Nematicity driven by hybridization in iron-based superconductors

Abstract

In this paper we study an effective model for the normal state of iron-based superconductors. It has separate, but interacting itinerant and localized degrees of freedom, originating from the dxz and dyz, and from dxy iron orbitals respectively. At low temperatures, below a mean-field phase transition, these different states condense together in an excitonic order parameter. We show that at even lower temperature, after another phase transition, this ordered state can spontaneously break the C4 lattice symmetry and become nematic. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds.Introduction. The discovery of high-temperature superconductivity in iron-based materials 1-3 is one of the most exciting recent developments in physics. It is almost certain that the origin of superconductivity is unconventional (i.e. driven by electron-electron interactions), and is very likely that the order parameter is unconventional as well 4,5 . The normal state properties of these materials, in contrast, at first seemed rather unremarkablea bad metal behavior at high temperatures, which can be followed by structural transition and antiferromagnetic phase at low temperatures. This simple picture was considerably complicated by the growing evidence of nematic fluctuations 6-8 or even long-range order 9 present in some compounds at temperatures well above the antiferromagnetic and structural transitions. Furthermore, this nematicity seems to be of purely electronic (rather than structural) origin 10 . Understanding this state and its connection to the other phases is an important milestone in the study of these materials.

Mott-Kondo insulator behavior in the iron oxychalcogenides

“…Since we also find that the imaginary parts of the orbital-resolved self-energies have a pole structure very close to ω = 0, it is very conceivable, as discussed in the text, that sizable orbital-selective spectral weight transfer upon suitable doping can lead to an OSMT [33], where a subset of orbital states remain (Mott-like) insulating while the remainder turn metallic. In the generic case, this will lead to bad-metallicity and breakdown of the Landau FL picture.…”

“…According to our LDA+DMFT treatment, the combined Mott-Kondo insulating state in FeOCh arises from the interplay between strong on-site (Mott-Hubbard) and inter-orbital Coulomb correlations in a system characterized by orbital polarization at the one particle level. On the other hand, it has been recently realized that inter-orbital (effectively Falicov-Kimball-like [32] in the selective metal where a co-existence of itinerant and localized d-states is found) electronic interactions together with inter-orbital one-electron hybridization effectively mixes different orbital degrees of freedom, providing a source of correlation induced hybridization band gaps [33]. Though this insulating state is thus due to the influence of one-electron hybridization and is adiabatically connected to a band insulator, it preserves specific features associated with strong correlations just above the gap.…”

Superconductivity in Fe-chalcogenides

“…The theoretical approaches combining both itinerant electrons with strong orbital characters and the local spin [131]. Other models of similar nature have been employed to understand the phase transitions [132,133].…”

Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

“…Note that these results are insensitive to the microscopic mechanism behind the tetragonal symmetry breaking, i.e. whether it arises due to orbital [28][29][30][31][32][33][34][35] or spin fluctuations 2,36-40 , or electron-phonon coupling. In making quantitative comparison with experiments, an important issue is the presence of twin domains, i.e.…”