  We present results from a detailed study of singlet free energies in full QCD
with realistic quark masses. An improved scheme for the non-perturbative
renormalization of the Polyakov loop is used and we compare its temperature
dependence for QCD with different flavor content. We also analyze screening
masses extracted from singlet free energies at various temperatures close to
and above the QCD transition temperature. We conclude that the temperature
dependence of screening masses is well described by perturbation theory up to a
non-perturbative pre-factor. An effective running coupling has been determined
for all temperature values giving additional insight into screening phenomena
at high temperature.
