Emergent Kondo Lattice Behavior in Iron-Based SuperconductorsAFe2As2(A=K, Rb, Cs)

Abstract

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) AFe 2 As 2 (A = K, Rb, Cs). Nuclear magnetic resonance on 75 As 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 AFe 2 As 2 (A = K, Rb, Cs) as the first material realization of d-electron HF superconductors.Superconductivity in heavy fermion (HF) materials is a conundrum in condensed matter physics. Conventional phonon-mediated pairing mechanism is failed in this case, suggesting an unconventional pairing mechanism similar to that in cuprates and organic superconductors (1). So far, only HF materials containing f electrons could be the hosts for unconventional superconductivity. Whether unconventional superconductivity could be achieved in HF materials without f electrons is still unknown. In f-electron HF materials, Kondo lattice has been widely accepted as the starting point for discussing the underlying physics (2). In two-fluid model of Kondo lattice (3, 4, 5), coherent state emerges below a characteristic temperature T* as the localized f-electrons collectively reduce their entropy by hybridizing with the itinerant electrons to form a new state of matter, an itinerant heavy-electron Kondo liquid (KL) that displays scaling behavior. The emergent KL coexists with the hybridized spin liquid that describes the lattice of local moments whose magnitude has been reduced by hybridization. "Hybridization effectiveness" has been proposed as the organizing principle responsible for the emergence of low-temperature order in HF systems, including unconventional superconductivity (5). Although Kondo picture for HF has achieved a great success in f-electron materials, the fate of Kondo picture in HF materials without f electrons, especially for d-electron systems, is still controversial. Besides Kondo scenario (6), many alternative scenarios, such as lightly doped Mott insulator (7), geometrical frustration via antiferromagnetic interactions (8) and spin-orbital fluctuations (9), have been proposed to responsible for HF behavior in d-electron systems. In addition, searching for unconventional superconductivity in the existing d-electron HF materials is still not successful.Recently, a remarkable mass enhancement was observed in heavily hole-doped Fe-based superconductors (FeSCs) AFe 2 As 2 (A = K, Rb, Cs). As sh...

Constraints on the total coupling strengthto bosons in the iron based superconductors

“…This puzzle of the missing VHS in Cs‐122 could be explained by substantial orbital fluctuations related to a locally broken tetragonal symmetry for which a splitting into two components one above and a counterpart below . Alternatively, it might be related to the opening of a pseudogap and or to a charge ordering suggested for an electronically ‐driven phase separation like in that suggested for Rb‐122 at K. The observation of charge ordering provides strong arguments against the vicinity of a Mott phase. As an experimental example illustrating that point, we refer to the A15 phase of CsC .…”

“…We would like to stress that the central theoretical problem in our opinion is not whether there are strongly correlated electrons (likely the 3 states) present in these compounds. The Curie–Weiss tails of the measured magnetic susceptibilities at high temperatures already evidences this fact . Instead, more subtle questions must be addressed, namely which of the subsystems is responsible for the QCP, which determine the nature of the magnetic phase beyond, and which form the bands where superconductivity occurs?…”

“…a). This observation, together with a claimed universal Knight ‐shift scaling anomaly in the NMR spin ‐lattice relaxation rate at (with = 165, 125, and 85 K for K‐, Rb‐, and Cs‐122, respectively), has lead to proposals for a related emergent Kondo ‐lattice like scenario for the whole series . But if one uses the DFT calculated DOS states/eV/f.u.…”

“…Starting from (K‐122) 1, and using the data from Ref. (Fig. S4 and Table S1), one estimates (Rb‐122)/(K‐122)0.727 and (Cs‐122)/ (K‐122)0.394, while less correlated FeSC exhibit ‐values in between 2 and 5.…”

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

“…For example, in CeCoIn 5 , a HF compound, it was shown that a tiny amount of Cd doping restores the long range antiferromagnetic (AF) order [4,5,55] and suppresses the superconducting dome developing around the quantum critical point separating the FL from the AF phase. In LaFe 1−x Mn x AsO 0.89 F 0.11 the Kondo coupling would involve itinerant and more localized 3d electrons [56][57][58] playing a role analogous to the f electrons in the HF. When these latter electrons finally localize the RKKY coupling among Mn impurities leads to the recovery of magnetism and the suppression of the metallic superconducting state.…”