  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 the anomalous coupling
parameters $\kappacLambda$ and $\kappauLambda$, where $\kappag$ define the
strength of $tcg$ and $tug$ couplings, and $\Lambda$ defines the scale of new
physics. The limits at 95% C.L. are: $\kappacLambda < 0.15 \rm TeV^{-1}$ and
$\kappauLambda < 0.037 \rm TeV^{-1}$.
