  Polyakov loop correlations at finite temperature in two-flavor QCD are
studied in lattice simulations with the RG-improved gluon action and the
clover-improved Wilson quark action. From the simulations on a $16^3 \times 4$
lattice, we extract the free energies, the effective running coupling $g_{\rm
eff}(T)$ and the Debye screening mass $m_D(T)$ for various color channels of
heavy quark--quark and quark--anti-quark pairs above the critical temperature.
The free energies are well approximated by the screened Coulomb form with the
appropriate Casimir factors. The magnitude and the temperature dependence of
the Debye mass are compared to those of the next-to-leading order thermal
perturbation theory and to a phenomenological formula given in terms of $g_{\rm
eff}(T)$. Also we made a comparison between our results with the Wilson quark
and those with the staggered quark previously reported.
