add a missing hbar

docs/7_tight_binding.md

@@ -357,7 +357,7 @@ $$
 and
 
 $$
-|v|^{-1} = \left|\frac{dk}{dE} \right| = \frac{1}{a}\frac{1}{\sqrt{4t^2 - (E - E_0)^2}}.
+|v|^{-1} = \hbar \left|\frac{dk}{dE} \right| = \frac{\hbar}{a}\frac{1}{\sqrt{4t^2 - (E - E_0)^2}}.
 $$
 
 You can get to this result immediately if you remember the derivative of arccosine. Otherwise you need to go the long way: compute $dE/dk$ as a function of $k$, express $k$ through $E$ as we did above, and take the inverse.