Orbital Order and Spin Nematicity in the Tetragonal Phase of Electron-doped Iron-Pnictides NaFe$_{1-x}$Co$_{x}$As

TL;DR

This study uses NMR to reveal that in electron-doped NaFeAs, orbital and spin nematicity emerge at temperatures well above structural transition, indicating an incommensurate orbital order driving spin nematicity.

Contribution

It demonstrates that orbital and spin nematicity occur above structural transition temperatures and introduces an incommensurate orbital order as the underlying mechanism.

Findings

Orbital and spin nematicity appear at temperatures above the structural transition.

Incommensurate orbital order sets in below a certain temperature and becomes commensurate below the structural transition.

NMR spectra splitting indicates the presence of orbital order influencing spin nematicity.

Abstract

In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42 K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on NaFe$_{1-x}$Co$_x$As (0 $\leq x \leq$ 0.042) that revealed orbital and spin nematicity occurring at a temperature $T^{\rm *}$ far above $T_{\rm s}$ in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below $T^{\rm *}$ and becomes commensurate below $T_{\rm s}$, which brings about the observed spin nematicity.