diff --git a/1_einstein_model_solutions.md b/1_einstein_model_solutions.md new file mode 100644 index 0000000000000000000000000000000000000000..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 diff --git a/mkdocs.yml b/mkdocs.yml index 8a11331100d6b277e82826b8683a1c28c786b533..7f687da3f0602c1e2a833acd755231adf4091317 100644 --- a/mkdocs.yml +++ b/mkdocs.yml @@ -32,6 +32,7 @@ nav: - Fermi surface periodic table: 'fermi_surfaces.md' - Extra exercises: 'extra_exercises.md' - Solutions: + - Lecture 0: '1_einstein_model_solutions.md' - Lecture 1: 'solutions/1_einstein_model.md' - Lecture 2: 'solutions/2_debye_model.md' diff --git a/src/1_einstein_model_solutions.md b/src/1_einstein_model_solutions.md new file mode 100644 index 0000000000000000000000000000000000000000..e549897bb1d42ae94f5f78f50291e53f77ca9881 --- /dev/null +++ b/src/1_einstein_model_solutions.md @@ -0,0 +1,85 @@ +# Solutions for lecture 1 exercises + +### Exercise 1: Heat capacity of a classical oscillator. + +1. + +$$ +Z = \int_{-\infty}^{\infty}dp \int_{-\infty}^{\infty} dx e^{-\frac{\beta}{2m}p^2-\frac{\beta k}{2}x^2} = \sqrt{\frac{2\pi m}{\beta}}\sqrt{\frac{2\pi}{\beta k}} = \frac{2\pi}{\beta}\sqrt{\frac{m}{k}}, +$$ + +where we used $\int_{-\infty}^{\infty}e^{-\alpha x^2} = \sqrt{\frac{\pi}{\alpha}}$. + +2. + +$$ +\langle E \rangle = -\frac{1}{Z}\frac{\partial Z}{\partial \beta} = \frac{1}{\beta} +$$ + +3. + +$$ +C = \frac{\partial\langle E\rangle}{\partial T} = k_B +$$ + +4. + +Since this is a 1D system, the prefactor is $1$ as expected. + +### Exercise 2: Quantum harmonic oscillator. + +1. + +Take a look into the graph from the notes. + +2. + +$$ +Z = \sum_{n = 0}^{\infty} e^{-\beta\hbar\omega(n + 1/2)} = e^{-\beta\hbar\omega/2}\frac{1}{1 - e^{-\beta\hbar\omega}} = \frac{1}{2\sinh(\beta\hbar\omega/2)}, +$$ + +where we used $\sum_{n = 0}^{\infty}r^n = \frac{1}{1 - r}$. + +3. + +$$ +\langle E\rangle = -\frac{1}{Z}\frac{\partial Z}{\partial\beta} = \frac{\hbar\omega}{2}\coth\frac{\beta\hbar\omega}{2} = \hbar\omega\left(\frac{1}{e^{\beta\hbar\omega} - 1} + \frac{1}{2}\right) = \hbar\omega\left(n_B(\beta\hbar\omega) + \frac{1}{2}\right). +$$ + +4. + +$$ +C = \frac{\partial \langle E\rangle}{\partial T} = \frac{\partial\langle E\rangle}{\partial\beta}\frac{\partial\beta}{\partial T} = k_B(\beta\hbar\omega)^2\frac{e^{\beta\hbar\omega}}{(e^{\beta\hbar\omega} - 1)^2}. +$$ + +In the high temperature limit $\beta \rightarrow 0$ and $e^{\beta\hbar\omega} \approx 1 + \beta\hbar\omega$, so $C \rightarrow k_B$ which is the same result as in Exercise 1.1. + +5. + +We can see that for $\beta\hbar\omega = 1$ we already have $C \approx 0.92k_B$, so we can define a characteristic temperature $T_E = \hbar\omega/k_B$ such that temperatures above this one can be considered high. + +6. + +$$ +\langle n\rangle = \frac{1}{Z}\sum_{n = 0}^{\infty} ne^{-\beta\hbar\omega(n + 1/2)} = 2\frac{e^{\beta\hbar\omega/2} - e^{-\beta\hbar\omega/2}}{2}e^{-\beta\hbar\omega/2}\frac{e^{-\beta\hbar\omega}}{(1 - e^{-\beta\hbar\omega})^2} = \frac{1}{e^{\beta\hbar\omega} - 1}, +$$ + +where we used $\sum_{n = 0}^{\infty}nr^n = \frac{r}{(1 - r)^2}$. + +### Exercise 3: Total heat capacity of a diatomic material. + +1. + +Use the formula $\omega = \sqrt{\frac{k}{m}}$. + +2. + +$E = N_{^6Li}\hbar\omega_{^6Li}(2 + 1/2)+N_{^7Li}\hbar\omega_{^7Li}(4 + 1/2)$. + +3. + +$E = N_{^6Li}\hbar\omega_{^6Li}\left(n_B(\beta\hbar\omega_{^6Li}) + \frac{1}{2}\right) + N_{^7Li}\hbar\omega_{^7Li}\left(n_B(\beta\hbar\omega_{^7Li}) + \frac{1}{2}\right)$. + +4. + +$C = \frac{N_{^6Li}}{N}C_{^6Li} + \frac{N_{^7Li}}{N}C_{^7Li}$ where the heat capacities are calculated with the formula from Excercise 2.4.