# Negative effect mass

## Main Question or Discussion Point

We have been shown that the Energy versus K curve has band gaps, and at the brillouin zone is where the standing waves occur.We were also shown the the mass is equal to h^2 divided by the gradient of this graph,but i have no idea what the negative effect mass actually means. Everything i have found on the net seems very complicated compared to what we are studying.Can anyone explain this concept to me?

Related Atomic and Condensed Matter News on Phys.org
Under the influence of periodic potential this negative mass concept does arise.
When an electron goes to zone boundary, it collides with crystal ions and loses its momentum.An energetic electron then loses momentum due to collision. So d/dE(dE/dK) becomes -ve and m comes out to be so.
As e- moves towards zone bondary it leaves back a vacant site-hole.More e- move toward boundary, more will be creation of holes, which then contributes to current. Remember after reaching zone boundary there will be no conduction of e- further.

The effective mass concept takes all the complex internal forces due to the periodic potential, and sweeps them under the rug allowing you to relate an external force (eg an electric field) to the acceleration of a charge.

Consider an electron in a semiconductor under the influence of a field in the negative x-direction. The force on the electron is F=-eE, so the external force is in the positive x-direction.

Far from the zone boundary, the effective mass is positive, so by Fext = m*a, the acceleration is also positive. The electron accelerates in the positive x-direction towards the zone boundary.

As the electron approaches the zone boundary, the effective mass becomes negative (this can be seen from the curvature of the dispersion relation). The external force is still positive, which means that the electron acceleration now becomes negative, ie the electron decelerates. The negative effective mass tells you that the electron responds to the field opposite to how a free electron would.

Physically, the fact that the electron accelerates opposite to the direction of the force is because the electron must reflect off the zone boundary. As it approaches the boundary, it must decelerate. This behavior is of course due to the complex interaction with the periodic potential, but the effective mass serves as a convenient tool to understand how the electron will behave without knowing the details of these internal forces.

P.S. In case you didn't learn it this way, a standing wave can be written as the sum of an equal forward and backward travelling wave, so it's just the same as saying the electron reflects from the boundary. And this may have just been a typo on your part, but it's the second derivitive and not the gradient of the dispersion relation that determines m*.

Last edited: