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Angel Rubio

Publications and source records attributed to Angel Rubio.

At least 433 records · Page 24Linked to original sources
arXiv2005

Raman spectra of BN-nanotubes: Ab-initio and bond-polarizability model calculations

We present it ab-initio calculations of the non-resonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are extracted from first-principles calculations of the polarizability tensor of a BN sheet. For light-polarization along the tube-axis, the agreement between model and it ab-initio spectra is almost perfect. For perpendicular polarization, depolarization effects have to be included in the model in order to reproduce the it ab-initio Raman intensities.

cond-mat.mtrl-sci↗
arXiv2005

Nonuniversality of the dispersion interaction: analytic benchmarks for van der Waals energy functionals

We highlight the non-universality of the asymptotic behavior of dispersion forces, such that a sum of inverse sixth power contributions is often inadequate. We analytically evaluate the cross-correlation energy Ec between two pi-conjugated layers separated by a large distance D within the electromagnetically non-retarded Random Phase Approximation, via a tight-binding model. For two perfect semimetallic graphene sheets at T=0K we find Ec = C D^{-3}, in contrast to the "insulating" D^{-4} dependence predicted by currently accepted approximations. We also treat the case where one graphene layer is replaced by a thin metal, a model relevant to the exfoliation of graphite. Our general considerations also apply to nanotubes, nanowires and layered metals.

cond-mat.soft↗
arXiv2004

The phonon dispersion of graphite revisited

We review calculations and measurements of the phonon-dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental data since the long-range character of the dynamical matrix is properly taken into account. Calculations with a plane-wave basis demonstrate that for the in-plane optical modes, the generalized-gradient approximation (GGA) yields frequencies lower by 2% than the local-density approximation (LDA) and is thus in better agreement with experiment. The long-range character of the dynamical matrix limits the validity of force-constant approaches that take only interaction with few neighboring atoms into account. However, by fitting the force-constants to the ab-initio dispersion relation, we show that the popular 4th-nearest-neighbor force-constant approach yields an excellent fit for the low frequency modes and a moderately good fit (with a maximum deviation of 6%) for the high-frequency modes. If, in addition, the non-diagonal force-constant for the second-nearest neighbor interaction is taken into account, all the qualitative features of the high-frequency dispersion can be reproduced and the maximum deviation reduces to 4%. We present the new parameters as a reliable basis for empirical model calculations of phonons in graphitic nanostructures, in particular carbon nanotubes.

cond-mat.mtrl-sci↗
arXiv2003

Band structure of boron doped carbon nanotubes

We present {\it ab initio} and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive ``acceptor''-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon $π$-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of acceptor and unoccupied carbon states above the Fermi edge.

cond-mat.mtrl-sci↗
arXiv2003

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first principles approach

We present results for the optical absorption spectra of small-diameter single-wall carbon and boron nitride nanotubes obtained by {\it ab initio} calculations in the framework of time-dependent density functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal BN sheets. In particular, we focus on the role of depolarization effects, anisotropies and interactions in the excited states. We show that already the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to which extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization such as optical and fluorescence spectroscopies as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy.

cond-mat.mtrl-sci↗
arXiv2003

Time and energy-resolved two photon-photoemission of the Cu(100) and Cu(111) metal surfaces

We present calculations on energy- and time-resolved two-photon photoemission spectra of images states in Cu(100) and Cu(111) surfaces. The surface is modeled by a 1D effective potential and the states are propagated within a real-space, real-time method. To obtain the energy resolved spectra we employ a geometrical approach based on a subdivision of space into two regions. We treat electronic inelastic effects by taking into account the scattering rates calculated within a GW scheme. To get further insight into the decaying mechanism we have also studied the effect of the variation of the classical Hartree potential during the excitation. This effect turns out to be small.

cond-mat.mtrl-sci↗
arXiv2002

Quasiparticle bandstructure effects on the {\it d} hole lifetimes of copper within the GW approximation

We investigate the lifetime of {\it d} holes in copper within a {\it first--principle} $GW$ approximation. At the $G_0W_0$ level the lifetime of the topmost {\it d} bands are in agreement with the experimental results and are four times smaller than those obtained in the ``on--shell'' calculations commonly used in literature. The theoretical lifetimes and bandstructure, however, worsen when further iterative steps of self-consistency are included in the calculation, pointing to a delicate interplay between self--consistency and the inclusion of vertex corrections. We show that the $G_0W_0$ success in the lifetimes calculation is due to the opening of new ``intraband'' decay channels that disappear at self--consistency.

cond-mat.str-el↗
arXiv2002

Excited states dynamics in time-dependent density functional theory: high-field molecular dissociation and harmonic generation

We present a theoretical description of femtosecond laser induced dynamics of the hydrogen molecule and of singly ionised sodium dimers, based on a real-space, real-time, implementation of time-dependent density functional theory (TDDFT). High harmonic generation, Coulomb explosion and laser induced photo-dissociation are observed. The scheme also describes non-adiabatic effects, such as the appearance of even harmonics for homopolar but isotopically asymmetric dimers, even if the ions were treated classically. This TDDFT-based method is reliable, scalable, and extensible to other phenomena such as photoisomerization, molecular transport and chemical reactivity.

cond-mat↗
arXiv2001

Can optical spectroscopy directly elucidate the ground state of C20?

The optical response of the lowest energy members of the C20 family is calculated using time-dependent density functional theory within a real-space, real-time scheme. Significant differences are found among the spectra of the different isomers, and thus we propose optical spectroscopy as a tool for experimental investigation of the structure of these important clusters.

physics.atm-clus↗
arXiv2001

Excitonic effects in solids described by time-dependent density functional theory

Starting from the many-body Bethe-Salpeter equation we derive an exchange-correlation kernel $f_{xc}$ that reproduces excitonic effects in bulk materials within time-dependent density functional theory. The resulting $f_{xc}$ accounts for both self-energy corrections and the electron-hole interaction. It is {\em static}, {\em non-local} and has a long-range Coulomb tail. Taking the example of bulk silicon, we show that the $- α/ q^2$ divergency is crucial and can, in the case of continuum excitons, even be sufficient for reproducing the excitonic effects and yielding excellent agreement between the calculated and the experimental absorption spectrum.

cond-mat↗
arXiv2001

Stochastic Heterostructures in B/N-Doped Carbon Nanotubes

Carbon nanotubes are one-dimensional and very narrow. These obvious facts imply that under doping with boron and nitrogen, microscopic doping inhomogeneity is much more important than for bulk semiconductors. We consider the possibility of exploiting such fluctuations to create interesting devices. Using self-consistent tight-binding (SCTB), we study heavily doped highly compensated nanotubes, revealing the spontaneous formation of structures resembling chains of random quantum dots, or nano-scale diode-like elements in series. We also consider truly isolated impurities, revealing simple scaling properties of bound state sizes and energies.

cond-mat.mtrl-sci↗
arXiv2001

Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects

We report ab initio calculation of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

cond-mat↗
arXiv2001

Assessment of exchange-correlation functionals for the calculation of dynamical properties of small clusters in TDDFT

We present a detailed study of different exchange-correlation (xc) functionals in describing the dynamical properties of finite systems. For that purpose, we calculated the static polarizabilities, ionization potentials and optical absorption spectrum of four small clusters, Na_2, Na_4, SiH_4 and Si_2H_6, using a real-space, real-time technique. The computed static polarizabilities and ionization potentials seem to be in rather good agreement with the available experimental data, once the proper asymptotics of the potential are taken into account. The same conclusion holds for the absorption spectra, although the xc kernels in use do not provide a sufficiently strong attractive interaction between electrons and holes, leading to spectra slightly shifted towards higher energies. This deficiency is traced back to the insufficient description of dynamical effects in the correlation functional. Furthermore, it is shown that the xc potential used to obtain the ground state is the key factor to get reasonable spectra, whereas the choice of the xc kernel just amounts to small, although important, quantitative changes.

cond-mat.soft↗
arXiv2001

Backscattering in carbon nanotubes : role of quantum interference effects

For similar disorder, the backscattering contribution to the conductivity, irrelevant for metallic single-walled carbon nanotubes, is proved to become more significant for doped semiconducting systems, as found in experiments. In the case of multi-walled nanotubes, the intershell coupling is further shown to enhance the contribution of backscattering for "metallic" double-walled, whereas it remains insignificant for "metallic/semiconducting" double-walled systems. This supports that MWNTs are long ballistic conductors close to the charge neutrality point.

cond-mat.dis-nn↗
arXiv2001

Conduction mechanism and magnetotransport in multi-walled carbon nanotubes

We report on a numerical study of quantum diffusion over micron lengths in defect-free multi-walled nanotubes. The intershell coupling allows the electron spreading over several shells, and when their periodicities along the nanotube axis are incommensurate, which is likely in real materials, the electronic propagation is shown to be non ballistic. This results in magnetotransport properties which are exceptional for a disorder free system, and provides a new scenario to understand the experiments (A. Bachtold et al. Nature 397, 673 (1999)).

cond-mat.mtrl-sci↗
arXiv2000

Solution of Poisson's equation for finite systems using plane wave methods

Reciprocal space methods for solving Poisson's equation for finite charge distributions are investigated. Improvements to previous proposals are presented, and their performance is compared in the context of a real-space density functional theory code. Two basic methodologies are followed: calculation of correction terms, and imposition of a cut-off to the Coulomb potential. We conclude that these methods can be safely applied to finite or aperiodic systems with a reasonable control of speed and accuracy.

physics.comp-ph↗
arXiv2000

A real-space, rela-time method for the dielectric function

We present an algorithm to calculate the linear response of periodic systems in the time-dependent density functional thoery, using a real-space representation of the electron wave functions and calculating the dynamics in real time. The real-space formulation increases the efficiency for calculating the interaction, and the real-time treatment decreases storage requirements and the allows the entire frequency-dependent response to be calculated at once. We give as examples the dielectric functions of a simple metal, lithium, and an elemental insulator, diamond.

cond-mat↗