Universal quasi-degenerate orbital origin of two-dome phases in iron pnictide superconductors

TL;DR

This work addresses the origin of two-dome magnetic and superconducting phases in iron-based superconductors by combining first-principles electronic structure, Wannier-based tight-binding modeling, mean-field orbital analysis, and multi-orbital RPA. It identifies a universal mechanism in which quasi-degenerate Fe-$3d$ orbitals drive the AFM-II/SC-II dome, while the AFM-I/SC-I dome is controlled by in-plane anisotropic $d_{xz/yz}$ orbitals; isotropic orbitals such as $d_{xy}$ or $d_{3z^{2}-r^{2}}$ govern the second dome, yielding an orbital-selective pairing scenario. A proposed orbital-spin mode matching rule links the active orbitals to specific spin fluctuation channels, providing a predictive framework for high-$T_c$ pairing in multi-orbital iron-based systems. The findings offer a cohesive explanation for the two-dome phase behavior across LaFeAsO$_{1-x}$H$_{x}$, LaFeAs$_{1-x}$P$_{x}$O, and KFe$_{2}$As$_{2}$, and have implications for discovering materials with enhanced superconducting temperatures by tuning orbital topology and spin-orbital coupling.

Abstract

A series of experiments revealed that novel bipartite magnetic and superconducting (SC) phases widely exist in the phase diagrams of iron pnictides and chalcogenides. Nevertheless, the origin of the two-dome magnetic and SC phases in iron-based compounds remains unclear. Here we theoretically investigated the electronic structures, magnetic and SC properties of three representative iron-based systems, i.e. LaFeAsO$_{1-x}$H$_{x}$, LaFeAs$_{1-x}$P$_{x}$O and KFe$_{2}$As$_{2}$. We propose a unified quasi-degenerate orbital mechanism for the emergence of the two-dome parent magnetic/structural phase and the subsequent two-dome SC phase. It is found that the degenerate in-plane anisotropic $d_{xz/yz}$ orbitals dominate the first magnetic/structural and SC phases, while in-plane isotropic orbitals $d_{xy}$ or $d_{3z^{2}-r^{2}}$ with quasi-degeneracy originating from quasi-symmetry drive the emergence of the second magnetic/SC dome phase. Moreover, a matching rule of spin and orbital modes for SC pairing state is proposed in multi-orbital iron-based systems. These results imply an orbital-driven mechanism as well as an orbital-selective scenario, and shed light on the understanding of the multi-dome magnetic and SC phases in multi-orbital systems.

Figures (16)

• Figure 1: (Color online) Band structures and Fermi surfaces for LaFeAsO ($x$=0) (a) and (b), and LaFeAsO$_{1-x}$H$_{x}$ ($x$=0.5) (c) and (d), respectively, with colors indicating majority orbital character.
• Figure 2: (Color online) Orbital occupancies and magnetic moments dependence on Coulomb interaction $U$ within the five-orbital Hubbard model for (a) LaFeAsO ($x$=0) and (b) LaFeAsO$_{1-x}$H$_{x}$ ($x$=0.5) with parameter $J_{H}$=0.1$U$.
• Figure 3: (Color online) Dependence of OO parameters on doping concentration $x$ (only valid for the AFM-II/SC-II phases with $x$ ranging from 0.3 to 0.6) within the five-orbital Hubbard model of the second parent phase LaFeAsO$_{0.5}$H$_{0.5}$ with $U$=1.7 eV and $J_{H}$=0.1$U$.
• Figure 4: (Color online) Dynamical spin susceptibilities of (a) LaFeAsO ($x$$=$ 0) with $U$$=$ 1.4 eV and $J_{H}$$=$ 0.1$U$, and (b) LaFeAsO$_{1-x}$H$_{x}$ ($x$$=$ 0.5) with $U$$=$ 0.9 eV and $J_{H}$$=$ 0.1$U$.
• Figure 5: (Color online) Orbital pairing vertices and two leading gap functions for LaFeAsO$_{1-x}$H$_{x}$, (a) and (b) $x$=0.125 with $U$=1.4 eV and $J_{H}$=0.1$U$, and (c) and (d) $x$=0.35 with $U$=0.8 eV and $J_{H}$=0.1$U$, respectively.