Spin Density Functional Theory Research Articles

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.

Radicals Formed in N-Acetylproline by Electron Attachment: Electron Spin Resonance Spectroscopy and Computational Studies

In this study, the reactions of electrons with N-acetylproline are investigated by electron spin resonance (ESR) spectroscopy and density functional theory. Electrons are produced by γ irradiation or by photoionization of K(4)Fe(CN)(6) in neutral 7.5 M LiCl-D(2)O aqueous glasses at low temperatures with identical results. Electrons are found to add to both the peptide bond and the carboxyl group of the acetyl-proline moiety at 77 K. On annealing, both the electron adducts undergo fragmentation of the peptide bond between the nitrogen and the α carbon of the peptide structure. However, the peptide bond electron adduct radical reacts much more rapidly than the carboxyl group electron adduct radical. The DFT calculations predict that the carboxyl adduct is substantially more stable than the peptide bond adduct, with the activation barrier to N-Cα cleavage 3.7 kcal/mol for the amide electron adducts and 23 kcal/mol for the carboxyl electron adducts in inagreement with the relative reactivity found by experiment.

Magnetic exchange couplings from constrained density functional theory: An efficient approach utilizing analytic derivatives

We introduce a method for evaluating magnetic exchange couplings based on the constrained density functional theory (C-DFT) approach of Rudra, Wu, and Van Voorhis [J. Chem. Phys. 124, 024103 (2006)]. Our method shares the same physical principles as C-DFT but makes use of the fact that the electronic energy changes quadratically and bilinearly with respect to the constraints in the range of interest. This allows us to use coupled perturbed Kohn-Sham spin density functional theory to determine approximately the corrections to the energy of the different spin configurations and construct a priori the relevant energy-landscapes obtained by constrained spin density functional theory. We assess this methodology in a set of binuclear transition-metal complexes and show that it reproduces very closely the results of C-DFT. This demonstrates a proof-of-concept for this method as a potential tool for studying a number of other molecular phenomena. Additionally, routes to improving upon the limitations of this method are discussed.

Ferromagnetism in Mo-doped TiO2 Rutile from Ab Initio Study

First-principles calculations based on spin density functional theory (DFT) within the general gradient approximation (GGA) are performed to study the spin-resolved electronic properties of TiO2 Rutile doped with 6.25 and 12.5% of Mo. The Mo impurity is found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The local moment of Molybdenum is slightly dependent on its concentration. The Fermi level is shifted to the bottom of the conduction band with increasing concentration of Mo. This leads to the increase of states density, just above the Fermi level, and consequently the 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region. The Mo-doped TiO2 favors ferromagnetic ground state which can be explained in terms of p–d hybridization mechanism for 6.25% of Mo, this mechanism depends on the Mo concentration. This suggests that the Mo-doped TiO2 is a promising dilute magnetic semiconductor and may find applications in the field of spintronics.

Itinerant and localized magnetic moments in ferrimagnetic Mn2CoGa thin films probed by x-ray magnetic linear dichroism: Experiment andab initiotheory

Epitaxial thin films of the half-metallic ${X}_{a}$ compound Mn${}_{2}$CoGa (Hg${}_{2}$CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of $1.95(5)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per unit cell were obtained. The ${L}_{3,2}$ x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.

Dependence of Structural and Electronic Properties of Uranium Monochalcogenides on Exchange–Correlation Energy Functionals

We study the dependence of the structural properties of uranium monochalcogenides, U X where X = S, Se, and Te, as well as their electronic ones on the exchange–correlation energy functionals within the spin density functional theory, carrying out all electron calculations by the fully relativistic full-potential linear-combination-of-atomic-orbitals method. We employ two functionals of the local spin density approximation (LSDA) and two functionals of the generalized gradient approximations (GGA); the former two are the Perdew–Zunger and Perdew–Wang functionals and the latter two are the Perdew–Burke–Ernzerhof (PBE) and PBEsol functionals. We also examine the effects of the relativistic correction to the LSDA exchange part of each functional. We find that, for lattice constants, bulk moduli, and cohesive energies, the results of the calculations using the PBE functional are in the best agreement with the experimental results. On the contrary, we find that calculated total magnetic moments and one-electro...

Electron theory of magnetoelectric effects in metallic ferromagnetic nanostructures

The physics of magnetoelectric effects in metallic films is investigated by use of the ab initio spin density functional theory for Fe films and Fe-Au-Fe heterofilms with various thicknesses. Based on the magnetic force theorem for the calculation of the magnetocrystalline anisotropy energy, ${E}_{\text{mca}}$, a method for the definition of layer-resolved contributions to ${E}_{\text{mca}}$ is introduced, and the reliability of this method is discussed in detail. The knowledge of such layer-resolved contributions is important for a theoretical modeling of magnetoelectric effects on an atomic level in systems that are too complicated for an ab initio treatment. Quantities that characterize the anisotropic electron charge density and its magnetoelectric modification are introduced, and these quantities are useful for the interpretation of the magnetoelectric effects on ${E}_{\text{mca}}$ in the spirit of Kittel's explanation of ${E}_{\text{mca}}$. Finally, it is shown that for certain average thicknesses of ultrathin Fe films in Fe-Au-Fe it is possible to switch the magnetization between in-plane and out-of-plane directions by rather small electric fields.

Exact-exchange spin-density functional theory of Wigner localization and phase transitionsin quantum rings

We apply exact-exchange spin-density functional theory in the Krieger–Li–Iafrateapproximation to interacting electrons in quantum rings of different widths. The rings arethreaded by a magnetic flux that induces a persistent current. A weak space and spinsymmetry breaking potential is introduced to allow for localized solutions. As theelectron–electron interaction strength described by the dimensionless parameterrS is increased, we observe—at a fixed spin magnetic moment—the subsequent transition of bothspin sub-systems from the Fermi liquid to the Wigner crystal state. A dramatic signature ofWigner crystallization is that the persistent current drops sharply with increasingrS. We observe simultaneously the emergence of pronounced oscillationsin the spin-resolved densities and in the electron localization functionsindicating a spatial electron localization showing ferrimagnetic order afterboth spin sub-systems have undergone the Wigner crystallization. The criticalrSc at the transition point is substantially smaller than in a fully spin-polarizedsystem and decreases further with decreasing ring width. Relaxing theconstraint of a fixed spin magnetic moment, we find that on increasingrS the stable phase changes from an unpolarized Fermi liquid to an antiferromagnetic Wignercrystal and finally to a fully polarized Fermi liquid.

Spin transport properties of single metallocene molecules attached to single-walled carbon nanotubes via nickel adatoms

The spin-dependent transport properties of single ferrocene, cobaltocene, and nickelocene molecules attached to the sidewall of a (4,4) armchair single-walled carbon nanotube via a Ni adatom are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with the spin density functional theory. Our calculations show that the Ni adatom not only binds strongly to the sidewall of the nanotube, but also maintains the spin degeneracy and affects little the transmission around the Fermi level. When the Ni adatom further binds to a metallocene molecule, its density of states is modulated by that of the molecule and electron scattering takes place in the nanotube. In particular, we find that for both cobaltocene and nickelocene the transport across the nanotube becomes spin-polarized. This demonstrates that metallocene molecules and carbon nanotubes can become a promising materials platform for applications in molecular spintronics.

Half-metallic ferromagnetism in ZnCrTe and CdCrTe: Ab initio study

Using the first-principles method, we investigate the structural, electronic and magnetic properties of the diluted magnetic semiconductors ZnCrTe and CdCrTe in the zinc blende phase with 25% of Cr. We have treated the ferromagnetic and antiferromagnetic phases. 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 new generalized gradient approximation GGA form and GGA+U. Structural properties are determined from the total energy calculations and we found that these compounds are stable in the ferromagnetic phase. We discuss the electronic structures, total and partial densities of states and local moments. Furthermore, we predict the values of spin-exchange splitting energies Δx(d) and Δx(pd) and exchange constants N0α and N0β produced by the Cr 3d states. Finally, we demonstrate that the half-metallicity is robust with respect to lattice contraction.

Negative Tunneling Magnetoresistance by Canted Magnetization inMgO/NiOTunnel Barriers

The influence of the insertion of an ultrathin NiO layer between the MgO barrier and the ferromagnetic electrodes in magnetic tunnel junctions has been investigated from measurements of the tunneling magnetoresistance and via x-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of up to -16% at 2.8 K. We attribute this effect to the formation of noncollinear spin structures at the interface of the NiO layer as inferred from XMCD measurements. The magnetic moments of the interface Ni atoms tilt from their easy axis due to exchange coupling with the neighboring ferromagnetic electrode, and the tilting angle decreases with increasing NiO thickness. The experimental observations are further supported by noncollinear spin density functional calculations.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Adiabatic approximation within time-dependent density functional theory using inversion of the ground-state spin-density Kohn–Sham formalism

It has recently been shown by Thiele et al. [M. Thiele, E. K. U. Gross, S. Kümmel, Phys. Rev. Lett. 100 (2008) 153004] that the exact adiabatic approximation in time-dependent density functional theory gives a good description of non-sequential double ionization in the one-dimensional helium atom. In this paper, we propose an adiabatic approximation based on the inversion of ground-state spin-density functional theory and apply it to several model systems. We demonstrate that our approach reproduces the derivative discontinuity and yields correlation potentials close to the exact correlation potentials for a strong-field ionization process as well as for the 1D H 2 and LiH molecules at large internuclear distance.

Constrained active space unrestricted mean-field methods for controlling spin-contamination

We have recently proposed a novel approach for obtaining high-spin restricted open-shell Hartree-Fock wave functions by imposing constraints on the unrestricted Hartree-Fock (UHF) method [T. Tsuchimochi and G. E. Scuseria, J. Chem. Phys. 133, 141102 (2010)]. We here extend these ideas to the case where the constraints are released in an active space but imposed elsewhere. If the active space is properly chosen, our constrained UHF (CUHF) method greatly benefits from a controlled broken-symmetry effect while avoiding the massive spin contamination of traditional UHF. We also revisit and apply Löwdin's projection operator to CUHF and obtain multireference wave functions with moderate computational cost. We report singlet-triplet energy splittings showing that our constrained scheme outperforms fully unrestricted methods. This constrained approach can be readily used in spin density functional theory with similar favorable effects.

Theoretical study on the interaction between C3 molecular wires and nanotubes

AbstractIn the present paper, the interaction between C3 molecular wires and nanotubes is studied by using first‐principles calculations based on spin density functional theory. We optimize the geometries structures of isolated C3 molecules. In addition, we present the bond lengths and bond angles for the C3 molecular wires that are inserted into different single‐walled carbon nanotubes (SWCNTs). In addition, the binding energies between C3 and SWCNTs are also calculated. Furthermore, the energy band structures and electron density of states (DOS) of C3@(4,4) and C3@(7,0) are also shown and compared with those of armchair SWCNT (4,4) and zigzag SWCNT (7,0). Then, we further investigate the interaction between C3 molecular wires and single‐walled boron nitride nanotubes (SWBNNTs) by the same method used in SWCNTs, and find that the presence of C3 molecules in BNNTs introduces an intermediate band (IB) consisting of two new energy bands. And the IB dramatically decreases the bandgaps of BNNTs.

Magnetic‐field‐induced electron transitions in concentric double quantum rings

AbstractWithin local spin‐density functional theory, we show that changes in the intensity of a magnetic field perpendicularly applied to a few‐electron concentric double quantum ring may alter the electron distribution in the constituent rings producing an observable effect on the infrared spectrum of the system. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Projector augmented-wave method: Application to relativistic spin-density functional theory

Applying the projector augmented-wave (PAW) method to relativistic spin-density functional theory (RSDFT) we derive PAW Dirac-Kohn-Sham equations for four-component spinor pseudo-wave-functions. The PAW freedom to add a vanishing operator inside the PAW spheres allows us to transform these PAW Dirac-type equations into PAW Pauli-type equations for two-component spinor pseudo-wave-functions. With these wave functions, we get the frozen-core energy as well as the charge and magnetization densities of RSDFT, with errors comparable to the largest between $1/{c}^{2}$ and the transferability error of the PAW data sets. Presently, the latter limits the accuracy of the calculations, not the use of the Pauli-type equations. The theory is validated by applications to isolated atoms of Fe, Pt, and Au, and to the band structure of fcc-Pt, fcc-Au, and ferromagnetic bcc-Fe.

Fully Relativistic Full-Potential Calculations of Magnetic Moments in Uranium Monochalcogenides with the Dirac Current

We study the orbital, spin, and total magnetic moments in uranium monochalcogenides, U X where X =S, Se, and Te, using the fully relativistic full-potential calculations based on the spin density functional theory. In particular, the orbital magnetic moments are calculated with the Dirac current. We employ two methods which adopt distinctly different basis sets; one is the fully relativistic full-potential linear-combination-of-atomic-orbitals (FFLCAO) method and the other is the fully relativistic full-potential mixed-basis (FFMB) method. Showing that the orbital magnetic moments calculated using the FFLCAO method and those calculated using the FFMB method agree very well with each other, we demonstrate that, in contrast to the conventional method, the method with the Dirac current enables us to calculate the orbital magnetic moments even if the basis set includes basis functions with no definite angular momenta, e.g., the plane waves in the FFMB method. Furthermore, it is found that the orbital magnetic...

Local atomic structure of superconductingFeSe1−xTex

The isovalent substitution of Te for Se in the superconducting $\ensuremath{\alpha}\text{-FeSe}$ raises ${T}_{C}$ where the average chalcogen--Fe bond angle decreases and the chalcogen-Fe distance increases. Locally, however, the Se and Te ions do not share the same site and have two distinct $z$ coordinates, in contrast to what is presumed in the $P4/nmm$ symmetry. The local bond angle between the chalcogens and Fe increases with the substitution, consistent with the rise in ${T}_{C}$, the Fe--Te bonds become shorter than in the binary FeTe, while the Fe--Se bonds stay the same as in the binary. Ab initio calculations based on spin density functional theory yielded an optimized structure with distinct $z$ coordinates for Se and Te, in addition to a stronger hybridization of Te with Fe.

Self-interaction Free Relativistic Spin-density Functional Theory

Abstract We review the progress in the recent development of the relativistic optimized effective potential (ROEP) method within spin-density functional theory. The ROEP-equations for spin-polarized systems are derived and their application to open-shell atoms using the exact exchange approximation for the exchange correlation-functional is presented. Further, we expand the ROEP framework to treat extended systems within the KKR-multiple scattering formalism. We illustrate the application of the theory to open-shell free atoms and solids.