Indications for Gluon Condensation from Nonperturbative Flow Equations
  We employ nonperturbative flow equations for the description of the effective action in Yang-Mills theories. We find that the perturbative vacuum with vanishing gauge field strength does not correspond to the minimum of the Euclidean effective action. The true ground state is characterized by a nonvanishing gluon condensate. 
M. Reuter, C. Wetterich