Thermodynamics of localized magnetic moments in a Dirac conductor
  We show that the magnetic susceptibility of a dilute ensemble of magnetic impurities in a conductor with a relativistic electronic spectrum is non-analytic in the inverse tempertature at $1/T\to 0$. We derive a general theory of this effect and construct the high-temperature expansion for the disorder averaged susceptibility to any order, convergent at all tempertaures down to a possible ordering transition. When applied to Ising impurities on a surface of a topological insulator, the proposed general theory agrees with Monte Carlo simulations, and it allows us to find the critical temperature of the ferromagnetic phase transition. 
V. Cheianov, M. Szyniszewski, E. Burovski, Yu. Sherkunov, and V.   Fal'ko
cond-mat.stat-mech cond-mat.dis-nn