We present the results of elastic and inelastic neutron scattering measurements on non-superconducting Ba(Fe${ _{ 0.957}}$Cu${ _{ 0.043}}$)${ _2}$As${ _2}$, a composition close to a quantum critical point between AFM ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low Cu composition as well as the parent compound BaFe$_2$As$_2$ and superconducting Ba(Fe$_{ 1-x}$Ni$_x$)$_2$As$_2$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe$_{ 0.957}$Cu$_{ 0.043}$)$_2$As$_2$, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length, ${ \xi(T)}$, increases rapidly as the temperature is lowered and find ${ \omega/T}$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.
Categories: cond-mat.str-el cond-mat.supr-con