Effective mass (solid-state physics)

In solid state physics, a particle's effective mass (often denoted m) is the mass that it seems to have when responding to forces, or the mass that it seems to have when en masse with other identical particles in a thermal distribution. One of the results from the band theory of solids is that the movement of particles in a periodic potential, over long distances larger than the lattice spacing, can be very different from their motion in a vacuum. The effective mass is a quantity that is used to simplify band structures by constructing an analogy to the behavior of a free particle with that mass. For some purposes and some materials, the effective mass can be considered to be a simple constant of a material. In general, however, the value of effective mass depends on the purpose for which it is used, and can vary depending on a number of factors.