Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface

Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001) surface F-center (FC) was studied by spin-density-functional molecular dynamics simulations. The incident clusters are steered by an attractive "funnel" created by the FC, resulting in adsorption of the cluster, with one of its atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their gas-phase structures, while for N>6 surface-commensurate isomers are energetically more favorable. Adsorbed clusters with N > 3 are found to remain magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte

Structural models for the Si(553)-Au atomic chain reconstruction

Recent photoemission experiments on the Si(553)-Au reconstruction show a one-dimensional band with a peculiar ~1/4 filling. This band could provide an opportunity for observing large spin-charge separation if electron-electron interactions could be increased. To this end, it is necessary to understand in detail the origin of this surface band. A first step is the determination of the structure of the reconstruction. We present here a study of several structural models using first-principles density functional calculations. Our models are based on a plausible analogy with the similar and better known Si(557)-Au surface, and compared against the sole structure proposed to date for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ]. Results for the energetics and the band structures are given. Lines for the future investigation are also sketched

Diffusion of Pt dimers on Pt(111)

We report the results of a density-functional study of the diffusion of Pt dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.} \protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations establish that the dimers are mobile at temperatures of interest for adatom diffusion, and thus contribute to mass transport. They also indicate that the diffusion path for dimers consists of a sequence of one-atom and (concerted) two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to Physical Review B; other papers of interest can be found at url http://www.centrcn.umontreal.ca/~lewi

Strain-Dependence of Surface Diffusion: Ag on Ag(111) and Pt(111)

Using density-functional theory with the local-density approximation and the generalized gradient approximation we compute the energy barriers for surface diffusion for Ag on Pt(111), Ag on one monolayer of Ag on Pt(111), and Ag on Ag(111). The diffusion barrier for Ag on Ag(111) is found to increase linearly with increasing lattice constant. We also discuss the reconstruction that has been found experimentally when two Ag layers are deposited on Pt(111). Our calculations explain why this strain driven reconstruction occurs only after two Ag layers have been deposited.Comment: 4 pages, 3 figures, Phys. Rev. B 55 (1997), in pres

Electrons in Dry DNA from Density Functional Calculations

The electronic structure of an infinite poly-guanine - poly-cytosine DNA molecule in its dry A-helix structure is studied by means of density-functional calculations. An extensive study of 30 nucleic base pairs is performed to validate the method. The electronic energy bands of DNA close to the Fermi level are then analyzed in order to clarify the electron transport properties in this particularly simple DNA realization, probably the best suited candidate for conduction. The energy scale found for the relevant band widths, as compared with the energy fluctuations of vibrational or genetic-sequence origin, makes highly implausible the coherent transport of electrons in this system. The possibility of diffusive transport with sub-nanometer mean free paths is, however, still open. Information for model Hamiltonians for conduction is provided.Comment: 8 pages, 4 figure

Variational finite-difference representation of the kinetic energy operator

A potential disadvantage of real-space-grid electronic structure methods is the lack of a variational principle and the concomitant increase of total energy with grid refinement. We show that the origin of this feature is the systematic underestimation of the kinetic energy by the finite difference representation of the Laplacian operator. We present an alternative representation that provides a rigorous upper bound estimate of the true kinetic energy and we illustrate its properties with a harmonic oscillator potential. For a more realistic application, we study the convergence of the total energy of bulk silicon using a real-space-grid density-functional code and employing both the conventional and the alternative representations of the kinetic energy operator.Comment: 3 pages, 3 figures, 1 table. To appear in Phys. Rev. B. Contribution for the 10th anniversary of the eprint serve

Nanotube Piezoelectricity

We combine ab initio, tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity of a heteropolar nanotube depends on its chirality and diameter and can be understood starting from the piezoelectric response of an isolated planar sheet, along with a structure specific mapping from the sheet onto the tube surface. We demonstrate that coupling between the uniaxial and shear deformation are only allowed in the nanotubes with lower chiral symmetry. Our study shows that piezoelectricity of nanotubes is fundamentally different from its counterpart in three dimensional (3D) bulk materials.Comment: 4 pages, with 3 postscript figures embedded. Uses REVTEX4 macros. Also available at http://www.physics.upenn.edu/~nsai/preprints/bn_piezo/index.htm

A mixed ultrasoft/normconserved pseudopotential scheme

A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the normconservation is released for only one or a few angular channels, is presented. Within this scheme some difficulties of the truly ultrasoft pseudopotentials are overcome without sacrificing the pseudopotential softness. i) Ghost states are easily avoided without including semicore shells. ii) The ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The number of nonlocal operators is reduced. The scheme will be most useful for transition metals, and the feasibility and accuracy of the scheme is demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure

Electron-Transport Properties of Na Nanowires under Applied Bias Voltages

We present first-principles calculations on electron transport through Na nanowires at finite bias voltages. The nanowire exhibits a nonlinear current-voltage characteristic and negative differential conductance. The latter is explained by the drastic suppression of the transmission peaks which is attributed to the electron transportability of the negatively biased plinth attached to the end of the nanowire. In addition, the finding that a voltage drop preferentially occurs on the negatively biased side of the nanowire is discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 figure

Magnetic susceptibility of insulators from first principles

We present an {\it ab initio} approach for the computation of the magnetic susceptibility $\chi$ of insulators. The approach is applied to compute $\chi$ in diamond and in solid neon using density functional theory in the local density approximation, obtaining good agreement with experimental data. In solid neon, we predict an observable dependence of $\chi$ upon pressure.Comment: Revtex, to appear in Physical Review Lette