# Brillouin zones and energy bands

1. Nov 14, 2009

### cavalier3024

In the semi-classical model, i noticed that all the electrons with values of k that are in the same brillouin zone are considered to be at the same energy band, but i cant quite understand why it is so.
i know that in each brillouin zone the number of allowed states (of k) is the same as the number of states at each band (N), but that doesn't really provide a full explanation.

so if you can briefly explain the idea behind this (or refer me to the relevant pages at Ashcroft) i would be thankful.

2. Nov 14, 2009

### Niles

There are 2N states in each band, and for a single k-vector, there are infinitely many energy bands (this can e.g. be seen when looking at the energy bands in the reduced zone scheme).

So when one energy band is filled, electrons start filling the next one, and so on. On what page in A&M have you read this?

3. Nov 14, 2009

### cavalier3024

you said that 'when one energy band is filled, electrons start filling the next one', but that's not always the case. sometimes bands overlap and one band may start filling before the previous one finished, giving two partially filled bands.
you can see what i mean at page 224 (chapter 12), figure 12.3. they show that 2N electrons fill part of the first brillouin zone and also part of the second BZ. then they say that this produces two partially filled bands. so what i understand is that all occupied states that are in the first brillouin zone are filling one band, and the ones at the second zone fill another band.
what i don't understand is why the brillouin zones determine the bands. why all the electrons that occupy a certain BZ are considered to be at the same band?

4. Jun 20, 2010

### kukitxu

In the Nearly Free Electron model energy gaps appear due to bragg reflections with atoms in the Brillouin zone limit.

In a one dimensional model you have then an energy gap at K=+-pi/a, K=+-2pi/a etc... Biger k values give biger energy values. So those energy values in the first BZ are the less energetic values, the 1st band. The energy values in the second BZ correspond to the 2nd band etc...

But every k value in the second BZ can be taken to the first BZ by substracting a G vector from the reciprocal lattice, every vector in the third BZ can be taken to the 1st one by substracting 2 G vectors etc... So they are "the same" k vector and every k value has an energy value in each energy band.

I hope this messy explanation clears your ideas. I like more Kittel than Ashcroft (chapter 7)