Paradigm for finding d-electron heavy fermions: the case of Cr-doped CsFe$_2$As$_2$

TL;DR

This paper proposes a new strategy to discover heavy-fermionic materials by doping Hund metals near half-filling, and demonstrates this approach experimentally with Cr-doped CsFe$_2$As$_2$, revealing a record-high Sommerfeld coefficient.

Contribution

The paper introduces a general method for finding d-electron heavy fermions through orbital-selective doping and confirms it experimentally in Cr-doped CsFe$_2$As$_2$ with unprecedented heavy fermionic behavior.

Findings

Achieved the highest Sommerfeld coefficient for Fe-pnictides to date (270 mJ/mol K$^2$).

Observed the emergence of a heavy fermionic state before antiferromagnetic ordering.

Validated the proposed doping strategy as effective for discovering new heavy-fermionic materials.

Abstract

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 inter-orbital hopping into heavy-fermion behaviour - 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 further 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.