Absence of the diffusion pole in the Anderson insulator
  We discuss conditions for the existence of the diffusion pole and its consequences in disordered noninteracting electron systems. Using only nonperturbative and exact arguments we find against expectations that the diffusion pole can exist only in the diffusive (metallic) regime. We demonstrate that the diffusion pole in the Anderson localization phase would lead to nonexistence of the self-energy and hence to a physically inconsistent picture. The way how to consistently treat and understand the Anderson localization transition with vanishing of the diffusion pole is presented. 
V. Janis and J. Kolorenc