Dynamic Effective Charge Research Articles

Dynamic effective charge in the continuum final state in the CDW-EIS model: ionisation of molecules by bare-ion impact

Dynamic effective charge in the continuum final state in the CDW-EIS model: ionisation of molecules by bare-ion impact

Giant piezoelectricity driven by Thouless pump in conjugated polymers

Piezoelectricity of organic polymers has attracted increasing interest because of several advantages they exhibit over traditional inorganic ceramics. While most organic piezoelectrics rely on the presence of intrinsic local dipoles, a highly nonlocal electronic polarisation can be foreseen in conjugated polymers, characterised by delocalised and highly responsive π-electrons. These 1D systems represent a physical realisation of a Thouless pump, a mechanism of adiabatic charge transport of a topological nature which results, as shown in this work, in anomalously large dynamical effective charges, inversely proportional to the bandgap energy. A structural (ferroelectric) phase transition further contributes to an enhancement of the piezoelectric response reminiscent of that observed in piezoelectric perovskites close to morphotropic phase boundaries. First-principles density functional theory (DFT) calculations performed in two representative conjugated polymers using hybrid functionals, show that state-of-the-art organic piezoelectric are outperformed by piezoelectric conjugated polymers, mostly thanks to strongly anomalous effective charges of carbon, larger than 5e—ordinary values being of the order of 1e—and reaching the giant value of 30e for band gaps of the order of 1 eV.

Dynamic effective charge in the continuum of the CDW-EIS model for ionization in ion–atom collisions: angular and energy dependence

In this work, a dynamic charge is employed in the continuum distorted wave–eikonal initial state theoretical model to describe the non-Coulomb potential of the residual target for single ionization in bare ion–multielectron atom collisions. A comparison between the well-known Belkić prescription and the effective charge depending on emission angle and energy in the final residual-target continuum state is shown. The obtained results show that this effective charge improves the description of double-differential cross sections for backward emission angles over the whole emission energy range allowing the calculation of simple differential and total cross sections.

Binary Encounter Electrons in Fast Dressed-Ion–H2 Collisions: Distorted Wave Theories and Experiment

We report measurements of double differential cross section for zero-degree binary encounter electrons emitted in collisions of 4.9 MeV and 13 MeV B(2−5)+ ions with H2 targets. The corresponding calculations based on continuum distorted-wave (CDW) theories are critically compared to the measurements. CDW in its post form exhibits a very good agreement with the measurements in all cases. The CDW theories utilized along with the well-known eikonal-initial-state (CDW-EIS) approximation are also examined and their results are compared to both the measurements and the CDW calculations. In particular, CDW-EIS using a recently proposed dynamic effective charge for the final channel projectile distortion exhibits a substantial improvement in comparison with an effective net-charge approximation.

Ion specific tuning of nanoparticle dispersion in an ionic liquid: a structural, thermoelectric and thermo-diffusive investigation.

Dispersions of charged maghemite nanoparticles (NPs) in EAN (ethylammonium nitrate) a reference Ionic Liquid (IL) are studied here using a number of static and dynamical experimental techniques; small angle scattering (SAS) of X-rays and of neutrons, dynamical light scattering and forced Rayleigh scattering. Particular insight is provided regarding the importance of tuning the ionic species present at the NP/IL interface. In this work we compare the effect of Li+, Na+ or Rb+ ions. Here, the nature of these species has a clear influence on the short-range spatial organisation of the ions at the interface and thus on the colloidal stability of the dispersions, governing both the NP/NP and NP/IL interactions, which are both evaluated here. The overall NP/NP interaction is either attractive or repulsive. It is characterised by determining, thanks to the SAS techniques, the second virial coefficient A2, which is found to be independent of temperature. The NP/IL interaction is featured by the dynamical effective charge ξeff0 of the NPs and by their entropy of transfer ŜNP (or equivalently their heat of transport ) determined here thanks to thermoelectric and thermodiffusive measurements. For repulsive systems, an activated process rules the temperature dependence of these two latter quantities.

Quantitative Investigation Nonequilibrium Inhomogeneous Plasma of the Heliosphere With Runaway Electrons

We prove that a nonequilibrium inhomogeneous giant gas discharge is realized in the heliosphere with huge values of the parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E/N$ </tex-math></inline-formula> , which determines the temperature of electrons. This quasi-stationary discharge determines the main parameters of the weak solar wind (SW) in the heliosphere. In connection with the development of space technologies and the human spacewalk, the problem of the nature of the SW is acute. The study of the interference of gravitational and electrical potentials at the earth’s surface began with the work of William Gilbert 1600. Such polarization effects–the interference of Coulomb and gravitational forces–have not been studied well enough even in the heliosphere. Our article is devoted to this problem. The Pannekoek–Rosselan–Eddington model does not consider the important role of highly energetic running (away from the Sun) electrons and, accordingly, the duality of electron fluxes in the heliosphere (from the Sun and to the Sun) According to an alternative model formulated by us, highly energetic (escaping from the positively charged Sun) electrons leave the Sun and the heliosphere, and weakly energetic ones, unable to leave the Coulomb potential well (hole–the positively charged Sun and the heliosphere–return to the positively charged Sun. The weak difference between the opposite currents of highly energetic (escaping from the Sun) electrons and weakly energetic (returning to the Sun) electrons is compensated by the current of positive ions and protons from the Su–SW. These dynamic processes maintain a quasi-constant effective dynamic charge of the Sun and the entire heliosphere all the way to the earth’s orbit. At the same time, quasi-neutrality (see discussion in the following) in the Sun and heliosphere is well performed up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−36</sup> . According to experiments and analytical calculations based on our model: 1) the plasma in the corona is nonequilibrium; 2) the maximum electron temperature is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e}$ </tex-math></inline-formula> 1–2 million degree; 3) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e}$ </tex-math></inline-formula> grows from 1000 km away from the Sun (see discussion in the following); and 4) the role of highly energetic electrons escaping from the plasma leads to a significant increase in the effective: solar charge and electric fields in the heliosphere in relation to the Pannekoek–Rosselan–Eddington model. This is due to the absence of a compensation layer that screens the effective charge of the Sun It is not formed due to the escape of highly energetic electrons (as in a conventional gas discharge) from the entire heliosphere with high temperatures exceeding the temperature of the Sun’s surface. Thus, the process of escape of highly energetic electrons forms the internal EMF of the entire heliosphere. Interference of gravitational and Coulomb potentials in the entire heliosphere is considered, and it is being manifested in generation of two opposite flows of particles: 1) that are neutral or with a small charge (to the Sun) and 2) in the form of high-energy electrons (escaping from the positively charged Sun) and an SW with positively charged ions with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z/M</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\ge0.107$ </tex-math></inline-formula> (from the positively charged Sun). The calculated values of the registered ion parameters in the SW were compared with experimental observations. Reasons for generating the ring current in inhomogeneous heliosphere and inapplicability of the Debye theory in describing processes in the SW (plasma with current) are considered.

Dynamic screening and two-center effects in neutral and partially dressed ion-atom collisions

In collisions involving projectiles with bound electrons, partial or total screening of their nuclear charge must be considered. Accordingly, a projectile dynamic effective charge, as a function of the collision momentum transfer, is included in the state-of-the-art continuum distorted wave with eikonal initial state model. This correction, applied in the exit channel, recovers a two-center description from collisions with neutral projectiles. Also, an improvement of both target and projectile ionization double differential cross sections is achieved, especially at the binary encounter and electron capture to the continuum peaks. The results calculated by our analytical model are shown in accordance with previous theoretical and experimental work.

Dynamic effective charge in the target continuum within the CDW-EIS model for ionisation in ion–atom collisions: angular dependence

Dynamic effective charge in the target continuum within the CDW-EIS model for ionisation in ion–atom collisions: angular dependence

Giant effective charges and piezoelectricity in gapped graphene

Since the first realization of reversible charge doping in graphene via field-effect devices, it has become evident how the induction of a gap could further enhance its potential for technological applications. Here we show that the gap opening due to a sublattice symmetry breaking has also a profound impact on the polar response of graphene. By combining ab initio calculations and analytical modelling we show that for realistic band-gap values ( eV) the piezoelectric coefficient and the Born effective charge of graphene attain a giant value, independent on the gap. In particular the piezoelectric coefficient per layer of gapped mono- and bilayer graphene is three times larger than that of a large-gap full polar insulator as hexagonal Boron nitride (h-BN) monolayer, and 30% larger than that of a polar semiconductor as MoS2. This surprising result indicates that piezoelectric acoustic-phonons scattering can be relevant to model charge transport and charge-carrier relaxation in gated bilayer graphene. The independence of the piezoelectric coefficient and of the Born effective charge on the gap value follows from the connection between the polar response and the valley Chern number of gapped Dirac electrons, a relation previously identified for polar h-BN and made possible by the effective gauge-field description of the electron-lattice/strain coupling in these systems. In the small gap limit, where the adiabatic ab initio approximation fails, we implement analytically the calculation of the dynamical effective charge, and we establish a universal relation between the complex effective charge and the so-called Fano profile of the phonon optical peak. Our results provide a general theoretical framework to understand and compute the polar response in narrow-gap semiconductors, but may also be relevant for the contribution of piezoelectric scattering to the transport properties in Dirac-like systems.

Zone-center phonons and elastic properties of ternary chalcopyrite ABSe2 (A = Cu and Ag; B = al, Ga and In)

Abstract The zone-center phonon frequencies and the elastic properties of six ternary chalcopyrite compounds represented by ABSe2 (A = Cu and Ag; B = Al, Ga and In) are systematically calculated by using the rigid ion model. In the present paper, we calculate four bond-stretching and three bond-bending interactions, along with three effective dynamical charges. The calculated results are compared and found to be in fairly good agreement with the available experimental and theoretical results. The elastic anisotropy of the studied compounds is analyzed and visualized by plotting three-dimensional surfaces representing the directional dependences of Young's modulus.

Phonon modes, dielectric properties, infrared reflectivity, and Raman intensity spectra of semiconducting silicide BaSi2: First principles study

In this study, we investigated the phonon modes, dielectric properties, infrared (IR) reflectivity, and Raman intensity spectrum of semiconducting silicide BaSi2 within the framework of the first principles density functional theory. The zone-center phonon mode frequencies formed two major bands in the ranges of 50–150 cm−1 and 270–500 cm−1. The modes in the frequency band 100–150 cm−1 had high Born effective dynamical charges and oscillator strengths, and they contributed significantly to the mode effective charge, static dielectric permittivity, IR activities, and reflectivity spectrum. These modes were attributable to displacements of isolated Si clusters relative to one another without internal distortions. The computed Raman activities and intensities were significant for modes in the frequency band of 270–500 cm−1. These modes were primarily associated with silicon atom vibrations in the Si clusters in BaSi2 and the internal modes of the isolated Si clusters with Td symmetry. The band gap was also computed for BaSi2 using the modified Becke–Johnson exchange potential and it was in good agreement with the previously reported experimental values. The computed Raman intensity and IR reflectivity spectrum can be used as benchmark first principles theoretical results to compare with experimental results.

Contribution of π-bonds to effective charges, cohesive energy, and force constants of graphene-like compounds

For 14 two-dimensional hexagonal compounds IV–IV and III–V, analytical expressions have been obtained using the Harrison bond-orbital method for the contribution from the π-interaction to the polarity of interatomic bonds, the effective atomic and transverse dynamical charges and their dependences on the deformation, as well as to the binding energy, the cohesive energy, and the central and non-central force constants.

Pressure dependence of effective dynamic charge in In Al1−As ternary alloys

Abstract Using Raman scattering data under high pressure, we show that In x Al 1− x As ternary alloys exhibit, depending on In molar fraction x , either decreasing or increasing effective dynamic charge for InAs-like mode with increasing pressure. The variation of the effective dynamic charge in ternary alloy is determined by two factors: (1) the charge transfer between two different vibrating pairs and (2) the tendency to reduce it under lattice contraction in binary case.

Influence of the volume fraction on the electrokinetic properties of maghemite nanoparticles in suspension

We used several complementary experimental and theoretical tools to characterise the charge properties of well-defined maghemite nanoparticles in solution as a function of the volume fraction. The radius of the nanoparticles is equal to 6 nm. The structural charge was measured from chemical titration and was found high enough to expect some counterions to be electrostatically attracted to the surface, decreasing the apparent charge of the nanoparticle. Direct-current conductivity measurements were interpreted by an analytical transport theory to deduce the value of this apparent charge, denoted here by ‘dynamic effective charge’. This dynamic effective charge is found to decrease strongly with the volume fraction. In contrast, the ‘static’ effective charge, defined thanks to the Bjerrum criterion and computed from Monte Carlo simulations turns out to be almost independent of the volume fraction. In the range of Debye screening length and volume fraction investigated here, double layers around nanoparticles actually interact with each other. This strong interaction between nanocolloidal maghemite particles is probably responsible for the experimental dependence of the electrokinetic properties with the volume fraction.

Ab initio calculation of fundamental properties of CaxMg1−xA (A=Se and Te) alloys in the rock-salt structure

We employed the density-functional perturbation theory (DFPT) within the generalized gradient approximation (GGA), the local density approximation (LDA) and the virtual-crystal approximation (VCA) to study the effect of composition on the structure, stability, energy gaps, electron effective mass, the dynamic effective charge, optical and acoustical phonon frequencies and static and high dielectric constants of the rock-salt CaxMg1−xSe and CaxMg1−xTe alloys. The computed equilibrium lattice constant and bulk modulus show an important deviation from the linear concentration. From the Voigt–Reuss–Hill approximation, CaxMg1−xSe and CaxMg1−xTe present lower stiffness and lateral expansion. For Ca content ranging between 0.25 and 0.75, the elastic constants, energy gaps, electron effective mass and dynamic effective charge are predictions. The elastic constants and computed phonon dispersion curves indicate that these alloys are mechanically stable.

Pressure-induced variation of effective dynamic charge in InP1−xAsx alloys due to charge transfer within cation sublattice

From the measurements of Raman spectra of ternary InP1−xAsx (x=0.4, 0.6, and 0.8) under high pressure, the pressure-dependence of the ionicity was found to be strongly affected by the As mole fraction x. In particular, for x=0.6 and 0.8 ternary alloys, the effective dynamic charge of InP-like mode strictly increases with pressure, in sharp contrast to the corresponding behavior satisfied by the majority of III–V and II–VI binary compounds including InP. This can be understood by the charge transfer from As to P in the cation sublattice when the ternary alloys are under pressure.

The Electronic Structures, Born Effective Charges, and Interatomic Force Constants in BaMO3 (M = Ti, Zr, Hf, Sn): A Comparative First‐Principles Study

The electronic structures, Born effective charges (BEC), and interatomic force constants of BaMO3 (M = Ti, Zr, Hf, Sn) in the cubic perovskite structure are computed using first‐principles density functional theory. Comparison with the Maximally‐localized Wannier function description of the electronic structures for BaTiO3 shows that the change of a single constituent (Ti to Zr, Hf, and Sn) has pronounce effects on the orbital hybridizations with the oxygen, with significant implications for the nature of the BEC anomalous of the compounds. Examination of the real‐space interatomic force constants shows that the key interatomic interactions are strongly dependent on the constituent's dynamical effective charge and the very delicate compensation between the short‐range interactions and long‐range dipole–dipole interactions gives rise to the ferroelectric structural instabilities.

Lattice Dynamics and Crystalline Properties of Wurtzite Zn1–xMgxO Powders under High Pressure

This investigation systematically studies the lattice dynamics and crystalline properties in Zn1–xMgxO using high-pressure Raman spectroscopy. The incorporation of Mg and the application of external pressure cause distinct phonon vibrational behaviors in ZnO. Accordingly, the 202.7, 332.7, and 511.5 cm–1 phonons, which have been controversially assigned, can be conclusively identified. Detailed Raman spectra reveal that the metallic phase transition of ZnO is complete by around 13.2 GPa, which pressure is found to decrease as the Mg content increases. Upon pressure release, an unusual hysteresis effect (>10.0 GPa) in Zn1–xMgxO is observed. The degree of crystal ionicity and anisotropy importantly affects the phase transition pressure of Zn1–xMgxO. Under ambient conditions, ZnO becomes more ionic upon the incorporation of Mg and becomes more covalent under higher pressure. These results are caused by the interplay between the pressure dependence of the high-frequency dielectric constant and Born’s transverse dynamical effective charge. The E1–A1 splitting of the longitudinal and transverse optical phonons is analyzed, yielding insight into the pressure-dependent crystal anisotropy of Zn1–xMgxO.

Ab initio calculation of the elastic properties and the lattice dynamics of the AlAs x Sb1−x alloy under pressure

The lattice dynamics and the elastic properties of the ternary AlAs x Sb1−x alloy have been studied using the density-functional perturbation theory within the local density approximation and employing the virtual-crystal approximation. We study the variation of the optical phonon frequencies (ωTO and ωLO), the high-frequency (ϵ∞) and static (ϵ0) dielectric coefficients, the dynamic effective charge (Z*) and the elastic constants (C 11, C 12, C 44) as a function of the composition (x) and the pressure. We have also predicted the behavior of the optical and acoustical phonons with composition x at the X and L high symmetry points under pressure and determined the Grüneisen parameter. We have found that no mechanical instabilities are associated with the structural transition at high pressures for all compositions.

Theoretical studies of structural, elastic, electronic and lattice dynamic properties of AlxYyB1−x−yN quaternary alloys

A theoretical study on the structural, elastic, electronic and lattice dynamic properties of AlxYyB1−x−yN quaternary alloys in zinc-blend phase has been carried out with first-principles methods. Information on the lattice parameter, the lattice matching to available substrates and energy band-gaps is a prerequisite for many practical applications. The dependence of the lattice parameter a, bulk modulus B, elastic constants C11, C12 and C44, band-gaps, optical phonon frequencies (ωTO and ωLO), the static and high-frequency dielectric coefficients ε (0) and ε (∞) and the dynamic effective charge Z⁎ were analyzed for y=0, 0.121, 0.241, 0.362 and 0.483. A significant deviation of the bulk modulus from linear concentration dependence was observed. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young's modulus, Poisson's ratio are numerically estimated in the frame work of the Voigt–Reuss–Hill approximation. The resistance to changes in bond length and lateral expansion in AlxYyB1−x−yN increase with increasing y concentration. We observe that at y concentration about 0.035 and 0.063, AlxYyB1−x−yN changes from brittle to ductile and Γ–X indirect fundamental gap becomes Γ–Γ direct fundamental gap. There is good agreement between our results and the available experimental data for the binary compound AlN, which is a support for those of the quaternary alloys that we report for the first time.