  We investigate quark potential by considering meson exchanges in the two
flavor Nambu--Jona-Lasinio model at finite temperature and density. There are
two kinds of oscillations in the chiral restoration phase, one is the Friedel
oscillation due to the sharp quark Fermi surface at high density, and the other
is the Yukawa oscillation driven by the complex meson poles at high
temperature. The quark-meson plasma is strongly coupled in the temperature
region $1\le T/T_c \lesssim 3$ with $T_c$ being the critical temperature of
chiral phase transition. The maximum coupling in this region is located at the
critical point.
