Flat band induced quantum criticality in a nonsuperconducting iron pnictide

Flat electronic bands at the Fermi energy $\left(E_F\right)$ can induce interaction-driven instabilities and further result in the emergence of a plethora of new quantum phases such as Mott insulators, ferromagnetism, fractional quantum Hall states even at high temperatures, and superconductivity. Except for flat bands in $f$-electron systems and special geometric lattices, however, the materials with quantum criticality induced by flat bands just at $E_F$ remain elusive. Here, by using angle-resolved photoemission spectroscopy and band structure calculations, we present a comprehensive study of the low-energy electronic structure of a nonsuperconducting iron pnictide, $\mathrm{Ba}{\left({\mathrm{Fe}}_{1/3}{\mathrm{Co}}_{1/3}{\mathrm{Ni}}_{1/3}\right)}_2{\mathrm{As}}_2$, which is a quantum-critical $3d$ transition metal alloy. We demonstrate the existence of a dispersionless flat band of $t_{2g}$ orbitals just at $E_F$ which is responsible for quantum critical behaviors. Our findings will promote studies of emergent physics induced by flat bands, such as non-Fermi-liquid behaviors and quantum critical phenomena in $3d$ transition metals.