# Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe$_{2}$As$_{2}$

## TL;DR

This study introduces a new strain-controlled NMR technique revealing how uniaxial strain induces spin-space anisotropy in BaFe$_{2}$As$_{2}$, uncovering a novel internal spin structure of nematic order in iron-based superconductors.

## Contribution

The paper demonstrates a method to control and probe spin anisotropy via strain, revealing a previously unobserved internal spin structure linked to nematic order.

## Key findings

- Strain induces anisotropic response in spin space.
- Revealed internal spin structure of nematic order.
- Electronic nematic materials can be magneto-mechanically controlled.

## Abstract

The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom [1-4]. The nematic state observed in many of these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion [5-7]. A more subtle and much less explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space into anisotropies in spin space. Here, we present a new technique enabling nuclear magnetic resonance under precise tunable strain control, which reveals that upon application of a tetragonal symmetry-breaking strain field, the magnetic fluctuation spectrum in the paramagnetic phase of BaFe$_{2}$As$_{2}$ also acquires an anisotropic response in spin-space. Our results unveil a hitherto uncharted internal spin structure of the nematic order parameter, indicating that similar to liquid crystals, electronic nematic materials may offer a novel route to magneto-mechanical control.