Impurities in Holography and Transport Coefficients
  We present a way to include impurities in AdS/CFT correspondence, in view of its application to condensed matter physics. Examples of these are the current impurity and spin impurity. We calculate electric conductivity and spin susceptibility of holographic superconductors, with doping of density/spin impurities. 
Koji Hashimoto, Norihiro Iizuka
hep-th cond-mat.str-el