Are There Any Cases of Non-Constant Particle Mass in Solid State Theory?

[TriKri]

In the book "The Physics of Semiconductors" by Marius Grundmann, there is a note on one of the pages, saying:

Generally, $\Psi'/m$ should be continuous, however, in the present example the mass is assumed constant throughout the structure.

I have never heard of a case where a particle would have a non-constant mass, is there any such case at all?

 

[DrDu]

Electrons and holes in semiconductors are quasi-particles. They have an "effective mass" which is given in terms of the second derivative of the band dispersion. This mass can vary if the band structure varies.

 

[TriKri]

In that case it would be more like a quasi-electron, not a real electron, right? Because the electron itself still has a constant mass.

 

[DrDu]

In Solid state theory, you take it usually for granted that the electrons you are talking about are quasi-electrons in reality.