  Let $\Gamma$ be a simplicial, finite, connected graph such that $\Gamma$ does
not decompose as a nontrivial join. We prove that two notions of strong
quasiconvexity and stability are equivalent in the right-angled Artin group
$A_\Gamma$ (except for the case of finite index subgroups). We also
characterize non-trivial strongly quasiconvex subgroups of infinite index in
$A_\Gamma$ (i.e. non-trivial stable subgroups in $A_\Gamma$) by quadratic lower
relative divergence. These results strengthen the work of
Koberda-Mangahas-Taylor on characterizing purely loxodromic subgroups of
right-angled Artin groups.
