremely useful, but it differs in several ways from the non-crystal "'k "'- space discussed above.
13.
Crystal momentum also earns its chance to shine in these types of calculations, for, in order to calculate an electron's trajectory of motion using the above equations one need only consider external fields, while attempting the calculation from a set of EOMs based on true momentum would require taking into account individual coulomb and Lorenz forces of every single lattice ion in addition to the external field.
