Stability of the Magnetic Monopole Condensate in three- and four-colour   QCD
  It is argued that the ground state of three- and four-colour QCD contains a monopole condensate, necessary for the dual Meissner effect to be the mechanism of confinement, and support its stability on the grounds that it gives the off-diagonal gluons an effective mass sufficient to remove the unstable ground state mode. 
M. L. Walker