# Momentum conservation in a direct transition

by hokhani
Tags: conservation, momentum, transition
 Sci Advisor P: 1,627 Momentum conservation in a direct transition Yes, but one should take the usual caveats into account like crystal momentum only being conserved up to a reciprocal lattice vector. An electron with crystal momentum $\hbar k$ can be described equivalently to a free particle with momentum $\hbar k$, if we assign it the right effective mass. The effective mass is given by the curvature of the parabola of the band. Therefore, momentum conservation works in that easy way as long as effective mass is a good approximation, that is, when the bands are approximately parabolic at the transition points in k-space. This is usually the case in direct band gap materials around the band gap. Total momentum is of course also conserved when this is not the case, but one cannot see it that easily.