Pairing and persistent currents - the role of the far levels
  We calculate the orbital magnetic response to Aharonov Bohm flux of disordered metallic rings with attractive pairing interaction. We consider the reduced BCS model, and obtain the result as an expansion of its exact solution to first order in the interaction. We emphasize the connection between the large magnetic response and the finite occupation of high energy levels in the many-body ground state of the ring. 
M. Schechter, Y. Imry, Y. Levinson, and Y. Oreg