Hidden robust presence of a hole Fermi surface in a heavily electron doped iron based superconductor LaFe${}_2$As${}_2$

We investigate the electronic structure of a recently discovered, heavily electron-doped iron based superconductor LaFe${}_2$As${}_2$. Although first principles calculation shows apparent absence of hole Fermi surfaces around the $\mathrm{\Gamma }$ point, we reveal, by hypothetically removing the La $d$ orbital contribution, that a hole Fermi surface around the $\mathrm{\Gamma }$ point is essentially present. In the collapsed phase of LaFe${}_2$As${}_2$, which is non-superconducting, the hole Fermi surface is found to be absent, and the difference from the uncollapsed superconducting phase can be naturally understood within the spin-fluctuation mediated pairing scenario.