Free electron dispersion relation, help?

In summary, the free electron dispersion relation is modified in the presence of a weak periodic potential. This modification results in the appearance of a set of parabolas shifted by the reciprocal lattice vector along the k-axis. These parabolas intersect a certain points, which are the points of degeneracy. As a result, the degeneracy is split and the amplitude of the periodic potential is twice the size.
  • #1
taffara_121
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Hi there,

Could anybody explain how the free electron dispersion relation would be modified by the presence of a periodic potential..? I'm struggling to get my head around it.

Thanks!
 
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  • #2
You will find figure 9.4 from Ashcroft and Mermin very useful in understanding how the dispersion of the free electron is modified in the presence of a weak periodic potential. In the so called repeated zone scheme (also sometimes called the extended zone scheme) the free electron dispersion in the periodic potential will look like a set of parabolas shifted by the reciprocal lattice vector along the k-axis. You can see that the parabolas will intersect a certain points. These are the points of degeneracy. According to Pauli’s exclusion principle you cannot have two electrons in the same state. As a result, this degeneracy will be split by the opening of a gap. Using degenerate perturbation theory you can show that the magnitude of this gap is twice the amplitude of the periodic potential.

Disclaimer: Please do not take what I have said here too literally. I strongly recommend you take a look at chapter 8 and 9 of Ashcroft and Mermin. The mathematical treatment of this problem presented in this book is, in fact, the simplest one you can use without getting into any trouble. The verbose description I gave above is so that you can develop an intuition for this phenomenon; I understand how overwhelming (and frustrating) it can be to get lost in the mathematical formalism.
 
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  • #3
tejas777 said:
In the so called repeated zone scheme (also sometimes called the extended zone scheme) the free electron dispersion in the periodic potential will look like a set of parabolas shifted by the reciprocal lattice vector along the k-axis.

Ignore the part in the parenthesis; that's not true.
 
  • #4
taffara_121 said:
Hi there,

Could anybody explain how the free electron dispersion relation would be modified by the presence of a periodic potential..? I'm struggling to get my head around it.

Thanks!

This could be helpful:
Specifically: Kronig-Penney Model
http://en.wikipedia.org/wiki/Particle_in_a_one-dimensional_lattice
 

Related to Free electron dispersion relation, help?

1. What is the free electron dispersion relation?

The free electron dispersion relation is a mathematical equation that describes the energy and momentum of an electron in a solid material. It shows how the energy of an electron changes as its momentum changes.

2. How is the free electron dispersion relation derived?

The free electron dispersion relation is derived from the Schrödinger equation, which describes the behavior of quantum particles such as electrons. It takes into account the periodicity of the crystal structure and the potential energy of the electrons.

3. What does the free electron dispersion relation tell us about the behavior of electrons in a solid?

The free electron dispersion relation tells us that the energy levels of electrons in a solid are quantized, meaning they can only have certain discrete values. It also shows that the energy of an electron increases as its momentum increases.

4. How does the free electron dispersion relation affect the electrical conductivity of a material?

The free electron dispersion relation plays a crucial role in determining the electrical conductivity of a material. It shows that in a material with a high density of free electrons, such as a metal, the electrons can easily move and conduct electricity. In contrast, in a material with a low density of free electrons, such as an insulator, the electrons are tightly bound and cannot move as easily, resulting in lower conductivity.

5. Can the free electron dispersion relation be used to predict the properties of different materials?

Yes, the free electron dispersion relation is an important tool in predicting the properties of different materials. By understanding the energy and momentum of electrons in a material, we can make predictions about its electrical conductivity, thermal conductivity, and other properties. This information is crucial in designing new materials for various applications.

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