use own version of einstein's figure

docs/lecture_1.md

@@ -49,7 +49,7 @@ This is for just one atom. In order to obtain the heat capacity of a full materi
 ### Debye model
 The Einstein model explained the experimental data quite well, but still slightly underestimated the observed values of $C$. Apparently the "each atom is an oscillator"-idea is too simplistic.
 
-![](figures/einstein.png)
+![](figures/einstein.svg)
 
 Peter Debye (1884 – 1966) suggested to instead consider _normal modes_: sound waves that propagate through a solid with velocity $v=\omega/k$, where $k=2\pi/\lambda$ is the _wave number_. Instead of just multiplying $\bar{\varepsilon}$ by $3N$, we now use: