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تسجيل مستخدم جديدNematicity, magnetic fluctuation and ferro-spin-orbital ordering in BaFe$_2$As$_2$ family
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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 family of Fe-based superconductors. Orbital ordering is strongly anisotropic and the temperature dependence of the corner zone orbital order maps to that of the orthorhombicity parameter. Orbital anisotropy results in two distinct spin density wave nesting wave vectors and causes inter-orbital charge and spin fluctuations. Temperature dependence of the orbital order is proportional to the nematic order and it sets in at a temperature where magnetic fluctuation starts building. Magnetic fluctuations in the orthorhombic phase is characterized through evolution of Stoner factor which reproduces experimentalfindings very accurately. Orbital ordering becomes strongly spin dependent in presence of magnetic interaction. Occupation probabilities of all the Fe-d-orbitals exhibit temperature dependence indicating their possible contribution in orbital fluctuation. This need to be contrasted with the usual definition of nematic order parameter (n$_{d_{xz}}$-n$_{d_{yz}}$). Relationship among orbital fluctuations, magnetic fluctuations and nematicity are established.
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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$_2$
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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 s
pin 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$_2$.
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T$-linear temperature dependence of the penetration depth $lambda(T)$ of these three compounds indicate extremely clean nodal gap superconductors, the thermal conductivity data $lim_{T,H rightarrow 0} kappa_S (H,T)/T$ unambiguously showed that LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ are extremely dirty, while KFe$_2$As$_2$ can be clean. This apparently conflicting experimental data casts a serious doubt on the nodal gap possibility on LaFePO and BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$.
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