A topological insulator is a material that respects time reversal invariance
but nevertheless has an effective theta term in the electromagnetic action.
Although the system is an insulator in the bulk, there can be currents at the
interface between a normal and a topological insulator. In this case one can
also observe a quantum Hall effect applying a magnetic field to the interface.
It has been recently argued that a fractional quantum Hall effect could also
be produced if quasiparticles carrying fractional charge exist in the
material, in this sense we talk about fractional topological insulators. This
is only possible if the theory has non-trivial and usually strong
interactions. I explain what kind of effective theories could describe
fractional topological insulators and give an specific example using a
holographic construction involving D7 probe branes in a D3 near horizon geometry.
