We present a study of the electronic and magnetic properties of the multiple-decker sandwich nanowires (CP-M) composed of cyclopentadienyl (CP) rings and 3d transition-metal atoms (M = Ti to Ni) using first-principles techniques. We demonstrate using density functional theory that structural relaxation plays an important role in determining the magnetic ground state of the system. Notably, the computed magnetic moment is zero in CP-Mn, while in CP-V a significant turn-up in magnetic moment is evidenced. Two compounds show a half-metallic ferromagnetic ground state CP-Fe/Cr with a gap within the minority/majority spin channel. In order to study the effect of electronic correlations upon the half-metallic ground states in CP-Cr, we introduce a simplified three-band Hubbard model which is solved within the variational cluster approach. We discuss the results as a function of size of the reference cluster and the strength of average Coulomb U and exchange J parameters. Our results demonstrate that for the range of studied parameters U = 2-4 eV and J = 0.6-1.2 eV the half-metallic character is not maintained in the presence of local Coulomb interactions. PACS number(s): 71.15.Mb H = H 0 + Rm Un Rm↑ n Rm↓ + R,m

We investigate half-metallicity in [001] stacked (CrAs)n/(GaAs)n heterostructures with n ≤ 3 by means of a combined many-body and electronic structure calculation. Interface states in the presence of strong electronic correlations are discussed for the case n = 1. For n = 2, 3 our results indicate that the minority spin half-metallic gap is suppressed by local correlations at finite temperatures, and continuously shrinks upon increasing the heterostructure period. Although around room temperature the magnetization of the heterostructure deviates by only 2% from the ideal integer value, finite temperature polarization at EF is reduced by at least 25%. Below the Fermi level the minority spin highest valence states are found to localize more on the GaAs layers while lowest conduction states have a many-body origin. Our results, therefore, suggest that in these heterostructures holes and electrons remain separated among different layers.

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We present the results of combined density functional and many-body calculations of the electronic and magnetic properties of the defect-free digital ferromagnetic heterostructures obtained by doping GaAs with Cr and Mn. While the local-density approximation +U predicts half-metallicity in these defect-free delta-doped heterostructures, we demonstrate that local many-body correlations captured by dynamical mean-field theory induce within the minority-spin channel nonquasiparticle states just above E F . As a consequence of the existence of these many-body states the half-metallic gap is closed and the carriers' spin polarization is significantly reduced. Below the Fermi level the minority-spin highest valence states are found to localize more on the GaAs layers, being independent of the type of electronic correlations considered. Thus, our results confirm the confinement of carriers in these delta-doped heterostructures, having a spin polarization that follows a different temperature dependence than the magnetization. We suggest that polarized hot-electron photoluminescence experiments might uncover evidence for the existence of many-body states within the minority-spin channel and elucidate their finite-temperature behavior.

Polycrystalline samples with a nominal composition Ru 1-x Sb x Sr 2 GdCu 2 O 8-d were prepared by a solid state reaction technique. A mixture of RuO 2, Sb 2 O 3, Gd 2 O 3, SrCO 3 and CuO was used to obtain the samples. We performed X-ray diffraction analyses, DC susceptibility and transport measurements and the studies showed that the samples were almost pure Ru -1212 phase. We also observed that Sb doping reduce the conductivity of the system and the transition temperature decreases with increasing Sb content. This may be due to a distortion of RuO 6 octahedral which is responsible for the increase of holes localization.