# Spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2 As2

## TL;DR

This study uses neutron polarization analysis to investigate how spin excitation anisotropy in BaFe2As2 evolves with temperature, revealing significant anisotropy near the magnetic transition that links spin-orbit coupling to superconductivity.

## Contribution

It provides the first detailed measurement of spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2As2, highlighting its similarity to doped superconductors and its implications for understanding spin-orbit coupling.

## Key findings

- Spin excitations are isotropic at 160 K in the tetragonal phase.
- Significant spin excitation anisotropy develops below 3 meV near T_N.
- In-plane spin excitation anisotropy is similar to doped superconductors.

## Abstract

We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe$_2$As$_2$, which has a tetragonal-to-orthorhombic structural distortion at $T_s$ and antiferromagnetic (AF) phase transition at $T_N$ with ordered moments along the orthorhombic $a$-axis below $T_s\approx T_N\approx 136$ K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with $M_a=M_b=M_c$, where $M_a$, $M_b$, and $M_c$ are spin excitations polarized along the $a$, $b$, and $c$-axis directions of the orthorhombic lattice, respectively. On cooling towards $T_N$, significant spin excitation anisotropy with $M_a>M_b\approx M_c$ develops below 3 meV with a diverging $M_a$ at $T_N$. The in-plane spin excitation anisotropy in the tetragonal phase of BaFe$_2$As$_2$ is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.