Band selection and disentanglement using maximally-localized Wannier functions: the cases of Co impurities in bulk copper and the Cu (111) surface

We have adapted the maximally-localized Wannier function approach of [I. Souza, N. Marzari and D. Vanderbilt, Phys. Rev. B 65, 035109 (2002)] to the density functional theory based Siesta method [J. M. Soler et al., J. Phys.: Cond. Mat. 14, 2745 (2002)] and applied it to the study of Co substitutional impurities in bulk copper as well as to the Cu (111) surface. In the Co impurity case, we have reduced the problem to the Co d-electrons and the Cu sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined density functional parameters in a fully orthonormal basis set. In order to test the quality of the Wannier approach to surfaces, we have studied the electronic structure of the Cu (111) surface by again transforming the density functional problem into the Wannier representation. An excellent description of the Shockley surface state is attained, permitting us to be confident in the application of this method to future studies of magnetic adsorbates in the presence of an extended surface state

Study of cycloplatinated complexes with isocyanide ligands: isomerism, optical properties and mechanochromism

Platinum(II) cyclometalated derivatives have attracted a great interest dueto their rich photophysical properties, with interesting applications aschemical sensors, photocatalysts or in light emitting diodes (OLEDs). Thepresence of planar ligands in these complexes improves the luminescenceand the ability to form aggregates through the formation of Pt···Pt and/or··· interactions. In this context, there are very scarce examples ofheteroleptic complexes with isocyanide and alkynyl ligands.In this work, we present the synthesis and characterization of a group ofPt(II) compounds featuring 2-phenylpyridine and 2-phenylquinoline ascyclometalated ligands and 2,6-dimethylphenyl isocyanide as auxiliaryligand. The p-tolylacetylide complexes [Pt(C^N)(CCTol)(CNXyl)] (C^N =ppy 3, pq 4) have been obtained from the chloride derivatives[Pt(C^N)Cl(CNXyl)] (C^N = ppy 1, pq 2). The isomerism of thesecompounds in which the isocyanide ligand can be trans to the nitrogen or tothe carbon of the cyclometalated ligand has been determined by differenttechniques, as NMR and X-ray diffraction.The photophysical properties (absorption and emission) of all complexeshave been studied with the aid of theoretical calculations. Interestingly, thephenylpyridine derivatives exhibit mechanical stimuli responsive colour andluminescence changes correlated with the formation of different aggregateswith ··· and/or Pt···Pt interactions

Nanoscale Smoothing and the Analysis of Interfacial Charge and Dipolar Densities

The interface properties of interest in multilayers include interfacial charge densities, dipole densities, band offsets, and screening-lengths, among others. Most such properties are inaccesible to direct measurements, but are key to understanding the physics of the multilayers. They are contained within first-principles electronic structure computations but are buried within the vast amount of quantitative information those computations generate. Thus far, they have been extracted from the numerical data by heuristic nanosmoothing procedures which do not necessarily provide results independent of the smoothing process. In the present paper we develop the theory of nanosmoothing, establishing procedures for both unpolarized and polarized systems which yield interfacial charge and dipole densities and band offsets invariant to the details of the smoothing procedures when the criteria we have established are met. We show also that dipolar charge densities, i. e. the densities of charge transferred across the interface, and screening lengths are not invariant. We illustrate our procedure with a toy model in which real, transversely averaged charge densities are replaced by sums of Gaussians.Comment: 30 pages, 15 figures, 4 table

Crossed-ratchet effects and domain wall geometrical pinning

The motion of a domain wall in a two dimensional medium is studied taking into account the internal elastic degrees of freedom of the wall and geometrical pinning produced both by holes and sample boundaries. This study is used to analyze the geometrical conditions needed for optimizing crossed ratchet effects in periodic rectangular arrays of asymmetric holes, recently observed experimentally in patterned ferromagnetic films. Geometrical calculations and numerical simulations have been used to obtain the anisotropic critical fields for depinning flat and kinked walls in rectangular arrays of triangles. The aim is to show with a generic elastic model for interfaces how to build a rectifier able to display crossed ratchet effects or effective potential landscapes for controlling the motion of interfaces or invasion fronts.Comment: 13 pages, 18 figure

Do Thiols Merely Passivate Gold Nanoclusters?

A Comment on the Letter by H. Hakkinen, R. N. Barnett, and U. Landman, Phys. Rev. Lett. 82, 3264 (1999)

Efficient index handling of multidimensional periodic boundary conditions

An efficient method is described to handle mesh indexes in multidimensional problems like numerical integration of partial differential equations, lattice model simulations, and determination of atomic neighbor lists. By creating an extended mesh, beyond the periodic unit cell, the stride in memory between equivalent pairs of mesh points is independent of their position within the cell. This allows to contract the mesh indexes of all dimensions into a single index, avoiding modulo and other implicit index operations.Comment: 2 pages, 0 figure

Why Some Interfaces Cannot be Sharp

A central goal of modern materials physics and nanoscience is control of materials and their interfaces to atomic dimensions. For interfaces between polar and non-polar layers, this goal is thwarted by a polar catastrophe that forces an interfacial reconstruction. In traditional semiconductors this reconstruction is achieved by an atomic disordering and stoichiometry change at the interface, but in multivalent oxides a new option is available: if the electrons can move, the atoms don`t have to. Using atomic-scale electron energy loss spectroscopy we find that there is a fundamental asymmetry between ionically and electronically compensated interfaces, both in interfacial sharpness and carrier density. This suggests a general strategy to design sharp interfaces, remove interfacial screening charges, control the band offset, and hence dramatically improving the performance of oxide devices.Comment: 12 pages of text, 6 figure

Mild sonochemical exfoliation of bromine-intercalated graphite: a new route towards graphene

A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads to the formation of suspensions of graphite flakes. The delamination is dramatically improved by intercalation of bromine into the graphite before sonication. The bromine intercalation was verified by Raman spectroscopy as well as by x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations show an almost ten times lower interlayer binding energy after introducing Br2 into the graphite. Analysis of the suspended material by transmission and scanning electron microscopy (TEM and SEM) revealed a significant content of few-layer graphene with sizes up to 30 $mu$m, corresponding to the grain size of the starting material.Comment: 10 pages 4 figure