  Iridate thin films are a prerequisite for any application utilizing their
cooperative effects resulting from the interplay of stron spin-orbit coupling
and electronic correlations. Here, heteroepitaxial Na2IrO3 thin films with
excellent (001) out-of-plane crystalline orientation and well defined in-plane
epitaxial relationship are presented on various oxide substrates. Resistivity
is dominated by a three-dimensional variable range hopping mechanism in a large
temperature range between 300 K and 40 K. Optical experiments show the onset of
a small optical gap of about 200 meV and a splitting of the Ir 5d-t2g manifold.
Positive magnetoresistance below 3 T and 25 K shows signatures of a weak
antilocalization effect. This effect can be associated with surface states in a
topological insulator and hence supports proposals for a topological insulator
phase present in Na2IrO3.
