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For an ideal Fermi gas in three-dimensions, the electron velocity \(v_{F}\) at the Fermi surface is related to electron concentration n as :

  • A.\(v_{F} \propto n^{2/3}\)
  • B.\(v_{F} \propto n\)
  • C.\(v_{F} \propto n^{1/2}\)
  • D.\(v_{F} \propto n^{1/3}\)

Answer: D

The Fermi velocity varies as \(n^{1/3}\).

For a free electron gas in three dimensions the Fermi wave vector is fixed by the electron density:

\[ \begin{aligned} k_{F} &= (3\pi^{2}n)^{1/3} \\ v_{F} &= \frac{\hbar k_{F}}{m} \propto n^{1/3} \end{aligned} \]

The Fermi energy, \(E_{F} = \hbar^{2}k_{F}^{2}/2m\), goes as \(n^{2/3}\), which is the trap in option A. The velocity is the square root of an energy, so it goes as \(n^{1/3}\).

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