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Register a new userFirst-principles study of the electronic and magnetic properties of cubic GdCu compound
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The structural, electronic, and magnetic properties of bulk GdCu (CsCl-type) are investigated using spin density functional theory, where highly localized $4f$ orbitals are treated within LDA+$U$ and GGA+$U$ methods. The calculated magnetic ground state of GdCu using collinear as well as spin spiral calculations exhibits a C-type antiferromagnetic configuration representing a spin spiral propagation vector $mathbf{Q}=frac{2pi}{a}(frac{1}{2},frac{1}{2},0)$. The parameters of the effective Heisenberg Hamiltonian are evaluated from a self-consistent electronic structure and are used to determine the magnetic transition temperature. The estimated N{e}el temperature of the cubic GdCu using GGA+$U$ and LDA+$U$ density functionals within the mean field and random phase approximations are in good agreement with the experimentally measured values. In particular, the theoretical understanding of the experimentally observed core Gd $4f$ levels shifting in photoemission spectroscopy experiments is investigated in detail. By employing the self-consistent constrained random-phase approximation we determined the strength of the effective Coulomb interaction (Hubbard $U$) between localized $4f$ electrons. We find that, the shift of Gd-$4f$ states in GdCu with respect to bulk Gd within DFT+$U$ is sensitive to choice of lattice parameter. The calculations for $4f$-level shifts using DFT+$U$ methods as well as Hubbard-1 approximation are not consistent with the experimental findings.
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First principles study of structural, elastic, and electronic properties of the cubic perovskitetype BaHfO$_3$ has been performed using the plane wave ultrasoft pseudo-potential method based on density functional theory with revised Perdew-Burke-Ernzerhof exchange-correlation functional of the generalized gradient approximation (GGA-RPBE). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported in the literatures. The independent elastic constants (emph{C}$_{11}$, emph{C}$_{12}$, and emph{C}$_{44}$), bulk modules emph{B} and its pressure derivatives $B^{prime}$, compressibility $beta$, shear modulus emph{G}, Youngs modulus emph{Y}, Poissons ratio $ u$, and Lam{e} constants ($mu, lambda$) are obtained and analyzed in comparison with the available theoretical and experimental data for both the singlecrystalline and polycrystalline BaHfO$_3$. The band structure calculations show that BaHfO$_3$ is a indirect bandgap material (R-$Gamma$ = 3.11 eV) derived basically from the occupied O 2emph{p} and unoccupied Hf 5emph{d} states, and it still awaits experimental confirmation. The density of states (total, site-projected, and emph{l}-decomposed) and the bonding charge density calculations make it clear that the covalent bonds exist between the Hf and O atoms and the ionic bonds exist between the Ba atoms and HfO$_3$ ionic groups in BaHfO$_3$. From our calculations, it is shown that BaHfO$_3$ should be promising as a candidate for synthesis and design of superhard materials due to the covalent bonding between the transition metal Hf 5emph{d} and O 2emph{p} states.
The complicated electronic, magnetic, and colossal magnetoresistant (CMR) properties of Sr and Ca doped lanthanum manganites can be understood by spin-polarized first-principles calculations. The electronic properties can be attributed to a detailed balancing between Sr and Ca induced metal-like O 2p and majority-spin (majority-spin) Mn eg delocalized states and the insulator-like minority-spin (minority-spin) Mn t2g band near the Fermi level (EF). The magnetic properties can be attributed to a detailed balancing between O mediated antiferromagnetic superexchange and delocalized majority-spin Mn eg-state mediated ferromagnetic spin-spin couplings. While CMR can be attributed to the lining up of magnetic domains trigged by the applied magnetic field, which suppresses the trapping ability of the empty Mn t2g states that resists the motion of conducting Mn majority-spin eg electrons.
Fully relativistic first-principles electronic structure calculations based on a noncollinear local spin density approximation (LSDA) are performed for pyrochlore iridates Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$. The all-in, all-out antiferromagnetic (AF) order is stablized by the on-site Coulomb repulsion $U>U_c$ in the LSDA+$U$ scheme, with $U_csim1.1$~eV and 1.3~eV for Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$, respectively. AF semimetals with and without Weyl points and then a topologically trivial AF insulator successively appear with further increasing $U$. For $U=1.3$~eV, Y$_2$Ir$_2$O$_7$ is a topologically trivial narrow-gap AF insulator having an ordered local magnetic moment $sim0.5mu_B$/Ir, while Pr$_2$Ir$_2$O$_7$ is barely a paramagnetic semimetal with electron and hole concentrations of $0.016$/Ir, in overall agreements with experiments. With decreasing oxygen position parameter $x$ describing the trigonal compression of IrO$_6$ octahedra, Pr$_2$Ir$_2$O$_7$ is driven through a non-Fermi-liquid semimetal having only an isolated Fermi point of $Gamma_8^+$, showing a quadratic band touching, to a $Z_2$ topological insulator.
The electronic structures of CeRhSn and CeRuSn are self-consistently calculated within density functional theory using the local spin density approximation for exchange and correlation. In agreement with experimental findings, the analyses of the electronic structures and of the chemical bonding properties point to the absence of magnetization within the mixed valent Rh based system while a finite magnetic moment is observed for trivalent cerium within the Ru-based stannide, which contains both trivalent and intermediate valent Ce.
Understanding the magnetic properties of graphenic nanostructures is instrumental in future spintronics applications. These magnetic properties are known to depend crucially on the presence of defects. Here we review our recent theoretical studies using density functional calculations on two types of defects in carbon nanostructures: Substitutional doping with transition metals, and sp$^3$-type defects created by covalent functionalization with organic and inorganic molecules. We focus on such defects because they can be used to create and control magnetism in graphene-based materials. Our main results are summarized as follows: i)Substitutional metal impurities are fully understood using a model based on the hybridization between the $d$ states of the metal atom and the defect levels associated with an unreconstructed D$_{3h}$ carbon vacancy. We identify three different regimes, associated with the occupation of distinct hybridization levels, which determine the magnetic properties obtained with this type of doping; ii) A spin moment of 1.0 $mu_B$ is always induced by chemical functionalization when a molecule chemisorbs on a graphene layer via a single C-C (or other weakly polar) covalent bond. The magnetic coupling between adsorbates shows a key dependence on the sublattice adsorption site. This effect is similar to that of H adsorption, however, with universal character; iii) The spin moment of substitutional metal impurities can be controlled using strain. In particular, we show that although Ni substitutionals are non-magnetic in flat and unstrained graphene, the magnetism of these defects can be activated by applying either uniaxial strain or curvature to the graphene layer. All these results provide key information about formation and control of defect-induced magnetism in graphene and related materials.
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