  We investigate the chiral phase transition at finite temperature (T) in
colour SU(3) Quantum Chromodynamics (QCD) with six species of fermions (Nf = 6)
in the fundamental representation. The simulations have been performed by using
lattice QCD with improved staggered fermions. The critical couplings (bc) for
the chiral phase transition are observed for several temporal extensions Nt,
and the two-loop asymptotic scaling of the dimensionless ratio Tc/Lambda_L
(Lambda_L = Lattice Lambda-parameter) is found to be achieved for Nt >= 6.
Further, we collect bc at Nf = 0 (quenched), and Nf = 4 at a fixed Nt = 6 as
well as Nf = 8 at Nt = 6 and 12, the latter relying on our earlier study. The
results are consistent with enhanced fermionic screening at larger Nf. The
ratio Tc/Lambda_L depends very mildly on Nf in the Nf = 0-4 region, begins
increasing at Nf = 6, and significantly grows up at Nf = 8, as Nf reaches to
the edge of the conformal window. We discuss the interrelation of the results
with preconformal dynamics in the light of a functional renormalization group
analysis.
