Local Spin Density Functional Theory Research Articles

Magnetic ground state of supported monatomic Fe chains from first principles

A new computational scheme is presented based on a combination of the conjugate gradient and the Newton–Raphson method to self-consistently minimize the energy within local spin-density functional theory, thus to identify the ground state magnetic order of a finite cluster of atoms. The applicability of the new ab initio optimization method is demonstrated for Fe chains deposited on different metallic substrates. The optimized magnetic ground states of the Fe chains on Rh(111) are analyzed in details and a good comparison is found with those obtained from an extended Heisenberg model containing first principles based interaction parameters. Moreover, the effect of the different bilinear spin–spin interactions in the formation of the magnetic ground states is monitored. In case of Fe chains on Nb(110) spin-spiral configurations with opposite rotational sense are found as compared to previous spin-model results which hints on the importance of higher order chiral interactions. The wavelength of the spin-spiral states of Fe chains on Re(0001) was obtained in good agreement with scanning tunneling microscopy experiments.

Onset of magnetic order in multilayers of Fe and Ni on and embedded in fcc-Cu(100) substrates

We investigate magnetic correlations and local magnetic moments at finite temperatures in some Fe and Ni multilayers on and embedded in Cu(100). Our study is based on a mean-field theory of magnetic fluctuations for layered materials within the first-principles local spin-density functional theory. We find that although there is no significant local moments formation in the Ni layers, the Ni layers are not magnetically dead in the sense that they mediate magnetic interactions between Fe layers separated by Ni layers. The Curie temperature of FenNim/Cu(100) and NinFem/Cu(100) films is almost independent of the Ni layer thickness, however, in Fe1NinFe1/Cu(100) and Fe1CunFe1/Cu(100) films, the Curie temperature exhibits an interesting oscillatory behaviour as the spacer layer thickness is increased. We show that there is a connection between this behaviour and the spin-reorientation transition observed in some Ni films. In the Ni1FenNi1/Cu(100) and Cu1Fen/Cu(100) films we found that the nature and strength of the magnetic correlations depends upon the Fe layer thickness.

Theoretical investigation of spin-filtering in CrAs/GaAs heterostructures

The electronic structure of bulk zinc-blende GaAs, zinc-blende and tetragonal CrAs, and CrAs/GaAs supercells, computed within linear muffin-tin orbital (LMTO) local spin-density functional theory, is used to extract the band alignment for the [1,0,0] GaAs/CrAs interface in dependence of the spin orientation. With the lateral lattice constant fixed to the experimental bulk GaAs value, a local energy minimum is found for a tetragonal CrAs unit cell with a longitudinal ([1,0,0]) lattice constant reduced by ≈2%. Due to the identified spin-dependent band alignment, half-metallicity of CrAs no longer is a key requirement for spin-filtering. Based on these findings, we study the spin-dependent tunneling current in [1,0,0] GaAs/CrAs/GaAs heterostructures within the non-equilibrium Green's function approach for an effective tight-binding Hamiltonian derived from the LMTO electronic structure. Results indicate that these heterostructures are promising candidates for efficient room-temperature all-semiconductor spin-filtering devices.

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.

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.

Ground state and infrared response of triple concentric quantum ring structures

Within local-spin density-functional theory, we study the ground state and infrared response of two-dimensional, triple concentric quantum ring nanostructures. Changes in their physical properties are presented as a function of the number of electrons or the intensity of a perpendicularly applied magnetic field. We discuss the addition spectrum of few-electron triple quantum rings at zero magnetic field, as well as the physical appearance of the ground state and dipole response of selected systems containing up to 50 electrons. We also investigate the ground state, persistent currents, and charge- and spin-density responses of a system made of 30 electrons.

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)

The electronic structure of UCoGe by ab initio calculations and XPS experiment

The crystal and electronic structures of the orthorhombic compound UCoGeare presented and discussed. It has been either refined by the x-ray diffractionon a single crystal or computed within the local spin density functional theory,employing the fully relativistic version of the full-potential local-orbital band structurecode, respectively. We particularly give our attention to investigating the Fermisurface and de Haas–van Alphen quantities of UCoGe. The calculated electronicdensity is then examined by x-ray photoelectron spectroscopy (XPS). Fairly goodagreement is achieved between theoretical and experimental XPS results in theparamagnetic state. A small difference in the position (in energy scale) of the U 5fbands is caused by the electron localization effect observed in the experimentalXPS. There is also some discrepancy for the Co 3d electron contributions belowEF. The Fermi surface in the non-magnetic state is of a semimetallic typewhile that in the ferromagnetic state, with the ordered moment of−0.47 μB/f.u. alongthe c axis, is more metallic, with nesting properties that may favour superconductivity.

Magnetic and structural properties of Cr-based diluted magnetic semiconductors and alloys

We applied supercell approach by using local spin density functional theory for Cr-doped GaN, CrN/ScN superlattices and the linear muffin-tin orbital method to predict the structural and magnetic properties of these novel diluted magnetic semiconductors (DMS), superlattices and their Cr x Sc 1− x N alloys. The global energy minimum of CrN is obtained for rocksalt (RS) structure if the compound is expanded by 8% and the phase becomes stable in the ferromagnetic (FM) state. The global energy minimum for the stable state occurs at in-plane lattice constant of 3.9 Å. In addition, the structural and optical properties of single crystal CrN/ScN superlattices and Cr 1− x Sc x N alloys are studied in detail. We report an isostructural phase transition from wurtzite (w-CrN) to hexagonal (h-ScN) at a hydrostatic pressure of 21 GPa which is associated with anomalous optical and piezoelectric properties.

Electronic structure and Fermi surface of strongly ferromagneticU2N2Z-type(Z=Sb,Te,Bi)compounds from first principles

The electronic structures of the uranium ternaries of the ${\text{U}}_{2}{\text{N}}_{2}Z$-type $(Z=\text{Sb},\text{Te},\text{Bi})$, crystallizing in the tetragonal $I4/mmm$ structure and having among uranium systems relatively high values of both their ferromagnetic (FM) transition temperatures and ordered magnetic moments (mms), have been investigated ab initio. They were calculated employing the local-spin density-functional theory with different versions of the orbital polarization correction (OPC) applied within the fully relativistic and full potential local-orbital band-structure code. The obtained results predict all these materials to be metallic both in the ferromagnetically ordered and nonmagnetically ordered (NMO) states. However, the bottoms of their conduction bands in the NMO states occur merely about 0.3 eV below the Fermi level, ${\text{E}}_{\text{F}}$, and a pronounced gap in ${\text{U}}_{2}{\text{N}}_{2}\text{Te}$ or pseudogaps in the two remaining compounds are opening just below these bands, which makes such a metallic state unstable. The considerable reduction in these gap or pseudogaps in the FM states cause a stabilization of a metallic behavior. A covalently metallic bonding character of these ternaries has been obtained in all calculations. It appeared that the only uranium atoms create predominantly metallic bonding due to a fairly strong hybridization between the U $5f$ and U $6d$ electrons. The U $5f$ electrons contributing also to the covalent bonding have somewhat dual character, i.e., partly localized and itinerant. Contrary to the Sb- and Bi-based ternaries ordered along the $c$ axis, as deduced from previous neutron powder diffraction studies, ${\text{U}}_{2}{\text{N}}_{2}\text{Te}$ has a canted FM structure with its mms tilted from the $c$ axis by about $70\ifmmode^\circ\else\textdegree\fi{}$. This fact has also been revealed by the present theoretical results. These data also point to the fact that the telluride, unlike the two other compounds, exhibits an almost half-metallic behavior. For all three ternaries, using the OPXC version of OPC the computed mms are in accord with the published experimental data. In turn, the Fermi surfaces of investigated ternaries have quasi-two-dimensional properties and quite unique nesting features along directions of their easy magnetization axes.

Magnetic ordering in Fe/Co sandwiches on Cu(100)

We investigate magnetic correlations and local magnetic moments at finitetemperatures of some Fe and Co multilayers on Cu(100) substrates, such asComFenCom/Cu(100) andFemConFem/Cu(100). We use an ab initio mean-field theory of magnetic fluctuations for layered materials basedon the first-principles local spin-density functional theory implemented throughthe screened Korringa–Kohn–Rostoker method. We find that the presence ofFe layers in the neighbourhood of a Co layer always leads to a reduction in themagnetic moment of the Co atoms, whereas that of the Fe atoms is enhanced. Ofparticular interest is the lack of local moment formation on the single fcc-Co layersandwiched between two fcc-Fe layers. However, a Co layer completely immersedin a Cu environment remains ferromagnetic. The Curie temperature of theComFenCom/Cu(100) system oscillates as the Fe layer thickness is increased whereas that of theFemConFem/Cu(100) system increases almost monotonically with Co layer thickness.

Interplay of negative pressure and hydrogen chemical effects in CeRhSn from first principles

Investigations within the local spin density functional theory (LSDF) of the intermetallic hydride system CeRhSnHx were carried out for discrete model compositions in the range 0.33 ≤xH ≤ 1.33 with the purpose of assessing the change of the cerium valence state in the neighborhood of the experimental hydride composition, CeRhSnH0.8. In agreement with experiment, the analyses of the electronic and magnetic structures and of the chemical bonding properties point to trivalent cerium for 1 ≤xH ≤ 1.33. In contrast, for lower hydrogen amounts the hydride system stays in an intermediate-valent state for cerium, like in CeRhSn. The influence of the insertion of hydrogen is addressed from both the volume expansion and chemical bonding effects. The latter are found to have the main influence on the change of Ce valence character. Spin polarized calculations point to a finite magnetic moment carried by the Ce 4f states; its magnitude increases with xH in the range 1 ≤xH ≤ 1.33.

Different Cerium Valence Transitions Observed by Hydrogenation of the Ternary Germanides CeRhGe and CeIrGe – Structure, Physical Properties and Chemical Bonding

The ternary germanides CeRhGe and CeIrGe which crystallize in the orthorhombic TiNiSi-type structure, absorb hydrogen at 523 K. X-Ray powder diffraction and transmission electron microscopy indicate that the hydrides CeRhGeH1.8 and CeIrGeH1.8 adopt the hexagonal ZrBeSi-type structure. Magnetization, electrical resistivity and thermoelectric power measurements reveal that these hydrides are intermediate-valence compounds. An unusual transition from antiferromagnetic to spin fluctuation behavior occurs upon hydrogenation of CeRhGe, while on the contrary, CeIrGeH1.8 presents a Kondo temperature of 285 K smaller than that observed for CeIrGe (610 K). In order to explain these opposite valence transitions, the electronic structures of the hydrides have been selfconsistently calculated within the local spin density functional theory (LSDF). The structures are compared to those reported previously by us for CeRhGe and CeIrGe.

Structural bistability of the oxygen-adsorbed graphene sheet

Structural and electronic properties for oxygen-adsorbed graphene sheets have been explored using first-principles total-energy calculations within the local spin density functional theory. It has been found that the structural bistability appears with regard to the oxygen adsorption. This bistability corresponds to the formation of epoxy group or ether group, where the ether group phase is more stable than the epoxy group one. Further, the relative stability for the one-side adsorption model to the both-sides one is explored; oxygen atoms prefer to adsorb on both sides of the graphene sheet, while the one-side adsorption structure becomes metastable.

Tuning the Structural and Physical Properties of Cr2Ti3Se8 by Lithium Intercalation: A Study of the Magnetic Properties, Investigation of Ion Mobility with NMR Spectroscopy and Electronic Band Structure Calculations

The room temperature intercalation of Cr2Ti3Se8 with butyl lithium yields a phase mixture of the starting material and of the new trigonal phase with composition Li0.4Cr0.5Ti0.75Se2. The phase pure fully intercalated trigonal phase is obtained at elevated temperature (80 degrees C) with the final composition Li0.62Cr0.5Ti0.75Se2. The line profile analysis (LPA) of the powder patterns shows that pronounced strain occurs in the intercalated material. The deintercalation of the material is realized by treatment of the fully intercalated sample with distilled water leading to the composition Li0.15Cr0.5Ti0.75Se2. The intercalation is accompanied by an electron transfer from the guest Li to the host material, and as a consequence significant changes of the interatomic distances are observed. The local environment and the dynamics of the Li+ ions in the fully intercalated sample were studied with 7Li magic angle spinning (MAS) NMR investigations. These reveal different environments of transition metal neighbors for the Li sites and a high mobility of the Li ions. Magnetic measurements show that in the pristine material antiferromagnetic interactions are dominating (theta = -113.5 K) with no long-range order at low temperatures. The magnetic ground state is characterized by a spin-glass behavior. With increasing Li content the antiferromagnetic character vanishes progressively, and the fully intercalated phase exhibits a positive Weiss constant (theta = 12 K) indicating dominating ferromagnetic exchange interactions; i.e., the magnetic properties can be significantly altered by lithiation. The interpretation of our experimental findings is supported by the results of accompanying band structure calculations done within the framework of local spin density functional theory. These demonstrate in particular the role of the charge transfer between the constituents as a function of the Li concentration and its impact on the exchange coupling.

Vertical homonuclear quantum ring molecules

AbstractWithin Local Spin Density Functional Theory, we have investigated the structure of homonuclear, fewelectron circular vertical semiconductor double quantum rings artificial molecules at zero magnetic field, and have obtained the addition spectrum as a function of inter‐ring distance. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic ground state of UCu 2X 2 (X=Si, Ge) from first principles

The electronic and magnetic structures of UCu 2X 2 germanide and silicide are revisited in view of existing controversy from experimental findings. From self-consistent calculations carried out within the local spin density functional theory using the augmented spherical wave method, the ground state is found to be ferromagnetic within simple and super cell setups. An analysis of the density of states and the chemical bonding shows the dominant role of Cu 2Ge 2-nearly planar like entities within the crystal lattice.

Electronic and magnetic properties of BNC ribbons

Both electronic and magnetic properties have been studied for hexagonally bonded honeycomb ribbons consisting of B, N, and C atoms, with zigzag edges terminated by H atoms. We have used first-principles total-energy electronic-structure calculations within the local spin-density functional theory. The energy gap of BN ribbons is dominated by the edge states and it decreases monotonically with increasing ribbon width. For metallic BNC ribbons with different zigzag edges, C and BN or NB, the ground state becomes ferrimagnetic; this originates from the coexistence of the border state and the edge state.

Magnetic properties of Co clusters deposited on Pt(111)

Using the fully relativistic version of the Korringa–Kohn–Rostoker method for electronic structure calculations within local spin density functional theory, the magnetic and spectroscopic properties of Co clusters deposited on Pt(111) have been investigated. Of central interest was the role of spin–orbit coupling, since it influences the spontaneous formation and orientation of magnetic moments and gives rise among other things to the occurrence of orbital magnetic moments, magnetic anisotropy energy and magnetic circular dichroism in X-ray absorption. The results have been complemented by calculations of the exchange coupling parameters Jij used within Monte Carlo simulations on the basis of the extended classical Heisenberg Hamiltonian. This allowed us to simulate the magnetic properties at finite temperatures, which are of central importance for applications.

Interaction effects in the mesoscopic regime: A quantum Monte Carlo study of irregular quantum dots

We address the issue of accurately treating interaction effects in the mesoscopic regime by investigating the ground state properties of isolated irregular quantum dots. Quantum Monte Carlo techniques are used to calculate the distributions of ground state spin and addition energy. We find a reduced probability of high spin and a somewhat larger even/odd alternation in the addition energy from quantum Monte Carlo than in local spin density functional theory. In both approaches, the even/odd effect gets smaller with increasing number of electrons, contrary to the theoretical understanding of large dots. We argue that the local spin density approximation over predicts the effects of interactions in quantum dots.