Showing papers by "Angel Rubio published in 2011"
TL;DR: In this article, an exact analytic form of the two-dimensional screened potential was provided for atomic thin layer insulating materials, which has strong implications for describing gap-impurity levels and also exciton binding energies.
Abstract: For atomic thin layer insulating materials we provide an exact analytic form of the two-dimensional (2D) screened potential. In contrast to three-dimensional systems where the macroscopic screening can be described by a static dielectric constant, in 2D systems the macroscopic screening is nonlocal ($q$ dependent) showing a logarithmic divergence for small distances and reaching the unscreened Coulomb potential for large distances. The crossover of these two regimes is dictated by 2D layer polarizability that can be easily computed by standard first-principles techniques. The present results have strong implications for describing gap-impurity levels and also exciton binding energies. The simple model derived here captures the main physical effects and reproduces well, for the case of graphane, the full many-body $\mathrm{GW}$ plus Bethe-Salpeter calculations. As an additional outcome we show that the impurity hole-doping in graphane leads to strongly localized states, which hampers applications in electronic devices. In spite of the inefficient and nonlocal two-dimensional macroscopic screening we demonstrate that a simple $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ approach is capable to describe the electronic and transport properties of confined 2D systems.
517 citations
TL;DR: In this article, the signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory using a single BN-sheet.
Abstract: The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the “free” exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra.
184 citations
TL;DR: In this paper, a variational Monte Carlo analysis of the linear and periodic chain of hydrogen atoms is presented, where the authors show that numerical accuracy comparable to that of benchmark density-matrix renormalization-group calculations can be achieved by using a highly correlated Jastrow-antisymmetrized geminal power variational wave function.
Abstract: In this paper, we report a fully ab initio variational Monte Carlo study of the linear and periodic chain of hydrogen atoms, a prototype system providing the simplest example of strong electronic correlation in low dimensions. In particular, we prove that numerical accuracy comparable to that of benchmark density-matrix renormalization-group calculations can be achieved by using a highly correlated Jastrow-antisymmetrized geminal power variational wave function. Furthermore, by using the so-called ``modern theory of polarization'' and by studying the spin-spin and dimer-dimer correlations functions, we have characterized in detail the crossover between the weakly and strongly correlated regimes of this atomic chain. Our results show that variational Monte Carlo provides an accurate and flexible alternative to highly correlated methods of quantum chemistry which, at variance with these methods, can be also applied to a strongly correlated solid in low dimensions close to a crossover or a phase transition.
98 citations
TL;DR: In this article, Maitra et al. proposed D-TDDFT, which includes explicit two-hole/two-particle states by adding a frequency-dependent term to adiabatic TDDFT.
93 citations
TL;DR: In this article, a local density approximation (LDA) for one-dimensional (1D) systems interacting via the soft-Coulomb interaction based on quantum Monte Carlo calculations is presented.
Abstract: We present a local density approximation (LDA) for one-dimensional (1D) systems interacting via the soft-Coulomb interaction based on quantum Monte Carlo calculations. Results for the ground-state energies and ionization potentials of finite 1D systems show excellent agreement with exact calculations obtained by exploiting the mapping of an $N$-electron system in $d$ dimensions onto a single electron in $N\ifmmode\times\else\texttimes\fi{}d$ dimensions, properly symmetrized by the Young diagrams. We conclude that 1D LDA is of the same quality as its three-dimensional (3D) counterpart, and we infer conclusions about 3D LDA. The linear and nonlinear time-dependent responses of 1D model systems using LDA, exact exchange, and the exact solution are investigated and show very good agreement in both cases, except for the well-known problem of missing double excitations. Consequently, the 3D LDA is expected to be of good quality beyond the linear response. In addition, the 1D LDA should prove useful in modeling the interaction of atoms with strong laser fields, where this specific 1D model is often used.
76 citations
TL;DR: In this article, it was shown that all adiabatic density functionals within time-dependent density-functional theory are not able to distinguish between resonant and nonresonant (detuned) Rabi oscillations.
Abstract: Through the exact solution of a two-electron singlet system interacting with a monochromatic laser we prove that all adiabatic density functionals within time-dependent density-functional theory are not able to discern between resonant and nonresonant (detuned) Rabi oscillations. This is rationalized in terms of a fictitious dynamical exchange-correlation (xc) detuning of the resonance while the laser is acting. The nonlinear dynamics of the Kohn-Sham system shows the characteristic features of detuned Rabi oscillations even if the exact resonant frequency is used. We identify the source of this error in a contribution from the xc functional to the set of nonlinear equations that describes the electron dynamics in an effective two-level system. The constraint of preventing the detuning introduces a new strong condition to be satisfied by approximate xc functionals.
63 citations
TL;DR: It is proved that clusters with anatase symmetry are energetically stable and can be considered as the starting seeds for the synthesis of larger nanostructures, which are of technological interest in photocatalysis and photovoltaics.
Abstract: Atomic clusters of TiO(2) are modeled by means of state-of-the-art techniques to characterize their structural, electronic and optical properties. We combine ab initio molecular dynamics, static density functional theory, time-dependent density functional theory, and many body techniques, to provide a deep and comprehensive characterization of these systems. TiO(2) clusters can be considered as the starting seeds for the synthesis of larger nanostructures, which are of technological interest in photocatalysis and photovoltaics. In this work, we prove that clusters with anatase symmetry are energetically stable and can be considered as the starting seeds to growth much larger and complex nanostructures. The electronic gap of these inorganic molecules is investigated, and shown to be larger than the optical gap by almost 4 eV. Therefore, strong excitonic effects appear in these systems, much more than in the corresponding bulk phase. Moreover, the use of various levels of theory demonstrates that charge transfer effects play an important role under photon absorption, and therefore the use of adiabatic functionals in time dependent density functional theory has to be carefully evaluated.
62 citations
TL;DR: In this article, the authors demonstrate that dissociative ionization of O2 can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field.
Abstract: We demonstrate that dissociative ionization of O2 can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.
60 citations
TL;DR: In this paper, the spin-symmetry breaking method was used for molecules composed of open-shell fragments, yielding accurate excitations at large separations when the acceptor effectively contains one active electron.
Abstract: Long-range charge-transfer excitations pose a major challenge for time-dependent density-functional approximations. We show that spin-symmetry breaking offers a simple solution for molecules composed of open-shell fragments, yielding accurate excitations at large separations when the acceptor effectively contains one active electron. Unrestricted exact-exchange and self-interaction-corrected functionals are performed on one-dimensional models and on the real LiH molecule within the pseudopotential approximation to demonstrate our results.
40 citations
TL;DR: The maximum thermostat coupling time is identified to guarantee proper thermostating for these simulations of the TIP4P/2005 water model, which predicts that anions induce faster formation of water clusters than cations of the same magnitude of charge.
Abstract: Extensive molecular dynamics simulations were conducted using the TIP4P/2005 water model of Abascal and Vega [J. Chem. Phys. 123, 234505 (2005)] to investigate its condensation from supersaturated vapor to liquid at 330 K. The mean first passage time method [J. Wedekind, R. Strey, and D. Reguera, J. Chem. Phys. 126, 134103 (2007); L. S. Bartell and D. T. Wu, 125, 194503 (2006)] was used to analyze the influence of finite size effects, thermostats, and charged species on the nucleation dynamics. We find that the Nose–Hoover thermostat and the one proposed by Bussi et al. [J. Chem. Phys. 126, 014101 (2007)] give essentially the same averages. We identify the maximum thermostat coupling time to guarantee proper thermostating for these simulations. The presence of charged species has a dramatic impact on the dynamics, inducing a marked change towards a pure growth regime, which highlights the importance of ions in the formation of liquid droplets in the atmosphere. It was found a small but noticeable sign pre...
37 citations
TL;DR: In this article, the anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states.
Abstract: The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with momentum transfer parallel to the BN layers is dominated by strongly bound, quasi-two-dimensional excitons. In contrast, excitations with momentum transfer perpendicular to the layers are influenced by weakly bound three-dimensional excitons. This striking phenomenon is revealed by a combined study using high-precision nonresonant inelastic x-ray scattering measurements supported by ab initio calculations. The results are relevant in general to layered insulating systems.
TL;DR: Copper and zinc phthalocyanines and porphyrins are used in organic light emitting diodes and dye-sensitized solar cells using near edge x-ray absorption fine structure (NEXAFS) spectroscopy, where the unoccupied valence states at the Cu and Zn atoms are probed and decomposed into 3d and 4s contributions with the help of density functional calculations.
Abstract: Copper and zinc phthalocyanines and porphyrins are used in organic light emitting diodes and dye-sensitized solar cells. Using near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the Cu 2p and Zn 2p edges, the unoccupied valence states at the Cu and Zn atoms are probed and decomposed into 3d and 4s contributions with the help of density functional calculations. A comparison with the N 1s edge provides the 2p states of the N atoms surrounding the metal, and a comparison with inverse photoemission provides a combined density of states.
TL;DR: In this article, the impact of electron correlations in the linear and non-linear response regimes of interacting many-electron systems exposed to time-dependent external fields was studied, and the performance of the recently derived 1D local density approximation as well as the exact-exchange orbital functional for those systems were analyzed.
TL;DR: Gas-phase action spectroscopy shows unambiguously that the low-energy absorptions by tetramethylthiotetrathiafulvalene and tetrathianaphthalene cations in solution phase are due to monomers and not π-dimers.
TL;DR: In this paper, the dynamic response of pristine and potassium-doped picene, the first example of a new family of organic molecular superconductors, was investigated by combining first-principles calculations and state-of-the-art experimental tools.
Abstract: We investigate the dynamic response of pristine and potassium-doped picene, the first example of a new family of organic molecular superconductors, by combining first-principles calculations and state-of-the-art experimental tools. We find that charge-carrier plasmons in K${}_{3}$ picene have a negative or almost negligible dispersion, which deviates from the traditional picture of metals based on the homogeneous electron gas. We show how this finding is the result of the competition between metallicity and electronic localization on the molecular units. Conduction electrons alone give rise to the negative dispersion, which is reduced by molecular polarization and crystal local-field effects. This analysis allows us to obtain a general picture of the plasmon dispersion in metallic molecular crystals.
TL;DR: Gas-phase infrared spectroscopy of the microhydrated suberate dianion is used together with quantum chemical calculations to establish relationships between conformational changes and spectroscopic features and to analyze how hydration can drive a conformational transition in a dianions and what role the hydrogen-bonded network plays.
Abstract: Dicarboxylate salts play an important role in many areas of science, including atmospheric chemistry, biochemistry, and synthetic chemistry. For example, they are used as antitumor drugs and as building blocks for metal–organic framework materials, and they are found in aerosol particles comprising photochemical smog. Isolated dicarboxylate dianions are stable in the gas phase and serve as model systems for multiply charged anions. 5] The presence of two charge centers separated by a hydrophobic, aliphatic chain also makes them ideal for studying charge screening and solventmediated effects. Herein, we use gas-phase infrared (IR) spectroscopy of the microhydrated suberate dianion (SA , OOC(CH2)6COO ) together with quantum chemical calculations to establish relationships between conformational changes and spectroscopic features. We then analyze how hydration can drive a conformational transition in a dianion and what role the hydrogen-bonded network plays. Gas-phase action spectroscopy is a powerful tool to study the effects of microhydration on the conformation of dicarboxylate dianions, adding one water molecule at a time. Anion photoelectron spectra in combination with quantum chemical calculations 8, 9] found a delicate dependence of the conformation of the dicarboxylate dianion on the degree of hydration, the aliphatic chain length, and the temperature. More detailed insight into the folding mechanism requires structural information, which is challenging to extract from the photoelectron data. IR photodissociation (IRPD) spectroscopy combined with high-level quantum chemical calculations on microhydrated anions is able to supply this information and thus leads to a considerably more detailed understanding of this hydration-mediated folding process at the molecular level. Recent IRPD spectra of SA and its monohydrate SA ·H2O revealed how the addition of a water molecule affects the spectroscopic signature of the dianion. The water molecule binds to one of the carboxylate groups of the quasi-linear dianion, thus causing characteristic shifts of the intense IR-active carboxylate stretching bands. The two symmetric (nS) and two antisymmetric (nA) carboxylate stretching modes in SA are in each case quasi-degenerate, because each pair of modes is weakly coupled as a result of the large distance between the carboxylate groups. Addition of a single water molecule lifts this degeneracy and leads to a characteristic splitting of both bands (compare the spectra for n = 0 and n = 1 in Figure 1).
TL;DR: In this article, the relative strength of the constituent resonances of the lowest-energy excitations of N${}_{2}, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, was investigated.
Abstract: Motivated by fundamental molecular physics and by atmospheric and planetary sciences, the valence excitations of N${}_{2}$ gas have seen several decades of intensive study, especially by electron-energy-loss spectroscopy (EELS). It was consequently surprising when a comparison of nonresonant inelastic x-ray scattering (NIXS) and nonresonant EELS found strong evidence for violations of the first Born approximation for EELS when leaving the dipole scattering limit. Here we reassess the relative strengths of the constituent resonances of the lowest-energy excitations of N${}_{2}$, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, expanding on the prior, qualitative interpretation of the NIXS results for N${}_{2}$ by both quantifying the generalized oscillator strength of the lowest-energy excitations and also presenting a time-dependent density functional theory (TDDFT) calculation of the $q$ dependence of the entire low-energy electronic excitation spectrum. At high $q$, we find that the LBH band has an unexpectedly large contribution from the octupolar $w{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Delta}}_{u}$ resonance exactly in the regime where theory and EELS experiment for the presumed-dominant $a{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{g}$ resonance have previously had substantial disagreement, and also where the EELS results must now be expected to show violations of the Born approximation. After correcting for this contamination, the $a{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Pi}}_{g}$ generalized oscillator strength from the NIXS results is in good agreement with prior theory. The NIXS spectra, over their entire $q$ range, also find satisfactory agreement with the TDDFT calculations for both bound and continuum excitations.
TL;DR: In this article, Mowbray et al. proposed a nano-bio Spectroscopy Group and ETSF Scientific Development Center, Departamento de Fisica de Materiales, Universidad del Pais Vasco, Centro de fisica of Materiales CSIC-UPV/EHU-MPC and DIPC, Avenida de Tolosa 72, E-20018 San Sebastian.
Abstract: D.J. Mowbray,1 P. Ayala,2 T. Pichler,2 and A. Rubio1, 3 Nano-bio Spectroscopy Group and ETSF Scientific Development Center, Departamento de Fisica de Materiales, Universidad del Pais Vasco, Centro de Fisica de Materiales CSIC-UPV/EHU-MPC and DIPC, Avenida de Tolosa 72, E-20018 San Sebastian, Spain University of Vienna, Faculty of Physics, 1090 Wien, Austria Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
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TL;DR: In this paper, a general approach to the computational design of nanostructured chemical sensors is presented based on identification and calculation of microscopic descriptors, which are used as input to a thermodynamic model to obtain the relevant macroscopic properties.
Abstract: We present a general approach to the computational design of nanostructured chemical sensors. The scheme is based on identification and calculation of microscopic descriptors (design parameters) which are used as input to a thermodynamic model to obtain the relevant macroscopic properties. In particular, we consider the functionalization of a (6,6) metallic armchair single-walled carbon nanotube (SWNT) by nine different 3d transition metal (TM) atoms occupying three types of vacancies. For six gas molecules (N_{2}, O_{2}, H_{2}O, CO, NH_{3}, H_{2}S) we calculate the binding energy and change in conductance due to adsorption on each of the 27 TM sites. For a given type of TM functionalization, this allows us to obtain the equilibrium coverage and change in conductance as a function of the partial pressure of the "target" molecule in a background of atmospheric air. Specifically, we show how Ni and Cu doped metallic (6,6) SWNTs may work as effective multifunctional sensors for both CO and NH_{3}. In this way, the scheme presented allows one to obtain macroscopic device characteristics and performance data for nanoscale (in this case SWNT) based devices.
TL;DR: In this article, the authors show how TDDFT can be used to design efficient gsBOMD algorithms, even if the electronic excited states are in this case not relevant.
Abstract: In principle, we should not need the time-dependent extension of density-functional theory (TDDFT) for excitations, and in particular not for Molecular Dynamics (MD) studies: the theorem by Hohenberg and Kohn teaches us that for any observable that we wish to look at (including dynamical properties or observables dependent on excited states) there is a corresponding functional of the ground-state density. Yet the unavailability of such magic functionals in many cases (the theorem is a non-constructive existence result) demands the development and use of the alternative exact reformulation of quantum mechanics provided by TDDFT. This theory defines a convenient route to electronic excitations and to the dynamics of a many-electron system subject to an arbitrary time-dependent perturbation. This is, in fact, the main purpose of inscribing TDDFT in a MD framework -the inclusion of the effect of electronic excited states in the dynamics. However, as we will show in this review, it may not be the only use of TDDFT in this context. In this manuscript, we review two recent proposals: In Section 1.2, we show how TDDFT can be used to design efficient gsBOMD algorithms -even if the electronic excited states are in this case not relevant. The work described in Section 1.3 addresses the problem of mixed quantum-classical systems at thermal equilibrium.
TL;DR: In this paper, the similarities of the two fields starting from the analysis of the Si6H6 molecule, the analogue of the benzene molecule but now being nonplanar, are analyzed.
Abstract: The delocalised nature of π-electrons in carbon-based compounds has opened a huge path for new fundamental and technological developments using carbon-based materials of different dimensionality (from clusters, to surfaces, nanotubes and graphene, among others). The success of this field has prompted the proposal that other inorganic structures based on B and N and more recently on Si and Ge could be formed with specific structural, mechanical, and electronic properties. In this paper we provide an analysis of the similarities of the two fields starting from the analysis of the Si6H6 molecule, the analogue of the benzene molecule but now being nonplanar. Then we move to the study of the two-dimensional (buckled) analogues of graphene but now formed by Si and Ge. Similarly, we look to nonplanar compounds based on boron and boron-carbon nitrogen composites. In particular, we focus on the mechanical properties of those new materials that exhibit a very high stiffness, resilience, and flexibility. Possible applications in the fields of catalysis, lubrication, electronic, and photonic devices now seem a likely by-product. We also address future directions triggered by the predicted superconducting properties of graphene, among other areas.
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TL;DR: Trabajo presentado a XXVth International Winterschool on Electronic Properties of Novel Materials: Molecular Nanostructures (IWEPNM 2011) 2011 as discussed by the authors.
Abstract: Trabajo presentado a XXVth International Winterschool on Electronic Properties of Novel Materials: Molecular Nanostructures (IWEPNM 2011).
Journal Article•
TL;DR: Wirtz et al. as mentioned in this paper showed that the phonon dispersion of Graphene displays two strong kinks in the highest optical branch (HOB) at the high-symmetry points G and K. This is due to the strong hybridization of the graphene p-bands with the Nickel d-bands which lifts the linear crossing of the p-band at K.
Abstract: Submitted for the MAR11 Meeting of The American Physical Society Tuning the Kohn Anomaly in the Phonon Dispersion of Graphene by Interaction with the Substrate and by Doping LUDGER WIRTZ, CNRS IEMN, Lille, France, ADRIEN ALLARD, CNRS IEMN, Lille, CLAUDIO ATTACCALITE, CNRS, Institut Neel, Grenoble, MICHELE LAZZERI, CNRS IMPMC, Paris, FRANCESCO MAURI, ANGEL RUBIO, ETSF/Univ. Basque Country, San Sebastian, Spain — The phonon dispersion of graphene displays two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points G and K. The slope of the HOB around K is a measure of the electron-phonon coupling (EPC) and determines the dispersion of the Raman D and 2D lines as a function of the laser energy. We show that the EPC can be strongly modified both due to interaction with a metallic substrate and due to doping. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around K becomes completely flat. This is due to the strong hybridization of the graphene p-bands with the Nickel d-bands which lifts the linear crossing of the p-bands at K. From experimental phonon dispersions one can therefore draw conclusions about the interaction strength between graphene and its different substrates. Furthermore, we present a new way to tune the EPC in graphene through electron/hole doping. We show that for the highest optical branch at K, the EPC is strongly dependent on the doping level. This dependency influences the dispersion of the Raman D and 2D lines and makes it possible to measure the charge state of graphene via resonant Raman spectroscopy. Ludger Wirtz CNRS IEMN, Lille, France Date submitted: 06 Dec 2010 Electronic form version 1.4
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TL;DR: In this paper, the initial stages of quasiparticle decay in a Fermi liquid are governed by a time-scale distinct from the scattering rates as derived from the Fermian golden rule approach.
Abstract: The initial stages of the quasiparticle decay in a Fermi liquid are governed by a time-scale distinct from the scattering rates as derived from the Fermi golden rule approach. We show that the initial decay is nonexponential and that it is determined by the zeroth spectral moment of the electron self-energy. We analyzed numerically a number of approximations for the self-energy by comparing with exact configuration interaction calculations for small finite system with fragmented states. A numerically simple approach for computing the spreading of the quasiparticle states for large systems is devised.
01 Jan 2011
TL;DR: The congreso "Frontiers in Interface Science: Theory and Experiment" celebrated in Berlin (Alemania) del 28 de Junio al 1 de Julio de 2011 as discussed by the authors.
Abstract: Trabajo presentado al congreso "Frontiers in Interface Science: Theory and Experiment" celebrado en Berlin (Alemania) del 28 de Junio al 1 de Julio de 2011.
10 Oct 2011
TL;DR: In this paper, state-of-the-art techniques for structural, electronic and optical properties of the TiO 2 ) 3 atomic cluster were studied by means of state of the art techniques.
Abstract: Atomic cluster (TiO 2 ) 3 is studied by means of state of the art techniques for structural, electronic and optical properties We combine molecular dynamics, density functional theory, time dependent density functional theory and many body techniques, to provide a deep and comprehensive characterization of the system Atomic clusters can be considered the starting seeds for the synthesis of larger nanostructures of technological interest Also, given the complexity of the material itself, a clear theoretical description of its basic properties provides interesting results both from the solid state physics and chemistry point of view