Asymptotic Freedom from Thermal and Vacuum Magnetization
  We calculate the effective Lagrangian for a magnetic field in spinor, scalar and vector QED. Connections are then made to $SU(N_C)$ Yang--Mills theory and QCD. The magnetization and the corresponding effective charge are obtained from the effective Lagrangian. The renormalized vacuum magnetization will depend on the renormalization scale chosen. Regardless of this, the effective charge decreasing with the magnetic field, as in QCD, corresponds to anti- screening and asymptotic freedom. In spinor and scalar QED on the other hand, the effective charge is increasing with the magnetic field, corresponding to screening. Including effects due to finite temperature and density, we comment on the effective charge in a degenerate fermion gas, increasing linearly with the chemical potential. Neglecting the tachyonic mode, we find that in hot QCD the effective charge is decreasing as the inverse temperature, in favor for the formation of a quark-gluon plasma. However, including the real part of the contribution from the tachyonic mode, we find instead an effective charge increasing with the temperature. Including a thermal gluon mass, the effective charge in hot QCD is group invariant (unlike in the two cases above), and decreases logarithmically in accordance to the vacuum renormalization group equation, with the temperature as the momentum scale. 
David Persson