  In this work, we construct a phenomenological effective model for the
heavy-light quark systems, which consist of (u,d,c,b) quarks, i.e. extended
nonlocal chiral-quark model (ExNLChQM) to compute the heavy-meson weak-decay
constants f_D and f_B. ExNLChQM is based on the heavy-quark effective field
theory as well as the dilute instanton-vacuum configuration. In ExNLChQM, a
certain portion of the heavy-meson mass is considered to be generated from the
nontrivial QCD vacuum contribution, being similar to that for the light quarks
in usual instanton approaches. Hence, the effective heavy- and light-quark
masses become momentum-dependent and play the role of a smooth UV regulator.
Employing a generic external-field method applied to the effective action from
ExNLChQM, we obtain f_D = (169.28 ~ 234.57) MeV and f_B = (165.41 ~ 229.21) MeV
from the numerical results, depending on different model parameters. These
values are in relatively good agreement with experimental data and various
theoretical estimations. We also discuss the heavy-quark effects on the QCD
vacuum, and the decay constants f_D^* and f_B^* in terms of the heavy-quark
spin symmetry.
