  Unconventional superconductivity from heavy fermion (HF) is always observed
in f-electron systems, in which Kondo physics between localized f-electrons and
itinerant electrons plays an essential role. Whether HF superconductivity could
be achieved in other systems without f electrons, especially for d-electron
systems, is still elusive. Here, we experimentally study the origin of
d-electron HF behavior in iron-based superconductors (FeSCs) AFe2As2 (A = K,
Rb, Cs). Nuclear magnetic resonance on 75As reveals a universal
coherent-incoherent crossover with a characteristic temperature T*. Below T*, a
so-called 'Knight shift anomaly' is first observed in FeSCs, which exhibits a
scaling behavior similar to f-electron HF materials. Furthermore, the scaling
rule also regulates the manifestation of magnetic fluctuation. These results
undoubtedly support an emergent Kondo scenario for the d-electron HF behavior,
which suggests the AFe2As2 (A = K, Rb, Cs) as the first material realization of
d-electron HF superconductors.
