Paradigm for Finding d-Electron Heavy Fermions: The Case of Cr-doped CsFe_{2}As_{2}

We define a general strategy for finding new heavy-fermionic materials without rare-earth elements: doping a Hund metal with pronounced orbital-selective correlations towards half-filling. We argue that in general band structures, a possible orbital-selective Mott transition is frustrated by interorbital hopping into heavy-fermion behavior-where d orbitals provide both the heavy and the light electrons-which is enhanced when approaching half-filling. This phase ultimately disappears due to magnetic correlations, as in a standard Doniach diagram. Experimentally we have hole doped CsFe_{2}As_{2}, a Hund metal with 0.5 electrons/Fe away from half-filling, and obtained a heavy fermionic state with the highest Sommerfeld coefficient for Fe pnictides to date (270  mJ/mol K^{2}), before signatures of an antiferromagnetic phase set in.