Spin Density Functional Theory Research Articles

The half-metallicity of zinc-blende CaC/GaAs(001) heterojunction: A density functional theory study

First principles calculations, based on spin density functional theory (SDFT) as implemented in the PWscf code of the QUANTUM-ESPRESSO package, are performed to study electronic and magnetic properties of CaC/GaAs(001) heterojunction. We simulate the interface of CaC in Zinc-Blende (ZB) structure and GaAs semiconductor to see whether this electrode could be a good choice for spin current injection or not. For this purpose, we calculate the spin-polarized density of states for ZB–CaC, in-plane strained ZB–CaC (tetragonal phase) and also for ZB–CaC/GaAs(001) heterojunction. It is shown that ZB–CaC, is a half-metal ferromagnet with large half-metallic gap. The half-metallicity is found to be robust with respect to the lattice deformation to the tetragonal phase and is maintained in the range of 0.59< c/a <1.22. Calculations with supercell scheme for simulation ZB-CaC/GaAs(001) heterojunction show that the spin polarization at the Fermi energy reduced at the CaC/GaAs(001) heterojunction. But due to large difference of density of states of spin channels around the Fermi energy, the spin polarization is large yet. So the CaC/GaAs(001) heterojunction can be used in spintronics applications as a spin current injector but it is not 100% spin-polarized.

GGA and GGA+U Description of Structural, Magnetic, and Elastic Properties of Rh2MnZ (Z=Ge, Sn, and Pb)

Structural, magnetic, electronic, and elastic properties of Rh2MnGe, Rh2MnSn, and Rh2MnPb Heusler compounds have been calculated using full potential linearized augmented-plane wave plus local orbitals (FP-L/APW+lo) method based on the spin density functional theory, within the generalized gradient approximation (GGA) and (GGA+U) (U is the Hubbard correction). Results are given for the lattice parameters, bulk moduli, spin magnetic moments and elastic constants. We have derived the bulk and the shear moduli, Young’s and Poisson’s ratio for Rh2MnZ (Z=Ge, Sn, and Pb). The elastic modulus of Rh2MnGe is predicted to be the highest. Also, we have estimated the Debye temperatures from the average sound velocity. We discuss the electronic structures, total and partial densities of states and local moments and we investigate the pressure effect on the elastic properties by calculating the elastic constants at various volumes.

Configuration interaction approach to Fermi liquid–Wigner crystal mixed phases in semiconductor nanodumbbells

Full configuration interaction calculations demonstrate the existence of mixed correlation phases in truly three-dimensional elongated nanocrystals subject to inhomogeneous spatial confining potentials. In such phases, the electron density behaves like a Fermi liquid in some regions, while, simultaneously, other more dilute regions display the typical quasi-classical Wigner distribution. The present results confirm and strengthen previous local spin-density functional theory predictions [Ballester et al., Phys. Rev. B 82, 115405 (2010)]. Additionally, simulation of the in-plane and z-polarized modes of the absorption spectra reveals the different correlation regimes occurring in these systems.

Clustering and magnetic anisotropy of Fe adatoms on graphene

Single Fe adatoms and clusters of Fe adatoms on graphene are studied through first-principles calculations using density functional theory (DFT) and spin density functional theory (sDFT). First, we consider computational cells containing various numbers of C atoms and one Fe adatom. We calculate the binding energy, adatom height, and magnetic moment of the adatom above a few high-symmetry positions in the cell. In all cases, the binding energy increases with decreasing cell size, suggesting that clustering of the Fe adatoms is energetically favored. We also calculate the energy of various clusters of two to four Fe atoms on graphene in computational cells of various sizes, using both DFT and sDFT. These calculations again show that, both in DFT and sDFT, the Fe adatoms strongly prefer to form clusters. The energy barrier for an isolated Fe adatom to diffuse from the center of one graphene hexagon is calculated to be 0.49 eV. This barrier is reduced for an Fe atom which is one of a pair of neighboring adatoms. Finally, by including spin-orbit interactions within sDFT, we calculate the magnetic anisotropy energy of a single Fe adatom on graphene. We find that the in-plane anisotropy energy is close to zero, while the out-of-plane anisotropy energy is $\ensuremath{\sim}$$D{S}^{2}{\mathrm{cos}}^{2}\ensuremath{\theta}$ where $S\ensuremath{\sim}2.0$, $\ensuremath{\theta}$ is the angle between the magnetic moment and the perpendicular to the graphene plane, and $D\ensuremath{\sim}0.25$ meV.

A New Approach to Noncollinear Spin Density Functional Theory beyond the Local Density Approximation.

The application of density functional theory to heavy elements and magnetic materials requires the generalization of existing functionals to the case of noncollinear spins. This letter describes a new idea to achieve such a generalization which, unlike previous efforts in this direction, (i) affords a nonvanishing local magnetic torque, (ii) is invariant with respect to spin-rotations, (iii) is free from numerical instabilities in regions of small magnetization, and (iv) reduces to the proper collinear limit.

Description of Substitutional Adsorption of Magnetic Ions on Metallic Surfaces with Formation of Monolayer Ferromagnetic Films Using the Spin-Density Functional Method

At present time, a large number of experimental works have been devoted to the study of magnetic ordering in Fe, Co, and Ni ultrathin films [. The investigation of a nature of magnetism in these structures has a large fundamental interest through an observable dimensionality crossover of magnetic characteristics from three dimensional values for films with thickness d 10 nm to two dimensional values for films with thickness d 1-2 nm. It has been established that the long-range ferromagnetic order arises in films at some effective film thickness. However, the nature and regularities of this phenomenon remain not quite clear. In our paper [ it was developed in terms of the spin-density functional theory the description of influence of the temperature and ferromagnetic ordering on the adsorption of Fe, Co, and Ni transition metal ions on a nonmagnetic substrate with the formation of a submonolayer films. For case of nonactivated adsorption the conditions for the formation of magnetic monoatomic films stable with respect to island adsorption with a change in the coverage parameter θ are revealed. It was demonstrated that the inclusion of the ferromagnetic ordering substantially affects the adsorption energy and leads to its considerable increase.

Full-Potential Study of Half-Metallic Ferromagnetism in CdTMO&lt;SUB&gt;2&lt;/SUB&gt; (TM=Fe, Co and Ni)

In this work, we present an ab initio calculation for the structural, electronic and magnetic properties of CdTMO2 (TM=Fe, Co and Ni) compound. The calculations are performed by a developed full-potential augmented plane wave plus local orbitals (FP-L/APW+lo) method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation GGA form and GGA+U (where U is the Hubbard correlation terms). We have performed total energy calculations of these compounds in the chalcopyrite structure. We present a comparative study between the electronic structures, total and partial densities of states and local moments calculated within GGA and GGA+U. Furthermore, we predict the values of spin-exchange splitting energies Δx(d) and Δx(p-d) and exchange constants N0α and N0β produced by the transition metal 3d states. Our calculations show the half-metallic behavior of these ferromagnetic compounds.

Few electron limit of n-type metal oxide semiconductor single electron transistors

We report the electronic transport on n-type silicon single electron transistors (SETs) fabricated in complementary metal oxide semiconductor (CMOS) technology. The n-type metal oxide silicon SETs (n-MOSSETs) are built within a pre-industrial fully depleted silicon on insulator (FDSOI) technology with a silicon thickness down to 10 nm on 200 mm wafers. The nominal channel size of 20 × 20 nm2 is obtained by employing electron beam lithography for active and gate level patterning. The Coulomb blockade stability diagram is precisely resolved at 4.2 K and it exhibits large addition energies of tens of meV. The confinement of the electrons in the quantum dot has been modeled by using a current spin density functional theory (CS-DFT) method. CMOS technology enables massive production of SETs for ultimate nanoelectronic and quantum variable based devices.

Oxidation State and Symmetry of Magnesia-Supported Pd13OxNanocatalysts Influence Activation Barriers of CO Oxidation

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.

Spin-polarized calculations of electronic structures in ferromagnetic and antiferromagnetic Zn0.75TM0.25Se (TM=Cr, Fe, Co and Ni)

In this work, we aim to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of Zn1−xTMxSe (TM=Cr, Fe, Co and Ni) diluted magnetic semiconductors in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, and with 25% of TM. The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation (GGA) form. We treated the ferromagnetic and antiferromagnetic phases and we found that all compounds are stable in the ferromagnetic structure. Structural properties are computed after total energy minimization. Our results show that the cohesive energies of Zn0.75TM0.25Se are greater than that of zinc blende ZnSe. We discuss the electronic structures, total and partial densities of states, local moments and the p–d exchange splitting. Furthermore, we found that p–d hybridization reduces the local magnetic moment of TM and produces small local magnetic moments on the nonmagnetic Zn and Se sites. We found also that in the AFM phase the TM local magnetic moments are smaller than in the FM phase; this is due to the greater interaction of the TM d-up and d-down orbitals.

Half-metallic ferromagnetism in Ag-doped ZnO: An ab initio study

The spin-resolved electronic structures of ZnO doped with 6.25% Ag have been studied with the first-principles calculations based on the spin density functional theory. The substitutional Ag impurities and their nearest neighbor O atoms are shown to be in a spin polarized state with a global magnetization of 1.0μB. Ag-doped ZnO is in a ferromagnetic ground state which can be explained by Zener's double exchange mechanism. Furthermore, band structure calculations show a half-metallic behavior of the Ag-doped ZnO. These results indicate that Ag-doped ZnO shows promise as a dilute magnetic semiconductor free of magnetic precipitation and may find applications in the field of spintronics.

Transport spin polarization of magnetic C28 molecular junctions

We present a theoretical study of spin transport through a magnetic C28 molecule sandwiched between two Au (111) electrodes. The ab initio modeling is performed by spin density functional theory and nonequilibrium Green’s function technique. The results clearly show that the spin-resolved transmission spectra of C28 molecular junctions exhibit robust transport spin polarization (TSP) characteristics, which depends on the contact configuration. At the small bias voltage, the conductance of C28 is mainly determined by the spin-down electrons. The TSP behavior can be effectively tuned by the gate. Our results indicate that C28 molecule holds promise in future molecular spintronics applications.

Surface properties of the clean and Au/Pd covered Fe3O4(111): DFT andDFT+Ustudy

The spin-density functional theory (DFT) and DFT+$U$ with Hubbard $U$ term accounting for on-site Coulomb interactions were applied to investigate structure, stability, and electronic properties of different terminations of the Fe$_3$O$_4$(111) surface. All terminations of the ferrimagnetic Fe$_3$O$_4$(111) surface exhibit very large (up to 90%) relaxations of the first four interlayer distances, decreasing with the oxide layer depth. Our calculations predict the iron terminated surface to be most stable in a wide range of the accessible values of the oxygen chemical potential. The adsorption of Au and Pd on two stable Fe- and O-terminated surfaces is studied. Our results show that Pd binds stronger than Au both to the Fe- and O-terminated surface. DFT+$U$ gives stronger bonding than DFT. The bonding of both adsorbates to the O-terminated magnetite surface is by 1.5-2.5 eV stronger than to the Fe-terminated surface.

First-principles studies of spin-orbit and Dzyaloshinskii-Moriya interactions in the {Cu3} single-molecule magnet

Frustrated triangular molecule magnets such as \{Cu$_3$\} are characterized by two degenerate S=1/2 ground-states with opposite chirality. Recently it has been proposed theoretically [PRL {\bf 101}, 217201 (2008)] and verified by {\it ab-initio} calculations [PRB {\bf 82}, 155446 (2010)] that an external electric field can efficiently couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). The SOI is nevertheless important, since it introduces a splitting in the ground-state manifold via the Dzyaloshinskii-Moriya interaction. In this paper we present a theoretical study of the effect of the SOI on the chiral states within spin density functional theory. We employ a recently-introduced Hubbard model approach to elucidate the connection between the SOI and the Dzyaloshinskii-Moriya interaction. This allows us to express the Dzyaloshinskii-Moriya interaction constant $D$ in terms of the microscopic Hubbard model parameters, which we calculate from first-principles. The small splitting that we find for the \{Cu$_3$\} chiral state energies ($\Delta \approx 0.02$ meV) is consistent with experimental results. The Hubbard model approach adopted here also yields a better estimate of the isotropic exchange constant than the ones obtained by comparing total energies of different spin configurations. The method used here for calculating the DM interaction unmasks its simple fundamental origin which is the off-diagonal spin-orbit interaction between the generally multireference vacuum state and single-electron excitations out of those states.

Structural, electronic, and magnetic properties of NinM clusters (M = Hf, Ta, W) with n = 1–12

We have studied structural, electronic, and magnetic properties of NinM (M=Hf, Ta, W) clusters with n=1–12 using spin density functional theory. The calculated results find that a single impurity M (M=Hf, Ta, W) enhances the binding energy of the nickel cluster, reduces their magnetic moment and decreases the ionization potential. Hf and Ta occupies the substitutional vertex site accompanied with substantial distortion in the nickel clusters. We find that the geometries of the host clusters do not change significantly after the doping of the W atom. After n>8, W atom prefers the interstitial site in the host cluster. The ability to delocalize the Ni-3d electrons decreases from Hf to W. The magnetic moments of doped clusters (NinHf, NinTa and NinW) are quenched because of the antiferromagnetic arrangement of the M (Hf, Ta, W) electrons.

1H NMR, Electron Paramagnetic Resonance, and Density Functional Theory Study of Dinuclear Pentaammineruthenium Dicyanamidobenzene Complexes

Paramagnetic (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and density functional theory (DFT) spin density calculations were selectively performed on the [{(NH(3))(5)Ru}(2)(μ-L)](3+,4+,5+) complexes, where L is 2,3,5,6-tetrachloro-, 2,5-dichloro-, 2,5-dimethyl-, and unsubstituted 1,4-dicyanamidobenzene dianion, to characterize the electronic structure of these complexes. EPR spectra of the [{(NH(3))(5)Ru}(2)(μ-L)](3+) complexes in N,N'-dimethylformamide at 4 K showed a ruthenium axial signal, and thus the complexes are [Ru(II),L(2-), Ru(III)] mixed-valence systems. DFT spin density calculations of [{(NH(3))(5)Ru}(2)(μ-L)](3+) where L = 1,4-dicyanamidobenzene dianion gave mostly bridging-ligand centered spin distribution for both vacuum and implicit solvent calculations, in poor agreement with EPR, but more realistic results were obtained when explicit electrostatic interactions between solute and solvent were included in modeling. For the [{(NH(3))(5)Ru}(2)(μ-L)](4+) complexes, EPR spectroscopy showed no signal down to 4 K. Nevertheless, solvent-dependent (1)H NMR data and analysis support a [Ru(III),L(2-), Ru(III)] state. Hyperfine coupling constants (A(c)/h) of trans- and cis-ammine and phenyl hydrogens were determined to be 17.2, 3.8, and -1.5 MHz respectively. EPR studies of the [{(NH(3))(5)Ru}(2)(μ-L)](5+) complexes showed a metal-radical axial signal and based on previously published (1)H NMR data, a [Ru(IV),L(2-), Ru(III)] state is favored over a [Ru(III),L(-), Ru(III)] state.

Structure, electronic configuration, and Mössbauer spectral parameters of an antiferromagnetic Fe2-peroxo intermediate of methane monooxygenase

Determining structures of reaction intermediates is crucial for understanding catalytic cycles of metalloenzymes. However, short life times or experimental difficulties have prevented obtaining such structures for many enzymes of interest. We report geometric and electronic structures of a peroxo intermediate in the catalytic cycle of methane monooxygenase hydroxylase (MMOH) for which there is no crystallographic characterization. The structure was predicted via spin density functional theory using (57)Fe Mössbauer spectral parameters as a reference. Computed isomer shifts (δ(Fe) = +0.68, +0.66 mm s(-1)) and quadrupole splittings (ΔE(Q) = -1.49, -1.48 mm s(-1)) for the predicted structure are in excellent agreement with experimental values of a peroxo MMOH intermediate. Predicted peroxo to iron charge transfer bands agree with UV-Vis spectroscopy. Peroxide binds in a cis μ-1,2 fashion and plays a dominant role in the active site's electronic structure. This induces a ferromagnetic to antiferromagnetic transition of the diiron core weakening the O-O bond in preparation for cleavage in subsequent steps of the catalytic cycle.

Valence-dependent analytic bond-order potential for magnetic transition metals

We extend the analytic bond-order potentials for transition metals [Phys. Rev. B 74, 174117 (2006)] to include ferro, antiferro, and noncollinear magnetism and charge transfer. This is achieved by first deriving a suitable tight-binding model through the expansion of the spin-density energy functional to second order with respect to magnetic and charge fluctuations. The tight-binding model is then approximated locally by the bond-order potential expansion, where the variational property of the bond-order potential expansion allows us to derive analytic expressions for the forces and torques on the atoms. From the bond-order potentials we then extract a hierarchy of multispin interactions beyond the conventional Heisenberg model. The explicit valence dependence of the bond-order potentials enables us to characterize the magnetic properties of the 3$d$ transition metals and to reproduce the trend from antiferromagnetic spin ordering close to the center of the $d$ band through noncollinear spin configurations to ferromagnetic ordering toward the edges of the $d$ band. The analytic representation of the energy within the bond-order potentials is then further expanded in the form of a Ginzburg-Landau expansion, deriving the prefactors explicitly from tight-binding and bond-order potentials. Thus, in this paper we present a coherent simplification from fundamental to empirical models of magnetism through coarse graining the electronic structure from spin-density functional theory to tight binding to bond-order potentials to the Ginzburg-Landau expansion.

First-principles calculation of X-ray dichroic spectra within the full-potential linearized augmented planewave method: An implementation into the Wien2k code

X-ray absorption and its dependence on the polarization of light is a powerful tool to investigate the orbital and spin moments of magnetic materials and their orientation relative to crystalline axes. Here, we present a program for the calculation of dichroic spectra from first principles. We have implemented the calculation of X-ray absorption spectra for left and right circularly polarized light into the Wien2k code. In this package, spin-density functional theory is applied in an all-electron scheme that allows to describe both core and valence electrons on the same footing. The matrix elements, which define the dependence of the photo-absorption cross-section on the polarization of light and on the sample magnetization, are computed within the dipole approximation. Results are presented for the L 2 , 3 and M 4 , 5 egdes of CeFe 2 and compared to experiments.

Electronic structure of EuN: Growth, spectroscopy, and theory

We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows that the samples contain predominantly Eu${}^{3+}$, but with a small and varying quantity of Eu${}^{2+}$ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K edge are compared to several different theoretical models, namely, local spin density functional theory with Hubbard $U$ corrections ($\text{LSDA}+U$), dynamic mean field theory (DMFT) in the Hubbard-I approximation, and quasiparticle self-consistent $GW$ (QS$GW$) calculations. The DMFT and QS$GW$ models capture the density of conduction band states better than does $\text{LSDA}+U$. Only the Hubbard-I model contains a correct description of the Eu $4f$ atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the lowest-lying ${}^{8}S$ multiplet. The Hubbard-I model is also in good agreement with purely atomic multiplet calculations for the Eu M-edge XAS. $\text{LSDA}+U$ and DMFT calculations find a metallic ground state, while QS$GW$ results predict a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV, and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow-band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu $4f$ multiplet raises the possibility of tuning both the magnetic and electronic states in this system.