Nonadiabatic Time-Dependent Spin-Density Functional Theory for strongly correlated systems

We propose a nonadiabatic time-dependent spin-density functional theory (TDSDFT) approach for studying the single-electron excited states and the ultrafast response of systems with strong electron correlations. The correlations are described by the correlation part of the nonadiabatic exchange-correlation (XC) kernel, which is constructed by using some exact results for the Hubbard model of strongly correlated electrons. We demonstrate that the corresponding nonadiabatic XC kernel reproduces main features of the spectrum of the Hubbard dimer and infinite-dimensional Hubbard model, some of which are impossible to obtain within the adiabatic approach. The theory may be applied for DFT study of strongly correlated electron systems in- and out-of-equilibrium, including the important case of nanostructures, for which it leads to a dramatic reduction of necessary computational power

Electronic Structure of the c(2x2)O/Cu(001) System

The locally self-consistent real space multiple scattering technique has been applied to calculate the electronic structure and chemical binding for the c(2x2)O/Cu(001) system, as a function of $d_{O-Cu1}$ -- the height of oxygen above the fourfold hollow sites. It is found that the chemical binding between oxygen and copper has a mixed ionic-covalent character for all plausible values of $d_{O-Cu1}$. Furthermore, the electron charge transfer from Cu to O depends strongly on $d_{O-Cu1}$ and is traced to the variation of the long-range electrostatic part of the potential. A competition between the hybridization of Cu1-$d_{xz}$ with O-$p_x,p_y$ and Cu1-$d_{x^2-y^2}$ with O-$p_z$ states controls modification of the electronic structure when oxygen atoms approach the Cu(001) surface. The anisotropy of the oxygen valence electron charge density is found to be strongly and non-monotonically dependent on $d_{O-Cu1}$.Comment: 14 pages, 7 figures, 1 tabl

Relationship between Electronic and Geometric Structures of the O/Cu(001) System

The electronic structure of the $(2\sqrt{2}\times\sqrt{2})R45^{\circ}$ O/Cu(001) system has been calculated using locally self-consistent, real space multiple scattering technique based on first principles. Oxygen atoms are found to perturb differentially the long-range Madelung potentials, and hence the local electronic subbands at neighboring Cu sites. As a result the hybridization of the oxygen electronic states with those of its neighbors leads to bonding of varying ionic and covalent mix. Comparison of results with those for the c(2x2) overlayer shows that the perturbation is much stronger and the Coulomb lattice energy much higher for it than for the $(2\sqrt{2}\times\sqrt{2})R45^{\circ}$ phase. The main driving force for the 0.5ML oxygen surface structure formation on Cu(001) is thus the long-range Coulomb interaction which also controls the charge transfer and chemical binding in the system.Comment: 17 pages, 8 figure

Friedel oscillations responsible for stacking fault of adatoms: The case of Mg(0001) and Be(0001)

We perform a first-principles study of Mg adatom and adislands on the Mg(0001) surface, and Be adatom on Be(0001), to obtain further insights into the previously reported energetic preference of the fcc faulty stacking of Mg monomers on Mg(0001). We first provide a viewpoint on how Friedel oscillations influence ionic relaxation on these surfaces. Our three-dimensional charge-density analysis demonstrates that Friedel oscillations have maxima which are more spatially localized than what one-dimensional average density or two-dimensional cross sectional plots could possibly inform: The well-known charge-density enhancement around the topmost surface layer of Mg(0001) is strongly localized at its fcc hollow sites. The charge accumulation at this site explains the energetically preferred stacking fault of the Mg monomer, dimer and trimer. Yet, larger islands prefer the normal hcp stacking. Surprisingly, the mechanism by which the fcc site becomes energetically more favorable is not that of enhancing the surface-adatom bonds but rather those between surface atoms. To confirm our conclusions, we analyze the stacking of Be adatom on Be(0001) - a surface also largely influenced by Friedel oscillations. We find, in fact, a much stronger effect: The charge enhancement at the fcc site is even larger and, consequently, the stacking-fault energy favoring the fcc site is quite large, 44 meV.Comment: Submitted to Physical Review

The crossover from collective motion to periphery diffusion for 2D adatom-islands on Cu(111)

The diffusion of two dimensional adatom islands (up to 100 atoms) on Cu(111) has been studied, using the self-learning Kinetic Monte Carlo (SLKMC) method [1]. A variety of multiple- and single-atom processes are revealed in the simulations, and the size dependence of the diffusion coefficients and effective diffusion barriers are calculated for each. From the tabulated frequencies of events found in the simulation, we show a crossover from diffusion due to the collective motion of the island to a regime in which the island diffuses through periphery-dominated mass transport. This crossover occurs for island sizes between 13 and 19 atoms. For islands containing 19 to 100 atoms the scaling exponent is 1.5, which is in good agreement with previous work. The diffusion of islands containing 2 to 13 atoms can be explained primarily on the basis of a linear increase of the barrier for the collective motion with the size of the island