  In this talk, we report our recent studies on an effective thermodynamic
potential (Omega_eff) at finite temperature (T>0) and zero quark-chemical
potential (mu_R=0), using the singular-gauge instanton solution and Matsubara
formula for N_c=3 and N_f=2 in the chiral limit, i.e. m_q=0. The
momentum-dependent constituent-quark mass is computed as a function of T,
together with the Harrington-Shepard caloron solution in the large-N_c limit.
In addition, we take into account the imaginary quark-chemical potential mu_I =
A_4, indentified as the traced Polayakov-loop (Phi) as an order parameter for
the Z(N_c) symmetry, characterizing the confinement (intact) and deconfinement
(spontaneously broken) phases. As a consequence, we observe the crossover of
the chiral (chi) order parameter sigma^2 and Phi. It also turns out that the
critical temperature for the deconfinement phase transition, T^Z_c is lowered
by about (5~10) % in comparison to the case with the constant constituent-quark
mass. This behavior can be understood by considerable effects from the partial
chiral restoration and nontrivial QCD vacuum on the Phi. Numerical results show
that the crossover transitions occur at (T^chi_c,T^Z_c) ~ (216,227) MeV.
