  We search for the production of single top quarks via flavor-changing neutral
current couplings of a gluon to the top quark and a charm (c) or up (u) quark.
We analyze 230 pb^{-1} of lepton+jets data from \ppbar collisions at a center
of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab
Tevatron Collider. We observe no significant deviation from standard model
predictions, and hence set upper limits on anomalous coupling parameters
\kappa^c_g/\Lambda and \kappa^u_g/\Lambda, where the \kappa_g define the
strength of the tcg and tug couplings, and \Lambda defines the scale of new
physics. The limits at 95% C.L. are: \kappa^c_g/\Lambda < 0.15 TeV^{-1} and
\kappa^u_g/\Lambda < 0.037 TeV^{-1}.
