Inelastic neutron scattering from superconducting (SC) Ba(Fe$_{ 0.926}$Co$_{ 0.074}$)$_2$As$_2$ reveals anisotropic and quasi-two-dimensional (2D) magnetic excitations close to \textbf{ Q}$_{ \texttt{ AFM}}=({ 1}{ 2}/{ 1}{ 2})$ - the 2D antiferromagnetic (AFM) wave-vector of the parent BaFe$_2$As$_2$ compound. The correlation length anisotropy of these low energy fluctuations is consistent with spin nematic correlations in the $J_1$-$J_2$ model with $J_1/J_2 \sim$ 1. The spin resonance at $\sim$8.3 meV in the SC state displays the same anisotropy. The anisotropic fluctuations experimentally evolve into two distinct maxima only along the direction transverse to \textbf{ Q}$_{ \texttt{ AFM}}$ above $\sim$80 meV indicating unusual quasi-propagating excitations.
Categories: cond-mat.supr-con cond-mat.str-el