Stability of the perturbative vacuum against spatial variations of the   Polyakov loop
  We investigate the effective action of the Polyakov loop with spatial variations. We expand the effective action not in powers of derivatives or momenta, but in powers of variational amplitudes. At one-loop order the results suggest that the instability towards the confining vacuum may be caused by the variational terms. 
Kenji Fukushima and Koichi Ohta