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تسجيل مستخدم جديدCrystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_{4} : role of the orbital degree of freedom
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The crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_4 (0<x<0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.
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We study a simplified model of the electronic structure of compounds of the type of La$_{1-x}$Sr$_x$MnO$_3$. The model represents each Mn$^{4+}$ ion by a spin S=1/2, on which an electron can be added to produce Mn$^{3+}$. We include two strong intrat
omic interactions in the Hamiltonian: exchange ($J$% ) and Coulomb ($U$). Finally, to represent the effect of Sr substitution by La in a simple way, we include a distribution of diagonal energies at the Mn sites. Then we use Green function techniques to calculate a mobility edge and the average density of states. We find that according to the amount of disorder and to the concentration of electrons in the system, the Fermi level can cross the mobility edge to produce a metal to insulator transition as the magnetization decreases (increase of temperature). If the disorder is large, the system remains insulating for all concentrations. Concentrations near zero or one favor the insulating state while intermediate values of concentration favor the metallic state.
We present the electronic structure of Sr_{1-(x+y)}La_{x+y}Ti_{1-x}Cr_{x}O_{3} investigated by high-resolution photoemission spectroscopy. In the vicinity of Fermi level, it was found that the electronic structure were composed of a Cr 3d local state
with the t_{2g}^{3} configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.
By using laboratory x-ray photoemission spectroscopy (XPS) and hard x-ray photoemission spectroscopy (HX-PES) at a synchrotron facility, we report an empirical semi-quantitative relationship between the valence/core-level x-ray photoemission spectral
weight and electrical conductivity in La_{1-x}Sr_{x}MnO_{3} as a function of x. In the Mn 2p_{3/2} HX-PES spectra, we observed the shoulder structure due to the Mn^{3+} well-screened state. However, the intensity at x=0.8 was too small to explain its higher electrical conductivity than x=0.0, which confirms our recent analysis on the Mn 2p_{3/2} XPS spectra. The near-Fermi level XPS spectral weight was found to be a measure of the variation of electrical conductivity with x in spite of a far lower energy resolution compared with the energy scale of the quasiparticle (coherent) peak because of the concurrent change of the coherent and incoherent spectral weight.
We study long wavelength magnetic excitations in lightly doped La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferr
omagnetic (AF) phase. The second magnetic resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state and led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the N{e}el temperature in x = 0.01.
We have undertaken a study of diluted magnetic semiconductors $Ga_{1-x}Mn_{x}N$ and $Ga_{1-x}Cr_{x}N$ with $x=0.0625, 0.125$, using the all electron linearized augmented plane wave method (LAPW) for different configurations of Mn as well as Cr. We st
udy four possible configurations of the impurity in the wurtzite GaN structure to predict energetically most favorable structure within the 32 atom supercell and conclude that the near-neighbor configuration has the lowest energy. We have also analyzed the ferro-magnetic as well as anti-ferromagnetic configurations of the impurity atoms. The density of states as well as bandstructure indicate half metallic state for all the systems. $T_c$ has also been estimated for the above systems.
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