Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering

Diffuse X-ray data for mixed stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure

Effect of dispersion interactions on the properties of LiF in condensed phases

Classical molecular dynamics simulations are performed on LiF in the framework of the polarizable ion model. The overlap-repulsion and polarization terms of the interaction potential are derived on a purely non empirical, first-principles basis. For the dispersion, three cases are considered: a first one in which the dispersion parameters are set to zero and two others in which they are included, with different parameterizations. Various thermodynamic, structural and dynamic properties are calculated for the solid and liquid phases. The melting temperature is also obtained by direct coexistence simulations of the liquid and solid phases. Dispersion interactions appear to have an important effect on the density of both phases and on the melting point, although the liquid properties are not affected when simulations are performed in the NVT ensemble at the experimental density.Comment: 8 pages, 5 figure

Static polarizability of molecular materials: environmental and vibrational contributions

Modeling the dielectric behavior of molecular materials made up of large pi-conjugated molecules is an interesting and complex task. Here we address linear polarizabilities, and the related dielectric constant, of molecular crystals and aggregates made up of closed-shell pi-conjugated molecules with either a non-polar or largely polar ground-state, and also examine the behavior of mixed-valence (or charge-transfer) organic salts. We recognize important collective phenomena due to supramolecular interactions in materials with large molecular polarizabilities, and underline large vibrational contributions to the polarizability in materials with largely delocalized electrons.Comment: 18 pages, including 9 figure

Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions

The dipole P(F) of systems with periodic boundary conditions (PBC) in a static electric field F is applied to one-dimensional Peierls-Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N = 14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid stacks with alternating transfer integrals t(1 +/- \delta) diverges at the neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions, \alpha_{vib}, that are related to \partial P/\partial \delta and that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response \kappa of models with neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with parameters used previously for variable ionicity \rho and vibrational spectra of CT salts. The calculated \kappa accounts for the dielectric response of CT salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small \rho and continuous crossover to large \rho, while stiff stacks such as TTF-CA have a first-order transition with discontinuous \rho that is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations.Comment: 10 pages, 9 figures; J.Chem.Phys., in pres

Non-additive electronic polarizabilities of ionic liquids: charge delocalization effects

Electronic charge delocalization on the molecular backbones of ionic liquid-forming ions substantially impacts their molecular polarizabilities. Density functional theory calculations of polarizabilities and volumes of many cations and anions are reported and applied to yield refractive indices of 1216 ionic liquids. A novel expression for the precise estimation of the molecular volumes of the ionic liquids from simulation data is also introduced, adding quadratic corrections to the usual sum of atomic volumes. Our significant findings include i) that the usual assumption of uniform, additive atomic polarizabilities is challenged when highly mobile electrons in conjugated systems are present, and ii) that cations with conjugated large carbon chains can be used together with anions for the design of ionic liquids with very high refractive indices. A novel relation for the polarizability volume is reported together with a refractive index map made up of the studied ionic liquidsThis work was supported by Ministerio de Economia y Competitividad (MINECO) and FEDER Program through the project MAT2017-89239-C2-1-P; Xunta de Galicia and FEDER (ED431D 2017/06, ED431E2018/08, GRC 508 ED431C 2020/10). C. D. R. F. thanks the support of Xunta de Galicia through the grant ED481A-2018/032. We also thank the Centro de Supercomputacion de Galicia (CESGA) facility, Santiago de Compostela, Galicia, Spain, for providing the computational resources employed in this workS

Understanding the role of van der Waals forces in solids from first principles

The study of cohesion in solids is among the most fundamental research subjects in condensed-matter physics. The search for a deeper understanding of cohesion has lead to a steady progress in electronic-structure methods, enabling us to better understand structural, electronic, and mechanical properties of solids. The quantitative description of cohesion in solids requires solving the many-body Schr¨odinger equation and such exact treatment remains an unsolved problem. In this context, the correct treatment of cohesive properties (lattice constants, cohesive energies, and bulk moduli) requires an accurate description of the long-range electron correlation. In particular, van der Waals (vdW) interactions, being ubiquitous and arising from correlations between electrons, have been proposed to affect the cohesion in solids since a long time. This leads to two unsolved questions: (1) How to properly and effectively model vdW interactions in solids?, and (2) What is the quantitative role of vdW interactions in the cohesive properties of different types of solids? In this thesis, we address both questions by developing novel methods for vdW interactions in solids and assessing the role of the long-range vdW energy for a wide variety of non-metallic solids in the context of density-functional theory (DFT). Among first-principles approaches to the many-body Schr¨odinger equation, DFT has become the method of choice for obtaining ground-state properties of molecules and materials. A great advantage of DFT is that it is in principle an exact theory and the complexity of the full many-body problem is replaced by the electronic exchange-correlation (XC) functional. However, this functional is only known approximately and all widely employed (semi-)local and hybrid functionals suffer from the so-called self-interaction errors and lack the longrange vdW energy tail, often yielding noticeable deviations from experimental data. This issue will be illustrated in my thesis by assembling a large database of 64 solids and employing the LDA, PBE, and M06-L functionals to study their cohesive properties. This assessment shows that none of these functionals is sufficient to describe the cohesion for a broad range of solids, leading us to propose that the missing long-range vdW interaction accounts for part of the deviations found in approximate XC functionals. To assess the role of vdW interactions in solids, we develop the so-called DFT+vdWTS+SCS method that accurately models the electrodynamic response effects in the polarizability and vdW coefficients. This method is essentially free of adjustable parameters; the only necessary ingredients are the electron density and reference polarizabilities for free (isolated) atoms in the gas phase. Together with a benchmark study based upon experimental and time-dependent DFT optical spectra, I show that the concept of atoms-in-solids can be successfully utilized to define polarizabilities for finite-gap materials. Remarkably, my analysis demonstrates the validity of the Clausius-Mossotti relation for linking the macroscopic dielectric function to the microscopic response in covalentlybonded semiconductors – a matter of long debate in the literature. Upon the inclusion of long-range vdW interactions on top of the nonempirical PBE functional, a factor-of-two improvement is found in the cohesive properties with respect to the standard PBE values. I conclude that the vdW energy plays a crucial role in the cohesion of semiconductors and ionic solids. The proposed DFT+vdWTS+SCS approach represents a promising way towards extending the applicability of standard density functionals, and thus will be useful for a wide variety of applications in molecules and materialsDie Bindungskräfte in Festkörpern sind von grundlegendem Interesse in der Physik kondensierter Materie. Eine quantitative Beschreibung von Kohäsion in Festkörpern bedarf der Lösung der Vielteilchen-Schrödinger-Gleichung, die allerdings meist nicht exakt lösbar ist. Für die Kohäsions-Eigenschaften ist eine genaue Beschreibung der langreichweitigen Korrelation der Elektronen maßgeblich. Insbesondere können van der Waals (vdW) Wechselwirkungen, die durch Korrelationen zwischen Elektronen auftreten, die Kohäsion in Festkörpern beeinflussen. Es stellen sich zwei Fragen: (1) Wie können vdW-Wechselwirkungen in Festkörpern präzise und effektiv modelliert werden?, und (2) Welche quantitative Rolle spielen sie? In dieser Arbeit werden beide Fragen behandelt, indem neue Methoden zur Beschreibung von vdW-Wechselwirkungen in Festkörpern entwickelt werden und die Rolle der langreichweitigen vdW-Energie für eine Vielzahl von nicht-metallischen Festkörpern im Kontext der Dichtefunktionaltheorie (DFT) untersucht wird. Unter den ab initio Ansätzen zur Lösung der Schrödinger-Gleichung hat sich die DFT zur Methode der Wahl entwickelt, um die Grundzustands-Eigenschaften von Molekülen und Materialien zu bestimmen. Ein bedeutender Vorteil der DFT liegt darin, dass es sich im Prinzip um eine exakte Theorie handelt, wobei die Komplexität des vollen Vielteilchen-Problems durch das elektronische Austausch-Korrelations- Funktional ersetzt wird. Allerdings ist dieses Funktional nur näherungsweise bekannt und alle (semi-)lokalen und Hybrid-Funktionale, die breite Anwendung finden, sind mit sogenannten Selbstwechselwirkungsfehlern behaftet und berücksichtigen außerdem nicht die langreichweitigen vdW-Energiebeiträge, was oft zu merklichen Abweichungen im Vergleich zu experimentellen Messwerten führt. Diese Problematik wird in meiner Arbeit erläutert, indem die Bindungseigenschaften von 64 Feststoffen unter Verwendung von LDA, PBE und M06-L Funktionalen untersucht werden. Es wird gezeigt, dass keines der Funktionale ausreichend ist, um Kohäsion in Festkörpern für einen weiten Bereich von Materialien zu beschreiben. Wir folgern, dass dies zum Teil auf das Fehlen der vdW-Wechselwirkung zurückzuführen ist. Zur Untersuchung der vdW-Wechselwirkungen in Festkörpern entwickeln wir die sogenannte DFT+vdWTS+SCS Methode für eine genaue Modellierung der elektrodynamischen response-Effekte in der Polarisierbarkeit und den vdW-Koeffizienten. Diese Methode ist im Wesentlichen frei von anzupassenden Parametern; einzig die Elektronendichte und Referenz-Polarisierbarkeiten für freie Atome in der Gasphase werden benötigt. Zusammen mit einer Benchmark-Studie, die auf experimentellen und mit zeitabhängiger DFT bestimmten, optischen Spektren basiert, zeigen wir, dass das Konzept atoms-in-solids (Atome im Festkörper) erfolgreich verwendet werden kann, um Polarisierbarkeiten für Materialien mit endlicher Bandlücke zu definieren. Besonders bemerkenswert ist, dass meine Analyse die Gültigkeit der Clausius-Mossotti Relation für die Verknüpfung der makroskopischen dielektrischen Funktion mit der mikroskopischen Antwort in kovalent gebundenen Halbleitern zeigt – dies war Gegenstand langer Diskussionen in der Literatur. Durch die Einbeziehung von langreichweitigen vdW-Wechselwirkungen wird eine Verbesserung um einen Faktor zwei in der Beschreibung der Bindungs-Eigenschaften mit Bezug auf die entsprechenden Standard-PBE-Ergebnisse erreicht. Wir schließen daraus, dass die vdW-Energie eine entscheidende Rolle für die Kohäsion in Halbleitern und ionischen Festkörpern spielt. Die vorgestellte DFT+vdWTS+SCS Methode zeigt einen vielversprechenden Weg auf, um die Anwendbarkeit von Standard-Dichtefunktionalen zu erweitern, und wird folglich für eine Vielzahl von Anwendungen in Molekülen und Materialien nutzbar sein

Accurate Determination of Ion Polarizabilities in Aqueous Solutions

We present a novel method for obtaining salt polarizabilities in aqueous solutions based on our recent theory for the refractive index of salt solutions, which predicts a linear relationship between the refractive index and the salt concentration at low concentrations, with a slope determined by the intrinsic values of the salt polarizability and the density of the solution. Here we apply this theory to determine the polarizabilities of 32 strong electrolyte salts in aqueous solutions from refractive index and density measurements. Setting Li^+ as the standard ion, we then determine the polarizabilities of seven cations (Na^+, K^+, Rb^+, Cs^+, Ca^(2+), Ba^(2+), and Sr^(2+)) and seven anions (F^–, Cl^–, Br^–, I^–, ClO_4^–, NO_3^–, and SO_4^(2–)), which can be used as important reference data. We investigate the effect of temperature on salt polarizabilities, which decreases slightly with increasing temperature. The ion polarizability is found to be proportional to the cube of bare ionic radius (r_(bare)^3) for univalent ions, but the relationship does not hold for multivalent ions. Contrary to findings of Krishnamurti, we find no significant linear relationship between ion polarizability and the square of the atomic number (N^2) for smaller ions

Spectroscopic ellipsometric investigation of the clean and oxygen exposed Ni(110) surface

Dynamical ellipsometric investigations of the initial oxidation of the Ni(110) surface have been performed. Ellipsometry appears to be well suited to distinguish between chemisorbed oxygen and nickel oxide on the surface. Annealing at 740 K causes the nucleation of nickel oxide to proceed faster than at 570 K. After equal exposures however, the nucleation is slower with an oxygen pressure of 2 × 10−7 Torr than with a pressure of 1.3 × 10−8 Torr. Spectroscopic ellipsometric measurements (400–800 nm) have been performed on clean and oxygen-exposed (at saturation) Ni(110) surfaces. The complex dielectric function of the clean surface has been determined. To explain the results of the oxygen exposed surface a model is discussed — a surface layer with a thickness of 6 Å and a mixture of 90% NiO and 10% Ni — in which changes with substrate optical properties are taken into account