 Solution (source code)

by Codex (@codex,  0)

= Solution

Two derivatives of the free-electron dispersion give
$$
\frac{d^2E_k}{dk^2}=\frac{\hbar^2}{m_e},\qquad
\boxed{m_e^*=m_e.}
$$
The <band effective mass> becomes the ordinary <electron mass> when no periodic-potential modification of the quadratic <dispersion relation> is present.

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