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A rich variety of competing phases is predicted for nitrogen at accessible pressures, including a new metallic chainlike phase very close in energy to the previously predicted cubic gauche phase, and other phases at slightly higher energies, e.g., one with N2 and N6 units. Large energy barriers between structures can account for recent observations of metastability, and we identify a low barrier transition path from the known ε phase to the chainlike metallic phase. In analogy to MgB2, the metal is anisotropic with multiple Fermi surfaces formed from π and σ states., We gratefully acknowledge funding from the Center for Simulation of Advanced Rockets at the University of Illinois Urbana-Champaign, supported by the U.S. DOE through the University of California, Subcontract No. B523819. The work was also supported by the DOD MURI Grant No. AA-5-7232-A1 managed by the Army Research Office. Computational support was provided by the Materials Computation Center and the National Center for Supercomputing Applications at the University of Illinois, and the Major Shared Research Center of the Army Research Laboratory. D. S.-P. acknowledges support from Spanish MCyT under the ‘‘Ramon y Cajal’’ program.

The use of quantum mechanics methods within the formalism of density functional theory requires a method to represent the electron wave functions. We compare the use of strictly localized basis functions based on atomic orbitals with the use of plane waves for the study of mineral properties and behaviour. Strictly localized functions enable the computational resources to scale linearly with the size of the system, whereas plane-wave methods scale more as the cube power of the system size, and for this reason the use of localized functions will be preferred for studies of large sizes. We present test results obtained from studies of cation ordering in spinel, garnet and amphibole phases, the high-pressure displacive phase transition in cristobalite, and the intercalation of organic molecules into pyrophyllite. We conclude that the use of localized basis sets provides a useful route forward for quantum mechanical studies of large-scale mineral problems.

We use time dependent density functional theory to study short-time dynamics of the screening of a negative charge suddenly introduced in a free electron gas. The electron gas is modeled with a finite-size jellium cluster. We find that the screening is built-up locally on a time scale well below the femtosecond for typical metallic densities. At this ultrashort time scale, the time evolution is not affected by the cluster boundary conditions, and our results apply to the infinite system as well. The results for different electronic densities can be understood in terms of universal scaling laws. © 2004 Elsevier B.V. All rights reserved., This work was supported in part by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/ EHU (Grant No. 9/UPV 00206.215-13639/2001), and the Spanish MCyT (Grant No. MAT2001-0946). AB acknowledges financial support from the DIPC, and DSP from the Spanish MCyT and CSIC.

A calculation of the inelastic scattering rate of Xe atoms on Cu(111) is presented. We focus on the regimes of low and intermediate velocities, where the energy loss is mainly associated to the excitation of electron–hole pairs in the substrate. We consider trajectories parallel to the surface and restrict ourselves to the Van der Waals contribution. The decay rate is calculated within a self-energy formulation. The effect of the response function of the substrate is studied by comparing the results obtained with two different approaches: the specular reflection model and the random phase approximation. In the latter, the surface is described by a finite slab and the wave functions are obtained from a one-dimensional model potential that describes the main features of the surface electronic structure while correctly retains the image-like asymptotic behaviour. We have also studied the influence of the surface state on the calculation, finding that it represents around 50% of the total probability of electron–hole pairs excitation., This work has been partially funded by the University of the Basque Country under Grant UPV/EHU (9/UPV 00206.215-13639/2001) and Spanish MCyT under Grant (MAT2001-0946). D.S.P. acknowledges support from Spanish CSIC and MCyT under the ‘Ramón y Cajal’ programme.

First-principles periodic density-functional theory calculations suggest ternary fluorides LiMgF3, NaCaF3 and LiNiF3 should adopt the ferroelectric LiNbO3 structure at low temperatures; LiMgF3 and LiNiF3 are predicted to have negative enthalpies of formation from the binary fluorides.

We report on a general method for the calculation of the frequency-dependent optical response of clusters based upon time-dependent density functional theory (TDDFT). The implementation is done using explicit propagation in the time domain and a self-consistent program that uses a linear combination of atomic orbitals (LCAO). Our actual calculations employ the SIESTA program, which is designed to be fast and accurate for large clusters. We use the adiabatic local density approximation to account for exchange and correlation effects. Results are presented for the imaginary part of the linear polarizability, $\mathrm{Im}\ensuremath{\alpha}(\ensuremath{\omega}),$ and the dipole strength function, $S(\ensuremath{\omega}),$ of ${\mathrm{C}}_{60}$ and ${\mathrm{Na}}_{8},$ and compared to previous calculations and to experiment. We also develop a method for the calculation of the integrated frequency-dependent second-order nonlinear polarizability for the case of a step function electric field, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}}_{\mathrm{step}}(\ensuremath{\omega}),$ and present results for ${\mathrm{C}}_{60}.$

We report a detailed study of the vibrational properties of BN single-walled nanotubes and of the BN monolayer. Our results have been obtained from a well-established tight-binding model complemented with an electrostatic model to account for the long-range interactions arising from the polar nature of the material and which are not included in the tight-binding model. Our study provides a wealth of data for the BN monolayer and nanotubes, such as phonon band structure, vibrational density of states, elastic constants, etc. For the nanotubes we obtain the behavior of the optically active modes as a function of the structural parameters, and we compare their frequencies with those derived from a zone-folding treatment applied to the phonon frequencies of the BN monolayer, finding general good agreement between the two., This work was supported in part by the Spanish Ministerio de Ciencia y Tecnologia Grant Nos. MAT2001-0946, BFM2000-1312-C02, and BFM2002-03278 and by the Basque Departamento de Educacion, Universidades e Investigacion, and the University of the Basque Country UPV/EHU Grant No. 9/UPV 00206.215-13639/ 2001. D.S.P. acknowledges support from the Spanish Ministerio de Ciencia y Tecnologia and CSIC under the ‘‘Ramon y Cajal’’ program. E.H. thanks the IST project SATURN IST-1999-10593.

We performed density-functional theory calculations in the local-density approximation of the structural, electronic, and optical properties of 4-Å-diameter single-walled carbon nanotubes. The calculated relaxed geometries show significant deviations from the ideal rolled graphene sheet configuration. We study the effect of the geometry on the electronic band structure finding the metallic character of the (5,0) nanotube to be a consequence of the high curvature of the nanotube wall. Calculations of the dielectric function and optical absorption of the isolated nanotubes were performed under light polarized parallel and perpendicular to the tube axis. We compare our results to measurements of the optical absorption of zeolite-grown nanotubes and are able to assign the observed maxima to the nanotube chiralities., We acknowledge the Ministerio de Ciencia y Tecnologia (Spain) and the DAAD (Germany) for a Spanish-German Research action (Grant No. HA 1999-0118). D.S.P. acknowledges support from the Spanish MCyT and CSIC under the ‘‘Ramon y Cajal’’ program. P.O. acknowledges support from Fundacion Ramon Areces (Spain), EU Project No. SATURN IST-1999-10593, and Spain-DGI Project No. BMF2000-1312-002-01.

``14-1-11'' phase compounds, based on magnetic Mn ions and typified by ${\mathrm{Ca}}_{14}{\mathrm{MnBi}}_{11}$ and ${\mathrm{Ba}}_{14}{\mathrm{MnBi}}_{11},$ show an unusual magnetic behavior, but the large number (104) of atoms in the primitive cell has precluded any previous full electronic structure study. Using an efficient, local-orbital-based method within the local-spin-density approximation to study the electronic structure, we find a gap between a bonding valence-band complex and an antibonding conduction-band continuum. The bonding bands lack one electron per formula unit of being filled, making them low carrier density p-type metals. The hole resides in the ${\mathrm{MnBi}}_{4}$ tetrahedral unit, and partially compensates for the high-spin ${d}^{5} \mathrm{Mn}$ moment, leaving a net spin near $4{\ensuremath{\mu}}_{B}$ that is consistent with experiment. These manganites are composed of two disjoint but interpenetrating ``jungle gym'' networks of spin-$\frac{4}{2}$ ${\mathrm{MnBi}}_{4}^{9\ensuremath{-}}$ units with ferromagnetic interactions within the same network, and weaker couplings between the networks whose sign and magnitude is sensitive to materials parameters. ${\mathrm{Ca}}_{14}{\mathrm{MnBi}}_{11}$ is calculated to be ferromagnetic as observed, while for ${\mathrm{Ba}}_{14}{\mathrm{MnBi}}_{11}$ (which is antiferromagnetic) the ferromagnetic and antiferromagnetic states are calculated to be essentially degenerate. The band structure of the ferromagnetic states is very close to half metallic.

The Si(557)-Au surface, containing monatomic Au wires parallel to the steps, has been proposed as an experimental realization of an ideal one-dimensional metal. In fact, recent photoemission experiments on this system (Nature 402, 504 (1999)) found two peaks that were interpreted in terms of the spin-charge separation in a Luttinger liquid. Our first-principles density functional calculations reveal two metallic bands associated with Au-Si bonds, instead of the single band expected from the Au 6s states, providing an alternative explanation for the experimental observations., Comment: 5 pages with 3 postscript figures. Submitted to PRL on May 10th 2001

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 figures

The excitation of a fullerene-cage vibrational mode by inelastic tunneling electrons has been observed on single C60 molecules adsorbed on Ag(110). The vibration is assigned to the Hg(ω2) mode. A critical enhancement of the inelastic signal was found on fullerenes oriented along one of their symmetry axis upon adsorption. In strong coincidence, those fullerenes maintain a resonance structure at the Fermi level that resembles the first unoccupied molecular orbital distribution of a free molecule, as determined by comparison with ab initio calculations. The degree of symmetry of the adsorbed fullerene is found to be crucial for resonant mechanism of vibrational excitation. We also propose that mode degeneracy splitting may reduce mode detection., J.I.P. thanks the EU commission for his contract under the Mary Curie scheme. J.G.H. thanks the Max-Planckfor support of his visit to the Fritz-Haber-Institut.

The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. Strict localization is maintained while ensuring the continuity of the basis-function derivative at the cutoff radius. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic, and metallic. Satisfactory convergence is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.

The adsorption of C60 on Si(111) has been studied by means of first-principles density functional calculations. A 2x2 adatom surface reconstruction was used to simulate the terraces of the 7x7 reconstruction. The structure of several possible adsorption configurations was optimized using the ab initio atomic forces, finding good candidates for two different adsorption states observed experimentally. While the C60 molecule remains closely spherical, the silicon substrate appears quite soft, especially the adatoms, which move substantially to form extra C-Si bonds, at the expense of breaking Si-Si bonds. The structural relaxation has a much larger effect on the adsorption energies, which strongly depend on the adsorption configuration, than on the charge transfer., 4 pages with 3 postscript figures, to appear in Surf. Science. (proceedings of the European Conference on Surface Science ECOSS-19, Sept 2000)

We have developed and implemented a self-consistent density functional method using standard norm-conserving pseudopotentials and a flexible, numerical LCAO basis set, which includes multiple-zeta and polarization orbitals. Exchange and correlation are treated with the local spin density or generalized gradient approximations. The basis functions and the electron density are projected on a real-space grid, in order to calculate the Hartree and exchange-correlation potentials and matrix elements, with a number of operations that scales linearly with the size of the system. We use a modified energy functional, whose minimization produces orthogonal wavefunctions and the same energy and density as the Kohn-Sham energy functional, without the need of an explicit orthogonalization. Additionally, using localized Wannier-like electron wavefunctions allows the computation time and memory, required to minimize the energy, to also scale linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, thus allowing structural relaxation and molecular dynamics simulations., Comment: 22 pages, 8 figures

We present results on a well known system, C60 on Si(111)(7×7), which show that the scanning tunneling microscope (STM) is probing a single molecular orbital (MO). This MO, after comparison of experimental intramolecular structure with ab initio calculations, arises from the highest occupied MO (HOMO). The adsorbed molecules keep their relevant electronic identity but suffer a decrease in their icosahedral symmetry. The nature of the chemical bonding is proposed as being responsible for selecting one single MO in the imaging mechanisms. Our results represent an example of a strongly interacting system that can be interpreted with a great level of detail, overcoming substrate complexities., We acknowledge financial support from Spain's DGES. DSP acknowledges support from grants DOE8371494 and DEFG02/96/ER 45439.

In this paper we discuss some of the details of the calculation of the macroscopic polarization, via the Berry-phase approach, using a general (numerical) local combination of atomic orbitals (LCAO) as a basis set. Previous implementations with a LCAO basis relied on the use of Gaussian expansions, where analytical formulas exist for the crucial matrix elements. However, our approach, which only requires the matrix elements of the position operator, can be more easily implemented in the case of numerical orbitals. This work is a necessary first step towards our main goal of applying the SIESTA code to the study of ferroelectric alloys. This code, which uses a numerical LCAO basis set, has been highly optimized for the treatment of large systems. We are confident that it will allow us to treat larger systems than the standard plane-waves methods, while keeping a reasonable accuracy. Test results for representative ferroelectric perovskites are presented.

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

Using first-principles density-functional calculations, gold monatomic wires are found to exhibit a zigzag shape which remains under tension, becoming linear just before breaking. At room temperature they are found to spin, which explains the extremely long apparent interatomic distances shown by electron microscopy. The zigzag structure is stable if the tension is relieved, the wire holding its chainlike shape even as a free-standing cluster. This unexpected metallic-wire stiffness stems from the transverse quantization in the wire, as shown in a simple free electron model., This work was supported by Grants No. DGES PB95-0202, No. DOE 8371494, and No. DEFG 02/96/ER 45439.

A brief review of the SIESTA project is presented in the context of linear-scaling density-functional methods for electronic-structure calculations and molecular-dynamics simulations of systems with a large number of atoms. Applications of the method to different systems are reviewed, including carbon nanotubes, gold nanostructures, adsorbates on silicon surfaces, and nucleic acids. Also, progress in atomic-orbital bases adapted to linear-scaling methodology is presented., Comment: 5 pages + 4 figures, submitted to Proc. Int. Conf. Solid State Spectroscopy (Sept 1999)

We apply first principles calculations to study the opening of single-wall carbon nanotubes (SWNT's) by oxidation. We show that an oxygen rim can stabilize the edge of the open tube. The sublimation of CO$_2$ molecules from the rim with the subsequent closing of the tube changes from endothermic to exothermic as the tube radius increases, within the range of experimental feasible radii. We also obtain the energies for opening the tube at the cap and at the wall, the latter being significantly less favorable., Comment: 4 pages, 3 figures. Submitted to Phys. Rev. B

A study based on ab initio calculations is presented on the estructural, elastic, and vibrational properties of single-wall carbon nanotubes with different radii and chiralities. We use SIESTA, an implementation of pseudopotential-density-functional theory which allows calculations on systems with a large number of atoms per cell. Different quantities like bond distances, Young moduli, Poisson ratio and the frequencies of different phonon branches are monitored versus tube radius. The validity of expectations based on graphite is explored down to small radii, where some deviations appear related to the curvature effects. For the phonon spectra, the results are compared with the predictions of the simple zone-folding approximation. Except for the known defficiencies of this approximation in the low-frequency vibrational regions, it offers quite accurate results, even for relatively small radii., Comment: 13 pages, 7 figures, submitted to Phys. Rev. B (11 Nov. 98)

The projection of the eigenfunctions obtained in standard plane-wave first-principle electronic-structure calculations into atomic-orbital basis sets is proposed as a formal and practical link between the methods based on plane waves and the ones based on atomic orbitals. Given a candidate atomic basis, ({\it i}) its quality is evaluated by its projection into the plane-wave eigenfunctions, ({\it ii}) it is optimized by maximizing that projection, ({\it iii}) the associated tight-binding Hamiltonian and energy bands are obtained, and ({\it iv}) population analysis is performed in a natural way. The proposed method replaces the traditional trial-and-error procedures of finding appropriate atomic bases and the fitting of bands to obtain tight-binding Hamiltonians. Test calculations of some zincblende semiconductors are presented., RevTex. 4 pages. 3 uuencoded compressed (tared) postscript figs. To appear in Solid St. Commun

The projection of the eigenfunctions obtained in standard plane-wave first-principle calculations is used for analyzing atomic-orbital basis sets. The "spilling" defining the error in such a projection allows the evaluation of the quality of an atomic-orbital basis set for a given system and its systematic variational optimization. The same projection allows obtaining the band structure and the Hamiltonian matrix elements in the previously optimized atomic basis. The spilling is shown to correlate with the mean square error in the energy bands, indicating that the basis optimization via spilling minimization is qualitatively equivalent to the energy-minimization scheme, but involves a much smaller computational effort. The spilling minimization also allows the optimization of bases for uses different than total-energy calculations, like the description of the band gap in semiconductors. The method is applied to the characterization of finite-range pseudo-atomic orbitals {[}O. F. Sankey and D. J. Niklewski, Phys. Rev. B {\bf 40}, 3979 (1989){]} in comparison to infinite-range pseudo-atomic and Slater-type orbitals. The bases are evaluated and optimized for several zincblende semiconductors and for aluminum. The quality of the finite-range orbitals is found to be perfectly comparable to the others with the advantage of a limited range of interacions. A simple scheme is proposed to expand the basis without increasing the range. It is found that a double-$z$ basis substantially improves the basis performance on diamond, whereas $d$-polarization is required for Si and Al for similar results. Finally, the projection allows the chemical analysis of the plane-wave results via population analysis on the previously optimized atomic basis., Comment: 11 pages, RevTeX, plus 13 uuencoded, compressed, tared figures

This is an Open Access article distributed under the terms of the Creative Commons Attribution License.-- This article is part of the series 12th Trends in NanoTechnology International Conference (TNT2011)., Recent advances in attosecond spectroscopy techniques have fueled the interest in the theoretical description of electronic processes taking place in the subfemtosecond time scale. Here we study the coupled dynamic screening of a localized hole and a photoelectron emitted from a metal cluster using a semi-classical model. Electron density dynamics in the cluster is calculated with Time-Dependent Density Functional Theory and the motion of the photoemitted electron is described classically. We show that the dynamic screening of the hole by the cluster electrons affects the motion of the photoemitted electron. At the very beginning of its trajectory, the photoemitted electron interacts with the cluster electrons that pile up to screen the hole. Within our model, this gives rise to a significant reduction of the energy lost by the photoelectron. Thus, this is a velocity dependent effect that should be accounted for when calculating the average losses suffered by photoemitted electrons in metals., NEK acknowledges support from the CSIC JAE-predoc program, co-financed by the European Science Foundation. We also acknowledge the support of the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Economía y Competitividad (Grant No. FIS2010-19609-CO2-02) and the ETORTEK program funded by the Basque Departamento de Industria and the Diputación Foral de Gipuzkoa.

arXiv:1203.2111, The transmission properties of armchair graphene nanoribbon junctions between graphene electrodes are investigated by means of first-principles quantum transport calculations. First the dependence of the transmission function on the size of the nanoribbon has been studied. Two regimes are highlighted: for a small applied bias transport takes place via tunneling and the length of the ribbon is the key parameter that determines the junction conductance; at a higher applied bias resonant transport through the HOMO and LUMO starts to play a more determinant role, and the transport properties depend on the details of the geometry (width and length) of the carbon nanoribbon. In the case of the thinnest ribbon it has been verified that a tilted geometry of the central phenyl ring is the most stable configuration. As a consequence of this rotation the conductance decreases due to the misalignment of the π orbitals between the phenyl ring and the remaining part of the junction. All the computed transmission functions have shown a negligible dependence on different saturations and reconstructions of the edges of the graphene leads, suggesting a general validity of the reported results. © 2012 the Owner Societies., C.M. thanks the CARIPLO Foundation for its support within the PCAM European Doctoral Programme and Pirelli-Corimav. D.S.P. acknowledges support from the Basque Departamento de Educación, UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-02), and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Gipuzkoa.

We investigate the electronic structure of carbon nanotubes functionalized by adsorbates anchored with single C-C covalent bonds. We find that, despite the particular adsorbate, a spin moment with a universal value of 1.0 $\mu_B$ per molecule is induced at low coverage. Therefore, we propose a mechanism of bonding-induced magnetism at the carbon surface. The adsorption of a single molecule creates a dispersionless defect state at the Fermi energy, which is mainly localized in the carbon wall and presents a small contribution from the adsorbate. This universal spin moment is fairly independent of the coverage as long as all the molecules occupy the same graphenic sublattice. The magnetic coupling between adsorbates is also studied and reveals a key dependence on the graphenic sublattice adsorption site., Comment: final version, improved discussion about calculations and defect concentration

3 pages, 3 figures.-- PACS nrs.: 68.35.Np, 61.46.Fg, 61.46.Df., Our high resolution transmission electronic microscopy studies of multiwall carbon nanotubes show, after the growth of zirconia nanoparticles by a hydrothermal route, the presence of surface Zr, forming an atomically thin layer. Using first-principles calculations we investigate the nature of the Zr–C interaction, which is neither ionic nor covalent, and the optimal coverage for the Zr metal in a graphene flake. This preferred coverage is in agreement with that deduced from electron energy loss spectra experiments. We show also that the amount of charge transferred to the C layer saturates as the Zr coverage increases and the Zr–C bond becomes weaker., We want to acknowledge the support by the ETORTEK (NANOMAT) program of the Basque government, the Intramural Special Project (Reference No. 2006601242), the Spanish Ministerio de Ciencia y Tecnología (MCyT) of Spain (Grant No. Fis 2007-66711-C02-C01), and the European Network of Excellence NANOQUANTA (NM4-CT-2004-500198). Y.S.P. gratefully acknowledges his DIPC grant.

The role of orbital polarization for ultrafast charge transfer between an atomic adsorbate and a substrate is explored. Core hole clock spectroscopy with linearly polarized x-ray radiation allows to selectively excite adsorbate resonance states with defined spatial orientation relative to the substrate surface. For c(4 x 2)S/Ru(0001) the charge transfer times between the sulfur 2s(-1)3p*+1 antibonding resonance and the ruthenium substrate have been studied, with the 2s electron excited into the 3p perpendicular* state along the surface normal and the 3p parallel* state in the surface plane. The charge transfer times are determined as 0.18+/-0.07 and 0.84+/-0.23 fs, respectively. This variation is the direct consequence of the different adsorbate-substrate orbital overlap.

High-quality graphene nanoribbons (GNRs) grown by on-surface synthesis strategies with atomic precision can be controllably doped by inserting heteroatoms or chemical groups in the molecular precursors. Here, we study the electronic structure of armchair GNRs substitutionally doped with di-boron moieties at the center, through a combination of scanning tunneling spectroscopy, angle-resolved photoemission, and density functional theory simulations. Boron atoms appear with a small displacement toward the surface, signaling their stronger interactions with the metal. We find two boron-rich flat bands emerging as impurity states inside the GNR band gap, one of them particularly broadened after its hybridization with the gold surface states. In addition, the boron atoms shift the conduction and valence bands of the pristine GNR away from the gap edge and leave unaffected the bands above and below, which become the new frontier bands and have a negligible boron character. This is due to the selective mixing of boron states with GNR bands according to their symmetry. Our results depict that the GNR band structure can be tuned by modifying the separation between di-boron moieties., We acknowledge financial support from the Basque Government (Departamento de Industria, grant no. PI-2015-1-42, and Departamento de Educación and UPV/EHU, grant no. IT-756-13), the Spanish Ministerio de Economía y Competitividad (grant nos. MAT2016-78293-C6, FIS2015-62538-ERC, FIS2017-83780-P), the Maria de Maeztu Units of Excellence Programme (MDM-2016-0618), the European Research Council (grant agreement no. 635919), and the European Regional Development Fund.

Nanometer-sized metallic necks have the unique ability to sustain extreme uniaxial loads (about 20 times greater than the bulk material). We present an experimental and theoretical study of the electronic transport properties under such extreme conditions. Conductance measurements on gold and aluminum necks show a strikingly different behavior: While gold shows the expected conductance decrease with increasing elastic elongation of the neck, aluminum necks behave in the opposite way. We have performed first-principles electronic-structure calculations which reproduce this behavior, showing that it is an intrinsic property of the bulk band structure under high uniaxial strain. This work has been supported by the Dirección General de Investigación Científica y Tecnológica of Spain (DGICYT) under Grants No. MAT95-1542, No. PB95-0202, and No. PB95-0061.

Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here, we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Ni sub). We have found that, although Ni sub defects are nonmagnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3-0.4 ο B per Ni sub, until a critical strain of ∿6.5% is reached. At this point, a sharp transition to a high-spin state (∿1.9 ο B) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene. 2011 American Chemical Society., We acknowledge support from Basque Departamento de Educacion and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2010-19609-CO2-02), and the ETORTEK program funded by the Basque Departamento de Industria and the Diputacion Foral de Guipuzcoa.

In this work we address the behaviour of electronic structure under uniaxial stress, by first-principles calculations and experiments of conductance in nanometer-sized metallic contactes of Au and Al. These contacts are shown to be specially suitable for this purpose. The conductance behaviour is related to the change with strain of Fermi surface. Both experimental and theoretically Au behaves like the free electron gas but Al has the opposite behaviour.

We simulate the liquid silicon surface with first-principles molecular dynamics in a slab geometry. We find that the atom-density profile presents a pronounced layering, similar to those observed in low-temperature liquid metals like Ga and Hg. The depth-dependent pair correlation function shows that the effect originates from directional bonding of Si atoms at the surface, and propagates into the bulk. The layering has no major effects in the electronic and dynamical properties of the system, that are very similar to those of bulk liquid Si. To our knowledge, this is the first study of a liquid surface by first-principles molecular dynamics., 4 pages, 4 figures, submitted to PRL

We studied the electronic band structure of isolated and bundled single‐walled nanotubes by ab initio calculations. Curvature effects on the electronic states depend on the chirality of the nanotube; the strongest deviations from the zone‐folding approximation are found in zig‐zag tubes. Bundling of the tubes to ropes further reduces the band gap in semiconducting tubes (20 – 30 %). We present first‐principles calculations of the optical absorption spectra in a (19,0) nanotube.

The time dependent screening of a negative charge suddenly introduced in a two-dimensional free electron gas is studied. With time-dependent density functional theory, we show that the screening is built up over time scales similar to those of a three dimensional electron gas. However, and in marked contrast with the three dimensional system, collective oscillations are rapidly damped, and the system quickly reaches the stationary limit. This can be understood from the dispersion of the plasmon modes in the long wavelength limit. Scaling laws are outlined for the dynamics of screening in 2D electron gases with different densities. © 2004 Elsevier B.V. All rights reserved., This work was supported in part by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/ EHU (Grant No. 9/UPV 00206.215-13639/2001), and the Spanish MCyT (Grant No. MAT2001-0946). AGB acknowledges financial support from the DIPC, and DSP from the Spanish MCyT and CSIC.

Physical review / B 55(24), R16065 - R16068 (1997). doi:10.1103/PhysRevB.55.R16065, We report an investigation on the electronic structure of metastable, epitaxial FeSi films grown on Si(111). The electronic structure of the metastable silicides was probed with angle-resolved photoemission, and compared with theoretical calculations. We identify the silicide as FeSi crystallizing in the CsCl structure. Its surface is Fe terminated, and presents a prominent, strongly dispersing surface state at a binding energy of -3.5 eV in Γ-bar. Its origin lies in the truncation of Fe bonds at the surface, and thus it has a major Fe dz2 content., Published by Inst., Woodbury, NY

We have recently developed a method to perform Density Functional Theory calculations in systems with a very large number of atoms, which is based on the use of numerical atomic orbitals as basis sets. The method incorporates Order-N techniques both in the calculation of the Kohn-Sham hamiltonian matrix elements and in the solution of the wave functions, which make the CPU time and memory to scale linearly with the number of atoms, allowing calculations in very large system. In this work, we present results on several test systems to show that the approach and the basis sets used with our method are able to provide an accuracy similar to that of other standard DFT techniques.