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What is the physical meaning of group velocity for elastic waves through crystals?

In a chapter about phonons they define group speeds for elastic waves in crystals as the derivative of the dispersion relation: $$v_g = \frac{d{\omega}}{d{k}} $$ I wonder though how they come to ...
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22 views

What are degenerate transversal oscillation modes?

This is just a question about terminology that is used in the beginning of a chapter about phonons. In a simple cubic crystal, we can consider elastic oscillations in f.i. the [100] direction. In ...
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515 views

Why do phonons cause excellent heat conduction in diamonds?

Phonons are the quantum of lattice vibrations in crystals and are not to be confused with photons, the gauge bosons of the electromagnetic force. Apparently, they contribute to heat conduction, but I ...