Inter-orbital nematicity and the origin of a single electron Fermi pocket in FeSe

TL;DR

This paper presents a theoretical study explaining the absence of an expected electron pocket in FeSe by inter-orbital nematic order, which leads to anisotropic Fermi surfaces and aligns with experimental superconducting gap observations.

Contribution

It introduces a model where inter-orbital nematic order causes Fermi surface anisotropy and single electron pocket formation in FeSe, explaining experimental puzzles.

Findings

Inter-orbital nematic order induces hybridization gaps at X and Y points.

The resulting electronic structure matches observed superconducting gaps.

Comparison with neutron data suggests the importance of self-energy effects.

Abstract

The electronic structure of the enigmatic iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1-Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. Here, we perform a theoretical study of the preferred nematic order originating from nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of inter-orbital $d_{xz}-d_{xy}$ and $d_{yz}-d_{xy}$ character, with spontaneously broken amplitudes for these two components. This inter-orbital nematic order naturally leads to distinct hybridization gaps at the $X$ and $Y$ points of the 1-Fe Brillouin zone, and may thereby produce highly anisotropic Fermi surfaces with only a single electron pocket at one of these momentum-space locations. The associated superconducting gap structure obtained with the generated low-energy electronic band structure from spin-fluctuation mediated pairing agrees well with that measured experimentally. Finally, from a comparison of the computed spin susceptibility to available neutron scattering data, we discuss the necessity of additional self-energy effects, and explore the role of orbital-dependent quasiparticle weights as a minimal means to include them.