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In the studies of iron-pnictides, a key question is whether their bad-metalstate from which the superconductivity emerges lies in close proximity with amagnetically ordered insulating phase. Recently it was found that at lowtemperatures, the heavily Cu-doped NaFe$_{1-x}$Cu$_x$As ($x > 0.3$)iron-pnictide is an insulator with long-range antiferromagnetic order, similarto the parent compound of cuprates but distinct from all other iron-pnictides.Using angle-resolved photoemission spectroscopy, we determined themomentum-resolved electronic structure of NaFe$_{1-x}$Cu$_x$As ($x = 0.44$) andidentified that its ground state is a narrow-gap insulator. Combining theexperimental results with density functional theory (DFT) and DFT+Ucalculations, our analysis reveals that the on-site Coulombic (Hubbard) andHund's coupling energies play crucial roles in formation of the band gap aboutthe chemical potential. We propose that at finite temperatures charge carriersare thermally excited from the Cu-As-like valence band into the conductionband, which is of Fe $3d$-like character. With increasing temperature, thenumber of electrons in the conduction band becomes larger and the hoppingenergy between Fe sites increases, and finally the long-range antiferromagneticorder is destroyed at $T > T_\mathrm{N}$. Our study provides a basis forinvestigating the evolution of the electronic structure of a Mott insulatortransforming into a bad metallic phase, and eventually forming asuperconducting state in iron-pnictidesa superconducting state iniron-pnictides.

NaFe$_{0.56}$Cu$_{0.44}$As: A pnictide insulating phase induced by on-site Coulomb interaction