$$ v_{\text g} $$ Unit $$ \frac{\mathrm m}{\mathrm s} $$

Velocity with which a lattice vibration propagates in the crystal. It follows from the derivative of the dispersion relation \(\omega(k)\) of the lattice vibration with respect to the wavenumber \(k\).

Spring constant

$$ D $$ Unit $$ \frac{\mathrm{kg}}{\mathrm{s}^2} $$

Spring constant for the coupling between neighboring lattice planes (atomic chains) of the crystal lattice.

Mass

$$ m $$ Unit $$ \mathrm{kg} $$

Mass of an atom inside a lattice chain.

Lattice constant

$$ a $$ Unit $$ \mathrm{m} $$

Lattice constant is the distance between adjacent lattice planes at equilibrium.

Wave number

$$ k $$ Unit $$ \frac{1}{\mathrm m} $$

Wavenumber represents the number of oscillations performed within the wavelength \( \lambda \): \( k = \frac{2\pi}{\lambda} \). The sign of the wavenumber indicates the direction of propagation of the oscillation.

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