  The electromagnetic Casimir effect has a fermionic counterpart in topological
insulators: Zero-point fluctuations of a massless Dirac fermion field mediate a
force between magnetic scatterers. The Casimir force is insensitive to disorder
that preserves the topological protection of an unpaired Dirac cone. The
protection may be broken if the Dirac equation is discretized, and an
exponential suppression of the Casimir effect will result if a gap opens at the
Dirac point. Here we show how this lattice artefact may be avoided, by applying
a recently developed local discretization of the Euclidean action that does not
suffer from the fermion-doubling obstruction of local discretizations of the
Hamiltonian.
