  We study the low-energy properties of three-dimensional (3D) topological Mott
insulators which can be viewed as strong topological insulators of spinons
interacting with a three-dimensional gauge field. The low-energy behavior of
such systems is dominated by the two-dimensional (2D) gapless surface spinons
coupled to the bulk gauge field. We find that a dimensional crossover from 3D
to 2D in the gauge field fluctuations may occur as the systems thickness and/or
temperature is varied. In the thin sample limit, the gauge field fluctuations
effectively become 2D and the problem becomes analogous to the standard 2D
spinon-gauge field theory. In the 3D limit, the bulk gauge field fluctuations
lead to a low-energy theory for the coupled system that is more controlled than
for the pure 2D case. We discuss various experimental signatures such as the
heat capacity scaling as T ln 1/T as well as modified
Ruderman-Kittel-Kasuya-Yoshida interactions on the surface.
