Orbital selective spin excitations and their impact on superconductivity of LiFe1-xCoxAs

TL;DR

This study reveals orbital-dependent spin excitations in LiFe$_{0.88}$Co$_{0.12}$As, showing their influence on superconductivity and highlighting the importance of multiple orbitals beyond Fermi surface nesting.

Contribution

It demonstrates orbital selective spin excitations in LiFeAs, contrasting with previous models, and links these excitations to superconductivity in iron pnictides.

Findings

Low-energy spin excitations mainly from $d_{xy}$ orbitals.

High-energy spin excitations from $d_{yz}$ and $d_{xz}$ orbitals.

Superconductivity depends on multiple orbital Fermi surface nesting conditions.

Abstract

We use neutron scattering to study spin excitations in single crystals of LiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the superconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT) and dynamical mean field theory (DMFT) calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the $d_{xy}$ orbitals, while high-energy spin excitations arise from the $d_{yz}$ and $d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting condition for the $d_{xy}$ orbital, the reduced superconductivity in LiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for the $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in iron pnictides.