Hund's physics as a key to understanding emerging quantum phases in iron-pnictides

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Understanding emerging phases in iron-pnictides
2022. 04. 29. 10:15
BME building F, seminar room of the Dept. of Theoretical Physics & Online
Angelo Valli (TU Wien)
The origin of high-temperature superconductivity is one of the most elusive puzzles of modern solid-state physics. The discovery of materials with unprecedently high critical temperatures (copper oxides, fullerides, iron pnictides) which defy the conventional theory of superconductivity, has fueled an intense research activity for decades. Unconventional superconducting states arise in the proximity of Mott insulators, i.e., states in which the electrons are localized by strong Coulomb repulsion, and in the iron pnictides, overwhelming evidence identifies the normal state as bad metal characterized by orbital-selective correlations arising from the Hund’s exchange interaction. This scenario suggests that a prominent role is played by many-body effects. At the same time, approaches based on the exchange of bosons of electronic origin (spin and orbital fluctuations being the most popular candidates) can predict the emergence of a variety of phenomena, thus pointing toward a more conventional pairing mechanism. 
We discuss a strategy to reconcile this evidence and show that the nature of the electronic correlations in the normal state has non-trivial repercussions on the ordered phases. The inclusion of dynamical correlations allows the simultaneous description of electronic excitations living on different energy scales. Within this framework, the Hund's metal is substantially different from both a weakly interacting metal and an ordinary correlated metal with a strong effective mass renormalization. We reveal that Hund’s correlations are counterintuitively beneficial to boson-mediated superconductivity [1] and that experimental features observed in the nematic photoemission spectra of FeSe and FeAs compounds naturally emerge from the orbital-selective behavior [2]. Our analysis leads to questioning the popular approaches that mainly focus on “who is the driver?” of the electronic instabilities and calls instead for a more coherent theoretical description of iron pnictides which must be able to capture the essence of the correlated metal. 
[1] L. Fanfarillo, A. Valli, and M. Capone, Phys. Rev. Lett. 125, 177001 (2020)
[2] L. Fanfarillo, A. Valli, and M. Capone, arXiv:2203.01273 (2022)