Acoustic and optical lattice branches
For a stable three-dimensional harmonic crystal with s atoms per primitive cell, the displacement dynamical matrix has 3s branches.
Key knowledge
For a stable three-dimensional harmonic crystal with s atoms per primitive cell, the displacement dynamical matrix has 3s branches. Translational invariance gives three acoustic branches with ω→0 as k→0; ordinary remaining branches have finite zone-center frequency and are called optical.
Conditions and scope
- Bulk three-dimensional crystal, positive masses, stable harmonic equilibrium and no extra soft mode; long-range electrostatic forces can split longitudinal/transverse optical frequencies.
- Optical is a branch label and does not guarantee optical activity. Low-dimensional systems and unstable/critical modes need separate counting and limiting behavior.
Sources · 1
References
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Evidence and review
Solid State Physics: third-year lecture notes (2012): Chapter 9 diatomic acoustic/optical branches and §12.3 six modes of a two-atom primitive cell, three acoustic limits and three optical modes.
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