Harmonic-chain dispersion and phonons
A monatomic chain with mass m, spacing a and nearest-neighbour spring constant κ has ω(k)=2√(κ/m)|sin(ka/2)|.
Key knowledge
A monatomic chain with mass m, spacing a and nearest-neighbour spring constant κ has ω(k)=2√(κ/m)|sin(ka/2)|. Quantization turns each normal mode into an oscillator with energy ħω(n+½); its quanta are phonons and its k labels repeat modulo 2π/a.
Values and equations
m ü_j=κ(u_{j+1}+u_{j−1}−2u_j); ω(k)=2√(κ/m)|sin(ka/2)|
- m
- Atomic mass kg
- κ
- Spring constant N m⁻¹
- u_j,a
- Displacement and equilibrium spacing m
- ω
- Angular frequency rad s⁻¹
- k
- Wavenumber m⁻¹
- j
- Site index
Conditions and scope
- Small longitudinal displacements, ideal harmonic nearest-neighbour chain and m,κ,a>0.
- Phonons carry crystal momentum; it is distinct from the full mechanical momentum of the entire crystal. Anharmonic interactions and higher-dimensional branches need extra structure.
Sources · 1
References
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Evidence and review
Solid State Physics: third-year lecture notes (2012): Chapter 8, equations 8.1–8.3, reciprocal periodicity §8.1 and quantum mode discussion §8.3.
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Entry details
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