  Two families of high temperature superconductors whose critical temperatures
are higher than 50K are known. One is the copper oxides and the other is the
iron-based superconductors. Comparisons of mechanisms between these two in
terms of common ground as well as distinctions will greatly help in searching
for higher Tc superconductors. However, studies on mechanisms for the iron
family based on first principles calculations are few. Here we first show that
the superconductivity emerges in the state-of-the-art numerical calculations
for an ab initio multi-orbital model of an electron-doped iron-based
superconductor LaFeAsO, in accordance with experimental observations. Then the
mechanism of the superconductivity is identified as enhanced uniform density
fluctuations by one-to-one correspondence with the instability toward
inhomogeneity driven by first-order antiferromagnetic and nematic transitions.
Despite many differences, certain common features with the copper oxides are
also figured out in terms of the underlying orbital selective Mottness found in
the iron family.
