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Demonstration that electric current at equilibrium is zero in crystals

Physics Asked by Gippo on December 28, 2020

As it is well known, electrons at equilibrium (no external field) do not conduct electric current, i.e.

$int_{BZ} dk,v_{k},f(epsilon_k)=0$

where $f(epsilon_k)$ is the Fermi-Dirac distribution

$f(epsilon)=frac{1}{e^{beta(epsilon-mu)}+1}$

$v_k$ is the velocity

$v_k=frac{partial epsilon_k}{partial k}$

and $BZ$ is the Brillouin zone.

Is it possible to demonstrate that just using the explicit expression
of the Fermi-Dirac distribution and the fact that we are considering functions periodic on the reciprocal lattice? I mean, is it possible to say that the integral is zero just from the general properties of the quantities involved? (for example, in the free-electron case it comes from the fact that it is the integral of an odd function in $k$).

One Answer

Good question, I am trying to answer this question(partially) from symmetry considerations.

If the system has the time reversal or inversion symmetry, then $varepsilon_{-k}=varepsilon_{k}$. From this $v(-k)=-v(k)$, and $f(varepsilon_{-k})=f(varepsilon_k)$. Since for any point $k$ in the BZ, we can always find its counterpart $-k$, therefore the integration over the entire BZ is zero.

I don't know how to argue if the the system breaks both inversion and time-reversal symmetry.

Answered by an offer can't refuse on December 28, 2020

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