Fermi distribution interpretation

In summary, the conversation discusses the use of the Fermi-Dirac distribution to describe a system of electrons and how it can be used to estimate the number of electrons in a specific state. The conversation also touches on the relationship between probability and the actual presence of particles in a system. The Fermi distribution is commonly used in various calculations, but its accuracy may vary in cases such as quantum dots where there are only a few electrons present.
  • #1
EmilyRuck
136
6
Hello!
Let [itex]E_1, E_2, \ldots, E_n[/itex] be [itex]n[/itex] allowed energy levels for a system of electrons. This system can be described by the Fermi-Dirac distribution [itex]f(E)[/itex].
Each of those levels can be occupied by two electrons if they have opposite spins.
Suppose that [itex]E_1, E_2, \ldots, E_n[/itex] are such that

[itex]\displaystyle 2 \sum_{k = 1}^n f(E_k) = 1[/itex]

where the [itex]2[/itex] is due to the degeneracy of states (two electrons allowed for each state). So, can it be stated that in such a system is actually present one electron, that is the result of the sum?
If someone could even explain why, it would be very appreciated.
In any case, thank you for having read.

Emily
 
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  • #2
If I understand it correctly, the Fermi factor is the mean number of electrons in a state in k-space. You sum over all the k-space states and it adds up to the number of electrons N in your system. Unless you are very selective with the k-space states that you sum over, you won't get the sum of the mean numbers to add up to 1. (if you pick a bunch of very highly energetic states, then maybe on the average only one of these states will be occupied.)
 
  • #3
Charles Link said:
If I understand it correctly, the Fermi factor is the mean number of electrons in a state in k-space.

Did you deduce this from my post or by your own?

Charles Link said:
Unless you are very selective with the k-space states that you sum over, you won't get the sum of the mean numbers to add up to 1. (if you pick a bunch of very highly energetic states, then maybe on the average only one of these states will be occupied.)

The first post was maybe not clear about this. Don't consider my problem as real and don't take its numbers as absolute: it was just an example to show how the Fermi distribution is used to count the electrons in a band of energies.
The strange fact about Fermi distribution is that we start talking that [itex]f(E)[/itex] is the probability that the quantum state at level E is occupied; then we talk about the number of electrons that populate the conduction band of a semiconductor by doing

[itex]\displaystyle \int_{E_c}^{+\infty} g_c(E) f(E) dE[/itex]

([itex]g_c(E)[/itex] is the density of states in the conduction band). So, how can we speak almost interchangeably about probability and actual presence of a particle?
 
  • #4
EmilyRuck said:
So, how can we speak almost interchangeably about probability and actual presence of a particle?

What do you mean by actual presence of a particle?
 
  • #5
Hyo X said:
What do you mean by actual presence of a particle?

I know that, in Quantum Mechanics, talking about "actual" quantities is inappropriate.
Anyway, it is common to compute the number of electrons in conduction band, for example, in a semiconductor (e.g. here, penultimate formula). During the computation, Fermi distribution and density of states are used. The resulting number [itex]n[/itex] is considered as if it were the number of actual particles populating that band.

Hoping to have clarified your doubt, this is what I meant.
 
  • #6
Until observation shows the model to be found significantly deficient, and/or until there is a better one, this is how n is estimated.
 
  • #7
How many electrons are "actually" present is not really a meaningful question in this case. The Fermi distribution tells you something about the statistical properties of a system; it can't be used to predict e.g. exactly how many electrons are going to be in a specific state at a given time.

Note that this is not really a practical problem, in most cases we are dealing with either so many electrons or such long times that Fermi distribution will give you a very accurate estimate of the occupancy as in the case of the formula for the semiconductor.

There are system where we are really dealing with a few electrons (e.g. quantum dots) and then you have to be more careful about really interpreting the results as an probability.
 
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  • #8
@EmilyRuck. What do you mean y actual presence ?
 

1. What is the Fermi distribution interpretation?

The Fermi distribution interpretation is a concept in statistical mechanics that describes the distribution of particles in a system at thermal equilibrium. It is based on the Fermi-Dirac statistics, which states that particles with half-integer spin, such as electrons, follow a different statistical behavior than particles with integer spin, such as photons.

2. How does the Fermi distribution interpretation differ from the Maxwell-Boltzmann distribution?

The Fermi distribution interpretation takes into account the exclusion principle, which states that no two particles can occupy the same quantum state at the same time. This leads to a different distribution for particles with half-integer spin, such as electrons, compared to particles with integer spin, such as photons. The Maxwell-Boltzmann distribution, on the other hand, assumes that particles can occupy the same quantum state without any restrictions.

3. What is the significance of the Fermi energy in the Fermi distribution interpretation?

The Fermi energy is the maximum energy of a particle at absolute zero temperature. It is a crucial parameter in the Fermi distribution interpretation as it determines the occupation of energy levels for particles with half-integer spin. The Fermi energy is also used to calculate various thermodynamic properties of a system, such as specific heat and entropy.

4. How does temperature affect the Fermi distribution interpretation?

As temperature increases, the Fermi distribution interpretation begins to resemble the Maxwell-Boltzmann distribution. This is because at higher temperatures, particles with half-integer spin are more likely to occupy higher energy states, leading to a more continuous distribution. At very high temperatures, the Fermi energy becomes insignificant, and the distribution follows the classical Maxwell-Boltzmann behavior.

5. What are some applications of the Fermi distribution interpretation?

The Fermi distribution interpretation is used in various fields such as condensed matter physics, astrophysics, and nuclear physics. It helps in understanding the behavior of electrons in metals, the energy distribution of particles in stars, and the properties of nuclear matter. It is also used in the design of electronic devices such as transistors and semiconductors, where the behavior of electrons is crucial for their functionality.

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