  In recent attempts to observe axion electrodynamics, much effort has focused
on trilayer heterostructures of magnetic topological insulators, and in
particular on the examination of a so-called zero Hall plateau, which has
misguidedly been overstated as direct evidence of an axion insulator state. We
investigate the general notion of axion insulators, which by definition must
contain a nontrivial volume to host the axion term. We conduct a detailed
magneto-transport analysis of Chern insulators comprised of a single magnetic
topological insulator layer of varying thickness as well as trilayer
structures, for samples optimized to yield a perfectly quantized anomalous Hall
effect. Our analysis gives evidence for a topological magneto-electric effect
quantized in units of e$^2$/2h, allowing us to identify signatures of axion
electrodynamics. Our observations may provide direct experimental access to
electrodynamic properties of the universe beyond the traditional Maxwell
equations, and challenge the hitherto proclaimed exclusive link between the
observation of a zero Hall plateau and an axion insulator.
