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

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 $T_N$ ($T_s\ge T_N$). 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.

6 pages, 4 figures