Flat band induced quantum criticality in a nonsuperconducting iron pnictide

Flat electronic bands at the Fermi energy (EF) 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 EF 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, Ba(Fe1/3Co1/3Ni1/3)2As2, which is a quantum-critical 3d transition metal alloy. We demonstrate the existence of a dispersionless flat band of t2g orbitals just at EF 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.