# Fermi energy condensed matter exam problem

1. Aug 1, 2014

### SMC

1. The problem statement, all variables and given/known data

Hello, I am preparing a condensed matter exam and I was wondering if I could get some help on the following question from a past exam paper:

Show that for the free electron gas at zero temperature the Fermi energy is given by:

$$ε_{F}=\frac{\hbar^{2}}{2m}(3π^{2}N)^{2/3}$$

and the fermi wave number by:

$$k_{F}=(3π^{2}N)^{1/3}$$

where N is the volume density of electrons.

2. Relevant equations

the previous question was to derive the density of states:

$$D(ε) = \frac{V}{2π^{2}}(\frac{2m}{\hbar^{2}})^{3/2} ε^{1/2}$$

but I don't know if I have to use that or not

3. The attempt at a solution

how I would have done it is derive $k_{F}$ from $$N=\frac{VK_{F}^{3}}{3π^{2}}$$ which is obtained by saying number of electrons N is equal to volume of fermi sphere over k-space volume element $(2π/L)^{3}$.

and then state that the dispersion for a free electron gas is
$$ε(k) = \frac{\hbar^{2}k^{2}}{2m}$$ to get fermi energy.

but this method gives a result with the V present in the equation while the equations in the question don't have the V for some reason. Also I derived wavenumber first then energy while the question asks to derive energy first then wavenumber.

Last edited: Aug 1, 2014
2. Aug 1, 2014

### Oxvillian

SMC - I think the $N$ in the question is the number of particles per unit volume. Otherwise their equations aren't dimensionally correct.

3. Aug 1, 2014

### SMC

yes ok that makes sense I should of noticed that, thank you Oxvillan. but I still don't understand why it asks me to derive fermi energy before wavenumber. is there a way of deriving fermi energy without deriving wavenumber first?

or maybe the questions are just in the wrong order

4. Aug 1, 2014

### Oxvillian

Actually I would have done the wavenumber first too

But you can also integrate the density of states in energy space from zero to whatever the Fermi energy is and set that equal to $N$.