Orbital magnetic response and the anisotropy of magnetic susceptibility in the Iron-based superconductors

TL;DR

This paper demonstrates that orbital magnetic susceptibility in iron-based superconductors is significant, anisotropic, and explains the observed magnetic anisotropy and Knight shift, highlighting the role of orbital angular momentum in their low-energy physics.

Contribution

It introduces a detailed analysis of orbital magnetic susceptibility in iron-based superconductors, revealing its dominance over spin susceptibility and its anisotropic nature, which explains experimental magnetic anisotropies.

Findings

Orbital magnetic susceptibility is several times larger than spin susceptibility.

Orbital susceptibility exhibits strong in-plane versus out-of-plane anisotropy.

Superconducting transition significantly reduces in-plane orbital susceptibility.

Abstract

We propose that the orbital angular momentum of the conduction electrons in the Iron-based superconductors is activated in their low energy physics. Using a five-band tight-binding model derived from fitting the LDA band structure, we find that the orbital magnetic susceptibility of the conduction electrons in such a multi-orbital system is several times larger than the Pauli spin susceptibility and is comparable in magnitude to the observed total magnetic susceptibility. The orbital magnetic susceptibility in the Fe-As plane($\chi^{x}_{L}$) is found to be larger than that perpendicular to the Fe-As plane($\chi^{z}_{L}$) by a factor about two and the total magnetic susceptibility in the normal state can be fitted with formula $\chi(T,\theta)\approx \chi_{s}(T)+\chi_{L}(\theta)$, where $\chi_{s}(T)$ is the temperature dependent isotropic part due to spin and $\chi_{L}(\theta)$ is the temperature independent anisotropic part due to orbital. In the superconducting state, $\chi^{x}_{L}$ is found to be significantly reduced as the pairing gap develops, while $\chi^{z}_{L}$ is almost not affected by the superconducting transition. We argue the large anisotropy observed in the bulk magnetic susceptibility and the Knight shift in the Iron-based superconductors should be attributed to the orbital magnetic response of their conduction electrons.