Temperature dependence of the paramagnetic spin excitations in BaFe${}_2$As${}_2$

We use inelastic neutron scattering to study temperature dependence of the paramagnetic spin excitations in iron pnictide BaFe${}_2$As${}_2$ throughout the Brillouin zone. In contrast to a conventional local moment Heisenberg system, where paramagnetic spin excitations are expected to have a Lorentzian function centered at zero energy transfer, the high-energy ($\mathrm{\hslash }\omega >100$ meV) paramagnetic spin excitations in BaFe${}_2$As${}_2$ exhibit spin-wave-like features up to at least 290 K ($T=2.1T_N$). Furthermore, we find that the sizes of the fluctuating magnetic moments $<m^2>\approx 3.6{\mu }_B^2$ per Fe are essentially temperature independent from the AF ordered state at $0.05T_N$ to $2.1T_N$, which differs considerably from the temperature dependent fluctuating moment observed in the iron chalcogenide Fe${}_{1.1}$Te [I. A. Zaliznyak {\it et al.}, Phys. Rev. Lett. {\bf 107}, 216403 (2011).]. These results suggest unconventional magnetism and strong electron correlation effects in BaFe${}_2$As${}_2$.