  Incorporated with twisted boundary condition, Polyakov loop correlators can
give a definition of the renormalized coupling. We employ this scheme for the
step scaling method (with step size s = 2) in the search of conformal fixed
point of SU(3) gauge theory with 12 massless flavors. Staggered fermion and
plaquette gauge action are used in the lattice simulation with six different
lattice sizes, L/a = 20, 16, 12, 10, 8 and 6. For the largest lattice size, L/a
= 20, we used a large number of Graphics Processing Units (GPUs) and
accumulated 3,000,000 trajectories in total. We found that the step scaling
function sigma (u) is consistent with u in the low-energy region. This means
the existence of conformal fixed point. Some details of our analysis and
simulations will also be presented.
