Effects of correlations on the electronic structure and pairing in Fe-chalcogenides

Research on iron based superconducting materials has found evidence that electronic correlations play a significant role in determining the electronic structure and therefore should also influence the superconducting pairing in these systems. Examples are band- and orbital-dependent mass renormalizations and energy shifts, the occurrence of nematic instabilities and orbital order. Here, I present some examples how spin-fluctuations and nonlocal correlation effects determine the electronic structure in Fe based superconductors [1] and lead to orbitally selective decoherence. In terms of the superconducting state, the continuing development of high-resolution experimental probes of superconducting gap structures allows us to quantitatively compare the detailed momentum structure of superconducting pairing. Effects of correlations within the spin-fluctuation pairing approach can change the symmetry of the pairing instability, shift between dominant orbital contributions or modify the momentum structure.
Correlations seem to be more important in the Fe-chalcogenides, which have attracted considerable attention due to their unusual low-energy electronic states. In particular, the compound FeSe enters a nematic phase without static magnetic order [2]. This leads to an electronic structure where the expected electron pocket at the Y-point cannot be detected with spectroscopic probes. This pocket, essential for an understanding of the superconducting state, is either absent or incoherent. I contrast the theoretical scenario for electronic anisotropy to that of orbital-selectivity based on orbital-dependent quasiparticle weights and summarize experimental evidences to distinguish these.
[1] S. Bhattacharyya, et al., Phys. Rev. B 102, 035109 (2020) “Non-local correlations in Iron Pnictides and Chalcogenides”
[2] A. Kreisel, P.J. Hirschfeld, B. M. Andersen, Symmetry, 12, 1402 (2020) “On the Remarkable Superconductivity of FeSe and its Close Cousins”