Solution to Set 5

I have no idea what I'm doing - KimberlyWynne 03:11, 2 March 2009 (EST)

Diatomic harmonic chain

Problem 1

Given:

• a chain of atoms
• with alternating masses $m_{1}\;$ and $m_{2}\;$
• connected with elastic springs with constant $K\;$
• moving only in the x-direction

Derive the dispersion relation $\omega ^{\alpha }(k)\;$ for this chain

• Index $\alpha =1\;$ for acoustic branch
• Index$\alpha =2\;$ for optical branch

Potential Energy $U=1,2,3,...n\;$

$U\cong {\frac {1}{2}}k\sum _{n}(U_{n}-U_{n-1})\;$

$\Rightarrow m{\ddot {U}}_{n}=-k[2U_{n}-U_{n-1}-U_{n+1}]\;$

Eigenvector Value

$u(t)=e^{i\omega t}u_{n}\;$

$\Rightarrow -m\omega ^{2}{\vec {u}}=-k\mathbf {M} {\vec {u}}$

Problem 2

Determine the speed of sound for this chain. What is the lowest frequency of long-wavelength sound corresponding to the optical branch?

From my lecture notes: $\omega _{\alpha }(k)\approx C_{\alpha }k\;$

where $C_{\alpha }$ = speed of sound

Problem 3

Sketch the motion of the atoms corresponding to the edge of the Brillouin zone, both for the optical and the acoustic branch.

Problem 4

Determine the Debye temperature for this system, and determine the form of the specific heat $c_{V}(T)$ in the limits of high and low temperatures.

Problem 5

Consider low temperatures ($T\ll T_{D}\;$) and determine the wavelength of the most abundant phonons $\lambda _{max}$ (Hint: note the analogy with Wien's Law!)