Adsorption and dissociation of molecular oxygen on α-Pu (0 2 0) surface: A density functional study

Molecular and dissociative oxygen adsorptions on the α-Pu (0 2 0) surface have been systematically studied using the full-potential linearized augmented-plane-wave plus local orbitals (FP-LAPW+lo) basis method and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Chemisorption energies have been optimized for the distance of the admolecule from the Pu surface and the bond length of O-O atoms for four adsorption sites and three approaches of O₂ admolecule to the (0 2 0) surface. Chemisorption energies have been calculated at the scalar relativistic level with no spin-orbit coupling (NSOC) and at the fully relativistic level with spin-orbit coupling (SOC). Dissociative adsorptions are found at the two horizontal approaches (O₂ is parallel to the surface and perpendicular/parallel to a lattice vector). Hor2 (O₂ is parallel to the surface and perpendicular to a lattice vector) approach at the one-fold top site is the most stable adsorption site, with chemisorption energies of 8.048 and 8.415 eV for the NSOC and SOC cases, respectively, and an OO separation of 3.70 Å. Molecular adsorption occurs at the Vert (O₂ is vertical to the surface) approach of each adsorption site. The calculated work functions and net spin magnetic moments, respectively, increase and decrease in all cases upon chemisorption compared to the clean surface. The partial charges inside the muffin-tins, the difference charge density distributions, and the local density of states have been used to investigate the Pu-admolecule electronic structures and bonding mechanisms.     

Density functional study of Pu<Subscript>2</Subscript>C<Subscript>3</Subscript>

The structural, magnetic, electronic, vibrational, thermodynamic and elastic properties of plutonium sesquicarbide (Pu₂C₃) are investigated based on density functional theory. The use of the Hubbard term to describe the 5f electrons of plutonium is discussed according the lattice parameters and magnetism. The calculated lattice constants, magnetism and density of states agree well with the experimental data or other theoretical calculations. The Pu-C bonds of Pu₂C₃ have a mixture of covalent character and ionic character, while covalent character is stronger than ionic character. The phonon frequencies and the assignment of infrared-active, Raman-active and silent modes at Γ point are obtained. Furthermore, the enthalpy difference H-H₂₉₈, entropy S, heat capacity and linear thermal expansion coefficient α of Pu₂C₃ have been calculated and compared with the available data. Lastly, the calculated elastic properties predict that Pu₂C₃ is ductile metal. In addition, the effect of spin-orbit coupling on the structural, magnetic, and electronic properties of Pu₂C₃ has been discussed. We hope that our results can provide a useful reference for further theoretical and experimental research on Pu₂C₃.     

The influence of defects on magnetic properties of fcc-Pu

The influence of vacancies and interstitial atoms on magnetism in Pu is considered in the framework of the density functional theory. The crystal structure relaxation arising due to various types of defects is calculated using the molecular dynamics method with a modified embedded atom model. The local density approximation with explicit inclusion of Coulomb and spin-orbit interactions is applied in matrix invariant form to describe correlation effects in Pu with these types of defects. The calculations show that both vacancies and interstitials give rise to local moments in the f-shell of Pu in good agreement with experimental data for aged Pu. Magnetism appears due to the destruction of a delicate balance between spin-orbit and exchange interactions.     

The Josephson effect in superconductors with heavy Fermions

The Josephson effect between a usual superconductor with even-parity pairing and a heavy-Fermion superconductor with odd-parity pairing would not be zero. Instead, due to spin-orbit coupling it would be of relative magnitude a/Ξ_o compared to the effect between two even-parity superconductors. a/Ξ_o, the ratio of lattice constant to pair size, is comparable to 0.1 for heavy-Fermion superconductors. Recent claims, that with strong spin-orbit coupling the even-parity to odd-parity Josephson effect should be of order unity times the even-even effect, are shown to be incorrect.     

Consequences of relativity for the hyperfine interactions — with applications to transition metals

The strong influence of relativity on the hyperfine interaction is demonstrated by non-, scalar andfully relativistic calculations for solid transition metal systems. By calculations of hyperfine fields andof the nuclear spin lattice relaxation rate of Ag in Ag _x Pt_1–x it is shown that scalar relativistic calculations, which neglect the influence of spin-orbit coupling, seem to be sufficient to account for relativistic effects in many cases. To account properly for the symmetry-breaking caused by spin-orbit coupling, which is reflected by many hyperfine interaction properties, a fully relativistic approach is demanded. The corresponding formalism in the framework of the KKR-GF (Korringa-Kohn-Rostoker-Green's function) method of band structure calculation for magnetic solids is outlined. As applications results for the hyperfine fields of pure transition metals andalloys are presented.     

Spin-orbit coupling and the dynamic Jahn-Teller effect in the C 60 − system

The dynamic Jahn-Teller effect is studied for a charged fullerene molecule C ₆₀ ⁻ with allowance for spin-orbit coupling. The system of self-consistent equations describing the interaction of an electron and the molecular vibrations in the event of spin-orbit splitting of the electronic level is solved analytically. A novel type of nonlinear vibrations occurring in such a system is described. It is shown that with spin-orbit coupling taken into account, the static Jahn-Teller configurations in the C ₆₀ ⁻ molecule are unstable even in the limit of strong electron-vibronc coupling and that the symmetry of the atomic configuration of the unperturbed C ₆₀ ⁻ molecule is restored under time averaging. Zh. éksp. Teor. Fiz. 116, 194–203 (July 1999)     

First-principles study of electronic structure, chemical bonding, and optical properties of cubic SrHfO3

Electronic structure and optical properties of SrHfO₃ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. From the density of states (DOS) as well as charge density studies, we find that the bonding between Sr and HfO₃ is mainly ionic and that HfO₃ entities bond covalently. The complex dielectric functions are calculated, which are in good agreement with the available experimental results. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small.     

Magnetoelastic effect in an exchange model

The effect of the interplay between magnetism, charge ordering and lattice distortion within a like double and super-exchange model is studied in low-dimensional systems. An important magnetoelastic effect that leads to a lattice contraction is presented in conjunction with an analytical minimization for a three-site one-dimensional model. The model is discussed in connection with the magnetism, charge ordering and the contraction of the rungs experimentally observed within the three-leg ladders (3LL) present in the oxyborate Fe₃O₂BO₃.     

The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling

We investigate the effect that Rashba spin-orbit coupling has on the low energy behaviour of a two dimensional magnetic impurity system. It is shown that the Kondo effect, the screening of the magnetic impurity at temperatures T<T _K , is robust against such spin-orbit coupling, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature T _K in such systems and the prospects of measuring this change are discussed. We conclude that many of the assumptions made in our analysis invalidate our results as applied to recent experiments in semi-conductor quantum dots but may apply to measurements made with magnetic atoms placed on metallic surfaces.     

Effect of well thickness on the Rashba spin splitting and intersubband spin-orbit coupling in AlGaN/GaN/AlGaN quantum wells with two subbands

By projecting the characteristic equation into the subspace of the conduction band, the Rashba spin splitting coefficient for the first two subbands (α₁, α₂) and the intersubband spin-orbit coupling coefficient (η₁₂) in AlGaN/GaN quantum well structure are obtained. Then sizable α₁, α₂ and η₁₂ in QWs are calculated by solving the Schrödinger and Poisson equations self-consistently. We find that the internal electric field is crucial for considerable spin-orbit coupling effect in III-nitride QWs and the spin-orbit coupling coefficient can be greatly modulated by the well thickness. Compared with the Rashba coefficient, the intersubband spin-orbit coupling coefficient is basically of the same order of magnitude. The results show the great possibility of spin manipulation in low-dimensional semiconductors, and III-nitride QWs are candidates for the design of spintronic devices.     

An all-electron LCGTO study of square and hexagonal plutonium monolayers

The linear combinations of Gaussian type orbitals fitting function (LCGTO-FF) method is used to study the electronic and geometrical properties of plutonium monolayers with square and hexagonal symmetry. The effects of several common approximations are examined: (1) scalar-relativity vs. full-relativity (i.e., with spin-orbit coupling included); (2) paramagnetic vs. spin-polarized; and (3) local-density approximation (LDA) vs. generalized- gradient approximation (GGA). The results indicate that spin-orbit coupling has a much stronger effect on the monolayer properties compared to the effects of spin-polarization. In general, the GGA is found to predict a larger lattice constant and a smaller cohesive energy compared to LDA predictions. We also find a significant compression of the monolayers compared to the bulk, contradicting the only other published result on a Pu monolayer. The current result supports the existence of a δ-like surface on α-Pu. Received 17 October 2001 Published online 6 June 2002     

Magnetic bloch states and carrier transport in two-dimensional semiconductor lattice structures with spin-orbit interaction

Quantization rules have been obtained for the Hall conductance of fully occupied Landau subbands of the two-dimensional electron gas with the Dresselhaus spin-orbit interaction in a periodic electrostatic field of a superlattice and a transverse magnetic field. The spin-orbit interaction mixes states of different magnetic subbands and changes the quantization rule for the Hall conductance compared to spinless particles. The calculations have been performed for the two-dimensional electrons in the structures with both a weak (AlGaAs/GaAs) and sufficiently strong (GaAs/In_0.23Ga_0.77As) spin-orbit interaction and Zeeman splitting. It has been found that the distribution of the Hall conductance among the magnetic subbands depends on the geometric parameters of the superlattices and promptly changes upon the touching of the adjacent subbands in the spectrum. The quantization rule for the Hall conductance in real semiconductor structures with relatively strong spin-orbit interaction has been shown to differ from that calculated by Thouless et al. [Phys. Rev. Lett. 49, 405 (1982)] for the systems without the spin-orbit interaction and Zeeman effect.     

Perturbative Handling of the Renner-Teller Effect and Spin-Orbit Coupling in Π Electronic States of Triatomic and Tetra-atomic Molecules

Second-order perturbative formulae for handling the Renner-Teller effect combined with the spin-orbit coupling in Π electronic states of triatomic and symmetric (ABBA-type) tetra-atomic molecules with linear equilibrium geometry are derived. Two schemes for partition of the model Hamiltonian are employed: In the first the spin-orbit coupling term is treated as a perturbation, in the second it is included in the zeroth-order Hamiltonian. It is demonstrated that both approaches lead to the same results when the spin-orbit coupling constant is small compared to the bending frequency, but much larger than the splitting of potential surfaces upon bending. The perturbative formulae derived for tetra-atomic molecules are used to compute the spectrum of the X²Π_u state of the acetylene ion, employing the parameters obtained in ab initio calculations. The results are compared with those generated in corresponding variational computations.     

Quantum charge density fluctuations and the phase transition in Ce

We have calculated the quantum quadrupolar interaction due to charge density fluctuations of localized 4f-electrons in Ce by taking into account the angular dependence, the degeneracy of the localized 4f -orbitals and the spin-orbit coupling. The calculated crystal field of 4 f electronic states is in good agreement with neutron diffraction measurements. We show that orientational ordering of quantum quadrupoles drives a phase transition at K which we assign with the transformation. In the phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory accounts for the first order character of the transition and for the cubic lattice contraction which accompanies the transition. The transition temperature increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the phase transition. Received 19 October 1998     

Dynamical properties of low-dimensional and spin-Peierls systems

Properties of low-dimensional spin-Peierls systems are described by using a one-dimensional S =1/2 antiferromagnetic Heisenberg chain linearly coupled to a single phonon mode of wave vector (whose contribution is expected to be dominant). By exact diagonalizations of small rings with up to 24 sites supplemented by a finite size scaling analysis, static and dynamical properties are investigated. Numerical evidences are given for a spontaneous discrete symmetry breaking towards a spin gapped phase with a frozen lattice dimerization. Special emphasis is put on the comparative study of the two inorganic spin-Peierls compounds CuGeO₃ and NaV₂O₅ and the model parameters are determined from a fit of the experimental spin gaps. We predict that the spin-phonon coupling is 2 or 3 times larger in NaV₂O₅ than in CuGeO₃. Inelastic neutron scattering spectra are calculated and similar results are found in the single phonon mode approximation and in the model including a static dimerization. In particular, the magnon S =1 branch is clearly separated from the continuum of triplet excitations by a finite gap. Received: 30 July 1997 / Revised: 16 September 1997 / Accepted: 10 October 1997     

Pressure effects on intermetallic actinide compounds

Abstract

Effects of volume compressions on the electronic structure have been studied theoretically for a number of different actinide systems. For the elemental metals the dhcp→fcc crystallographic transformation for Am, Cm, Bk and Cf is calculated and compared with experiment. The magnetism of actinide compounds has also been studied. The spin and orbital 5f moments in UFe₂ are found to be almost equal in magnitude but of opposite directions, which now has been confirmed by neutron experiments. Calculation of the 5f spin polarization in UW, suggests that the 5f localization remains stable even at high pressures. The orbital and spin composition of the magnetic moment in NpOs₂ is studied as a function of volume.     

Theoretical Interpretation of Luminescent Properties of Nitroanilines

Abstract

The energies of low-lying singlet and triplet ππ?-, and nπ?- states of nitroanilines and their methylated derivatives have been calculated with the aid of the PPP-method including the superposition of singly excited configurations. For these systems the spin-orbit coupling matrix elements between the ππ?- and nπ?- states of different multiplicities were estimated using one-electron and one-center approximations. The interpretation of the luminescent properties of nitroanilines in a dense medium is given. The dependence of the luminescence on temperature is predicted for molecules having the T_nπ?- state located somewhat above the lowest Sππ?- state.     

The dipnictenes R-E=E′-R′ (E=P,As, Sb,Bi) revisited: a TDDFT and multi-reference study of some aspects of its electronic spectroscopy

Compounds with double bonds Terp-E=E′-Terp (E, E'=P, As, Sb or Bi; Terp=-C₆H₃-2,6 (C₆H₅)₂) have been studied using DFT. These compounds have been used as a model for other compounds with larger substituents. Some bands of its electronic spectroscopy have been calculated using TDDFT, and the results show high correlation with experimental data, when available. In Bi-containing compounds, the origin of some weak bands in the calculated spectra and also in experimental data is unclear. An assignation of these bands is suggested, wedding the experimental data with calculated and theoretical results. These bands could be weak singlet–triplet transitions, partially allowed by spin-orbit coupling only in Bi-containing compounds. A theoretical estimation of the energy of these bands is achieved using CASSCF and MR-MP2.     

Ferromagnetism induced by intrinsic defects and nitrogen substitution in SnO2 nanotube

The possibilities of magnetism induced by intrinsic defects and nitrogen substitution in (5,5) single-wall SnO₂ nanotube are investigated by ab initio calculations. The calculated results indicate that a stoichiometric SnO₂ nanotube is nonmagnetic. The tin (Sn) vacancy can induce the magnetic moments rather than oxygen vacancy, which is originated from the polarization of O 2p electrons. A couple of tin vacancies can lead to the ferromagnetic coupling. A nitrogen substitution for oxygen also produces magnetic moments. When substituting two nitrogen atoms, the characteristics of exchange coupling depend upon the distance of two nitrogen atoms. The longer distance of two nitrogen atoms prefers the ferromagnetic coupling, whereas the short distance leads to the antiferromagnetic coupling.