  The surface states of a topological insulator are described by an emergent
relativistic massless Dirac equation in 2+1 dimensions. In contrast to
graphene, there is an odd number of Dirac points, and the electron spin is
directly coupled to the momentum. We show that a magnetic impurity opens up a
local gap and suppresses the local density of states. Furthermore, the Dirac
electronic states mediate an RKKY interaction among the magnetic impurities
which is always ferromagnetic, whenever the chemical potential lies near the
Dirac point. These effects can be directly measured in STM experiments. We also
study the case of quenched disorder through a renormalization group analysis.
