The evolution of stellar metallicity gradients of the Milky Way disk
  from LSS-GAC main sequence turn-off stars: a two-phase disk formation
  history?
  We use 297 042 main sequence turn-off stars selected from the LSS-GAC to
determine the radial and vertical gradients of stellar metallicity of the
Galactic disk in the anti-center direction. We determine ages of those turn-off
stars by isochrone fitting and measure the temporal variations of metallicity
gradients. Our results show that the gradients, both in the radial and vertical
directions, exhibit significant spatial and temporal variations. The radial
gradients yielded by stars of oldest ages (>11 Gyr) are essentially zero at all
heights from the disk midplane, while those given by younger stars are always
negative. The vertical gradients deduced from stars of oldest ages (>11Gyr) are
negative and show only very weak variations with the Galactocentric distance in
the disk plane, $R$, while those yielded by younger stars show strong
variations with $R$. After being essentially flat at the earliest epochs of
disk formation, the radial gradients steepen as age decreases, reaching a
maxima (steepest) at age 7-8 Gyr, and then they flatten again. Similar temporal
trends are also found for the vertical gradients. We infer that the assemblage
of the Milky Way disk may have experienced at least two distinct phases. The
earlier phase is probably related to a slow, pressure-supported collapse of
gas, when the gas settles down to the disk mainly in the vertical direction. In
the later phase, there are significant radial flows of gas in the disk, and the
rate of gas inflow near the solar neighborhood reaches a maximum around a
lookback time of 7-8 Gyr. The transition of the two phases occurs around a
lookback time between 8 and 11 Gyr. The two phases may be responsible for the
formation of the Milky Way thick and thin disks, respectively. And, as a
consequence, we recommend that stellar age is a natural, physical criterion to
distinguish thin and thick disk stars. ... (abridged)
