  We investigate the angular momentum of mono-abundance populations (MAPs) of
the Milky Way thick disk by using a sample of 26,076 giant stars taken from
APOGEE DR17 and Gaia EDR3. The vertical and perpendicular angular momentum
components, $L_Z$ and $L_P$, of MAPs in narrow bins have significant variations
across the [$\alpha$/M]-[M/H] plane. $L_Z$ and $L_P$ systematically change with
[M/H] and [$\alpha$/M] and can be alternatively quantified by the chemical
gradients: $d[{\rm M/H}]/dL_Z = 1.2\times 10^{-3}
$\,dex\,kpc$^{-1}$\,km$^{-1}$\,s, $d{\rm [M/H]}/dL_P = -5.0\times
10^{-3}$\,dec\,kpc$^{-1}$\,km$^{-1}$\,s, and $d[\alpha/{\rm M}]/dL_Z =
-3.0\times 10^{-4} $\,dex\,kpc$^{-1}$\,km$^{-1}$\,s, $d[\alpha/{\rm M}]/dL_P =
1.2\times 10^{-3}$\,dec\,kpc$^{-1}$\,km$^{-1}$\,s. These correlations can also
be explained as the chemical-dependence of the spatial distribution shape of
MAPs. We also exhibit the corresponding age dependence of angular momentum
components. Under the assumption that the guiding radius ($R_g$) is
proportional to $L_Z$, it provides direct observational evidence of the
inside-out structure formation scenario of the thick disk, with $dR_g/dAge =
-1.9$\,kpc\,Gyr$^{-1}$. The progressive changes in the disk thickness can be
explained by the upside-down formation or/and the consequent kinematical
heating.
