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Re: [Phys-l] solid state physics



Yet even the electron with "only one mass" has a different *effective* mass depending on the band structure. Since the light and heavy hole effective masses arise from motion in different directions within the crystal why doesn't the same thing happen to the electron?



-----Original Message-----
From: Stefan Jeglinski <jeglin@4pi.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Mon, 20 Feb 2006 10:11:31 -0500
Subject: Re: [Phys-l] solid state physics


Why aren't there light and heavy electrons just as there are light
and heavy holes?

An electron is just an electron, so it has only one mass. A hole,
although generally described as an electron analog with an opposite
charge, is actually better described as an entire electron (valence)
band, minus one electron (which has been promoted to the conduction
band). Hence the hole's "positive" charge. And, since the hole is
actually an entire band of electrons, minus one, its "mass" is
typically much larger in magnitude.

The "band" is a solid-state concept, and doesn't generally exist
without the presence of a lot of electrons, eg in a crystal. The band
structure depends a great deal on the [crystal] environment, so you
can imagine the variance that is possible. When an electron is
promoted to the conduction band, a variety of holes can be created.
Commonly though, there are 2 valence bands of importance, denoted
"light" and "heavy," due in part to different transport properties
possessed when an electron is lost to the conduction band.

This is basically Solid State 101, and I've glossed over quite a bit.
It can get as complicated as you like. The only "real" particle in
this field is an electron. But there are plenty of pseudoparticles,
of which the hole is one (the polaron is another). With our limited
minds, these pseudoparticles serve useful pedagogical and
calculational purposes.


Stefan Jeglinski

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