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The group velocity of an electron wavepacket in a homogeneous lattice is

v_{group}(k) = ∇_{k}E_{k},

where E_{k}is the dispersion. I have just read an article, where they use this to find the group velocity of a wavepacket in an inhomogeneous lattice, but they use the homogeneous dispersion. I don't quite understand why this is allowed. Can you clarify this?

Best,

Niles.

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# Group velocity of wavepacket

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