We present the results of elastic and inelastic neutron scattering measurements on nonsuperconducting $\mathrm{Ba}\left(\mathrm{Fe}{}_{0.957}\mathrm{Cu}{}_{0.043}\right){}_2\mathrm{As}{}_2$, a composition close to a quantum critical point between antiferromagnetic (AFM) ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low-Cu composition as well as the parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and superconducting $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{As}}_2$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.957}{\mathrm{Cu}}_{0.043}\right)}_2{\mathrm{As}}_2$, 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 $\xi \left(T\right)$ increases rapidly as the temperature is lowered and find $\omega /T$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

• Received 13 October 2015

DOI:https://doi.org/10.1103/PhysRevB.92.214404