Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe$_{2-x}$Ni$_x$As$_2$

TL;DR

This study uses neutron scattering and transport measurements to observe spin excitation anisotropy in detwinned BaFe$_{2-x}$Ni$_x$As$_2$, revealing nematic behavior in the paramagnetic orthorhombic phase consistent with spin nematic theory.

Contribution

It provides direct experimental evidence of spin excitation anisotropy above $T_s$ in iron pnictides, supporting the spin nematic scenario without requiring a separate nematic phase transition.

Findings

Spin excitation anisotropy appears below $T_s$ in detwinned samples.

Uniaxial pressure induces anisotropy above $T_s$.

Results align with spin nematic theory predictions.

Abstract

We use transport and inelastic neutron scattering measurements to investigate single crystals of iron pnictide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03$), which exhibit a tetragonal-to-orthorhombic structural transition at $T_s$ and stripe antiferromagnetic order at $T_N$ ($T_s\geq T_N$). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave vector $S_1(1,0)$ and its 90$^\circ$ rotated wave vector $S_2(0,1)$ under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain single domain orthorhombic antiferromagnetic phase of BaFe$_{2-x}$Ni$_{x}$As$_{2}$ induces resistivity and spin excitation anisotropy at temperatures above zero pressure $T_s$. In uniaxial pressure-free detwinned sample, spin excitation anisotropy between $S_1(1,0)$ and $S_2(0,1)$ first appear in the paramagnetic orthorhombic phase below $T_s$. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above $T_s$ in iron pnictides.