Topological Entropy of Left-Invariant Magnetic Flows on 2-Step Nilmanifolds

We consider magnetic flows on 2-step nilmanifolds $M=\mathrm{\Gamma }\mathrm{\backslash }G$, where the Riemannian metric $g$ and the magnetic field $\sigma$ are left-invariant. Our first result is that when $\sigma$ represents a rational cohomology class and its restriction to $\mathfrak{g}=T_{e}G$ vanishes on the derived algebra, then the associated magnetic flow has zero topological entropy. In particular, this is the case when $\sigma$ represents a rational cohomology class and is exact. Our second result is the construction of a magnetic field on a 2-step nilmanifold that has positive topological entropy for arbitrarily high energy levels.