'Pi State' Induced by Impurities with a Repulsive Interaction
  We study the properties of a quantum impurity embedded in a superconducting host. The superconductor is described by the negative U Hubbard model while the impurity introduces a repulsive interaction to the system. We discuss the influence of this `on-impurity' Coulomb repulsion on the local properties (density of states, electron pairing) of the superconductor. We show the condition of pi - like behaviour, defined as two subsystems having a phase difference of pi, in the system by using a proper combination of attractive pairing interaction and repulsive one located at impurity site. 
Grzegorz Litak and Mariusz Krawiec