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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 ($hbaromega>100$ 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 $<m^2>approx 3.6 mu^2_B$ 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$_2$.
Understanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as $A$Fe$_2$
Inelastic neutron scattering measurements on Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ manifest a neutron spin resonance in the superconducting state with anisotropic dispersion within the Fe layer. Whereas the resonance is sharply peaked at Q$_{AFM}$ a
We use electronic Raman scattering to study the low-energy excitations in BaFe$_2$(As$_{0.5}$P$_{0.5}$)$_2$ ($T_c approx 16$ K) samples. In addition to a superconducting pair breaking peak (2$Delta=6.7$ meV) in the A$_{1g}$ channel with a linear tail
We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two
Through detailed electronic structure simulations we show that the electronic orbital ordering (between d$_{yz}$ and d$_{xz}$ bands) takes place due to local breaking of in-plane symmetry that generates two non-equivalent $a$, $b$ directions in 122 f