  Whether the newly discovered KxFe2-ySe2 systems are doped Mott or band
insulators is key to how superconductivity emerges at lower temperature. With
extant theoretical studies supporting conflicting scenarios, a more realistic
approach is urgently called for. Here, we use LDA+DMFT to study this issue in
KxFe2-ySe2. We find that the undoped KFe1.6Se2 system is a new kind of
Mott-Kondo insulator (MKI). Electron doping this MKI drives a Mott transition
to an orbital-selective non-Fermi liquid metal. Good agreement with spectral
and transport responses supports our view, implying that superconductivity
arises from a doped Mott insulator, as in the high-Tc cuprates.
