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_(based on chapter 2 of the book)_
Exercises: 2.3, 2.4, 2.5, 2.6, 2.8
In this lecture we will:
!!! summary "Learning goals"
- discuss specific heat of a solid based on atomic vibrations (_phonons_)
- disregard periodic lattice $\Rightarrow$ consider homogeneous medium
- _(chapter 9: discuss phonons in terms of atomic masses and springs)_
- discuss the Einstein model
- discuss the Debye model
- introduce reciprocal space, periodic boundary conditions and _density of states_
After this lecture you will be able to:
- Explain quantum mechanical effects on the heat capacity of solids (Einstein model)
- Compute occupation number, energy and heat capacity of a bosonic particle
- Write down the total thermal energy of a material
### Einstein model
Before solid state physics: heat capacity per atom $C=3k_{\rm B}$ (Dulong-Petit). Each atom is (classical) harmonic oscillator in three directions. Experiments showed that this law breaks down at low temperatures, where $C$ reduces to zero ($C\propto T^3$).
@@ -128,6 +127,13 @@ $$E=\int\limits_0^\infty\left(\frac{1}{2}\hbar\omega+\frac{\hbar\omega}{ {\rm e}
$g(\omega)$ is the _density of states_: the number of normal modes found at each position along the $\omega$-axis. How do we calculate $g(\omega)$?
!!! summary "Learning goals"
After this lecture you will be able to:
- Describe the concept of reciprocal space and allowed momenta
- Write down the total energy of phonons given the temperature and the dispersion relation
- Estimate heat capacity due to phonons in high temperature and low temperature regimes
#### Reciprocal space, periodic boundary conditions
Each normal mode can be described by a _wave vector_ ${\bf k}$. A wave vector represents a point in _reciprocal space_ or _k-space_. We can find $g(\omega)$ by counting the number of normal modes in k-space and then converting those to $\omega$.
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