add QHE example

content/index.txt

@@ -70,6 +70,29 @@ wave function is peaked near the zigzag segments of the boundary, as `expected
 Taken from the Kwant `plotting tutorial <doc/1.0/tutorial/tutorial6.html>`_.
 
 
+Quantum Hall effect
+...................
+
+.. container:: rightside
+
+   .. image:: qhe-edgestate.png
+
+.. container:: leftside
+
+   .. image:: qhe-plateaus.png
+
+One of the most common applications of Kwant is to calculate the conductance of
+a nonoelectronic system.  The plot on the left shows the conductance through a
+2-d electron gas as a function of magnetic flux.  The quantization of
+conductance that is visible (plateaus) is the hallmark of the quantum Hall
+effect.  The third plateau around 1 / Φ = 40 does not develop due to a
+constriction in the system that leads to backscattering.  The situation at this
+value of magnetic field is shown on the right.
+
+Taken from example 6 of the `Kwant paper
+<http://downloads.kwant-project.org/doc/kwant-paper.pdf>`_.
+
+
 3-d system: Majorana states
 ...........................