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Register a new userWhy magnetism in CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs is different
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S. Sharma
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Using state-of-the-art first-principles calculations we study the magnetic behaviour of CeOFeAs. We find the Ce layer moments oriented perpendicular to those of the Fe layers. An analysis of incommensurate magnetic structures reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs show marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by a reduction in magnetic frustration. Finally, we investigate the impact of electron doping upon CeOFeAs and show that while the ground state Fe moment remains largely unchanged by doping, the stability of magnetic order goes to zero at a doping that corresponds well to the vanishing of the Neel temperature.
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Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_{1-x}$F$_x$FeAs, and the E$_g$ mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450 cm$^{-1}$ and shows a very sharp peak. Electronic scattering background is low and electron-phonon coupling is not evident for the observed phonon modes. Three features are found above 500 cm$^{-1}$, which may be associated with multi-phonon process. Nevertheless it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals considering their energies.
We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3mu_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new superconductivity mechanism, mediated by the spin fluctuations. An abrupt change on the Hall measurement is further predicted for the parent compound LaOFeAs.
We report density functional theory calculations for the parent compound LaOFeAs of the newly discovered 26K Fe-based superconductor LaO$_{1-x}$F$_x$FeAs. We find that the ground state is an ordered antiferromagnet, with staggered moment about 2.3$mu_B$, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on Fe 3d and As 4p orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident proximity of superconductivity to antiferromagnetism and the Mott transition, we suggest that the system may be an analog of the electron doped cuprates, where antiferromagnetism and superconductivity coexist.
We report the theoretical study of the flux-lattice melting in the novel iron-based superconductor $LaO_{0.9}F_{0.1}FeAs$ and $LaO_{0.925}F_{0.075}FeAs$. Using the Hypernetted-Chain closure and an efficient algorithm, we calculate the two-dimensional one-component plasma pair distribution functions, static structure factors and direct correlation functions at various temperatures. The Hansen-Verlet freezing criterion is shown to be valid for vortex-liquid freezing in type-II superconductors. Flux-lattice meting lines for $LaO_{0.9}F_{0.1}FeAs$ and $LaO_{0.925}F_{0.075}FeAs$ are predicted through the combination of the density functional theory and the mean-field substrate approach.
A series of layered CeO$_{1-x}$F$_x$FeAs compounds with x=0 to 0.20 are synthesized by solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F-doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. The very high superconducting transition temperature strongly challenges the classic BCS theory based on the electron-phonon interaction. The very closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuations play a key role in the superconducting paring mechanism. The study also reveals that the Ce 4f electrons form local moments and ordered antiferromagnetically below 4 K, which could coexist with superconductivity.
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