  We propose a novel geometric model of three-dimensional topological
insulators in presence of an external electromagnetic field. The gapped
boundary of these systems supports relativistic quantum Hall states and is
described by a Chern-Simons theory with a gauge connection that takes values in
the Maxwell algebra. This represents a non-central extension of the Poincar\'e
algebra and takes into account both the Lorentz and magnetic-translation
symmetries of the surface states. In this way, we derive a relativistic version
of the Wen-Zee term, and we show that the non-minimal coupling between the
background geometry and the electromagnetic field in the model is in agreement
with the main properties of the relativistic quantum Hall states in the flat
space.
