Multipolar nematic state of nonmagnetic FeSe based on the DFT+$U$

TL;DR

This study reveals that in FeSe, a multipolar nematic state emerges due to electronic interactions, characterized by complex multipole orders, which are crucial for understanding nematicity in iron-based superconductors.

Contribution

The paper demonstrates the multipolar nature of the nematic state in FeSe using DFT+$U$ and analyzes the role of multipole couplings in nematicity.

Findings

Fermi surface topology changes with increasing U

Nematic ground state involves antiferrohexadecapoles and ferromultipoles

Multipole coupling influences orbital splitting in FeSe

Abstract

Clarifying the origin of nematic state in FeSe is one of urgent problems in the field of iron-based superconductivity. Motivated by the discovery of a nematic solution in the density-functional theory implemented by on-site Coulomb interaction (DFT+$U$) [npj Quantum Mater. \textbf{5}, 50 (2020)], we reexamine the $U$ dependence of electronic states in the nonmagnetic normal state of FeSe and perform full multipolar analyses for the nematic state. We find that with increasing $U$ the normal state experiences a topological change in the Fermi surfaces before the emergence of a nematic ground state. The resulting nematic ground state is a multipolar state having both antiferrohexadecapoles in the $E$ representation and ferromultipoles in the $B_2$ representation on each Fe site. Cooperative coupling between the $E$ and the $B_2$ multipoles in the local coordinate with the $D_{2d}$ point group will play an important role in the formation of the $d_{xz},~d_{yz}$ orbital-splitting nematic state not only in FeSe, but also in other iron pnictides.