Repulsive Coulomb Interaction Research Articles

Effect of long-range repulsive Coulomb interactions on packing structure of adhesive particles.

The packing of charged micron-sized particles is investigated using discrete element simulations based on adhesive contact dynamic model. The formation process and the final obtained structures of ballistic packings are studied to show the effect of interparticle Coulomb force. It is found that increasing the charge on particles causes a remarkable decrease of the packing volume fraction ϕ and the average coordination number 〈Z〉, indicating a looser and chainlike structure. Force-scaling analysis shows that the long-range Coulomb interaction changes packing structures through its influence on particle inertia before they are bonded into the force networks. Once contact networks are formed, the expansion effect caused by repulsive Coulomb forces are dominated by short-range adhesion. Based on abundant results from simulations, a dimensionless adhesion parameter Ad*, which combines the effects of the particle inertia, the short-range adhesion and the long-range Coulomb interaction, is proposed and successfully scales the packing results for micron-sized particles within the latest derived adhesive loose packing (ALP) regime. The structural properties of our packings follow well the recent theoretical prediction which is described by an ensemble approach based on a coarse-grained volume function, indicating some kind of universality in the low packing density regime of the phase diagram regardless of adhesion or particle charge. Based on the comprehensive consideration of the complicated inter-particle interactions, our findings provide insight into the roles of short-range adhesion and repulsive Coulomb force during packing formation and should be useful for further design of packings.

Understanding Oxygen-Vacancy Migration in the Fluorite Oxide CeO2: An Ab Initio Study of Impurity-Anion Migration

The site exchange of an anion moiety (O, N, F, Ne, P, S, Cl, and Ar) with an oxygen vacancy in fluorite-structured CeO2 was studied by means of density functional theory (DFT) calculations. The obtained activation energies of migration vary between 0.2 and 0.9 eV, and increase with the formal valence of the migrating ion; the size of the migrating ion appears to play a minor role. An analysis of ion displacements suggests that repulsive Coulombic interactions between the migrating anion and oxygen ions as the next-nearest neighbors in the saddle-point configuration provide the dominant contribution to the activation energy of migration. As well as emphasizing the ease with which anion moieties are mobile in AO2 fluorite materials, these results suggest a new paradigm for understanding fast oxygen-ion conducting materials.

Cation-cation clusters in ionic liquids: Cooperative hydrogen bonding overcomes like-charge repulsion.

Direct spectroscopic evidence for H-bonding between like-charged ions is reported for the ionic liquid, 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate. New infrared bands in the OH frequency range appear at low temperatures indicating the formation of H-bonded cation-cation clusters similar to those known for water and alcohols. Supported by DFT calculations, these vibrational bands can be assigned to attractive interaction between the hydroxyl groups of the cations. The repulsive Coulomb interaction is overcome by cooperative hydrogen bonding between ions of like charge. The transition energy from purely cation-anion interacting configurations to those including cation-cation H-bonds is determined to be 3–4 kJmol−1. The experimental findings and DFT calculations strongly support the concept of anti-electrostatic hydrogen bonds (AEHBs) as recently suggested by Weinhold and Klein. The like-charge configurations are kinetically stabilized with decreasing temperatures.

Tunable magnetic interaction in hydrogenated epitaxial graphene modulated by the SiC substrate

We show that the ${d}^{0}$ ferromagnetism with high Curie temperature $({T}_{c})$ can be achieved in the electron-doped hydrogenated epitaxial graphene on certain SiC substrates through first-principles calculations. The pristine systems are found to be Mott insulators regardless of SiC polytype $(2H,4H$, or $6H)$ which, however, plays a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double-exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A ${T}_{c}$ of around 400 K is predicted for graphene on the $2H$-SiC substrate. We employ a nondegenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.

Long-range Coulomb interaction effects on polarons in conjugated polymers

Combining the Su–Schrieffer–Heeger and Pariser–Parr–Pople model (SSH+PPP), we have studied the influence of electron–electron interactions on the motion and dissociation of a polaron in the presence of an electric field, with emphasis on the long-range Coulomb interactions. The multiconfigurational time-dependent Hartree–Fock (MCTDHF) formalism was used to compute the electron–electron interactions. How the saturated velocity and the critical dissociation electric field of the polaron are related to the on-site Coulomb repulsion and long-range Coulomb interactions has been investigated. It was found that the on-site Coulomb interaction does not favor the motion of a polaron. There is a critical strength of the long-range Coulomb interaction for which the polaron takes on the most localized configuration. Comparing with the results obtained using the extended Hubbard model, we found that competition between the long-range Coulomb interactions and the on-site Coulomb interaction leads to a non-monotonic dependence of both the saturated velocity and the critical dissociation electric field on the long-range Coulomb interactions.

Effects of the defects on the structural, electronic and magnetic properties of Sr2FeMoO6

From the point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the imperfect Sr2FeMoO6 compound containing seven different defects of the FeMo or MoFe antisites, Fe–Mo interchange, VFe, VMo, VO or VSr vacancies have been studied by using first-principles projector augmented wave (PAW) within generalized gradient approximation (GGA) as well as taking into account on-site Coulomb repulsive interaction (GGA + U). No obvious structural changes are observed for the imperfect Sr2FeMoO6 compound containing FeMo or MoFe antisites, Fe–Mo interchange or VSr vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO) vacancies. The half-metallic character is maintained for the imperfect Sr2FeMoO6 compound containing FeMo antisite, VFe, VO or VSr vacancies, while it vanishes when MoFe antisite, Fe–Mo interchange or VMo vacancy are presented. So the MoFe antisite, Fe–Mo interchange or VMo vacancy defects have to be avoided in order to preserve the half-metallic character of the Sr2FeMoO6 compound and thus usable in spintronics devices. In FeMo or MoFe antisites, Fe–Mo interchange cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe, VMo, VO or VSr vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice, while two cation sublattices are coupled antiferromagnetically. The total magnetic moments μtot (μB/f.u.) of the imperfect Sr2FeMoO6 compound containing seven different defects decrease in the sequence of VSr vacancy (4.38), VMo vacancy (3.92), VO vacancy (3.51), VFe vacancy (3.40), FeMo antisite (3.40), MoFe antisite (3.33), Fe–Mo interchange (2.47) and the mechanisms of the saturation magnetization reduction have been discussed.

High-temperature superconductivity and long-range order in strongly correlated electronic systems

A selective review of the question of how repulsive electron correlations might give rise to off-diagonal long-range order (ODLRO) in high-temperature superconductors is presented. The article makes detailed explanations of the relevance to superconductivity of reduced electronic density matrices and how these can be used to understand whether ODLRO might arise from Coulombic repulsions in strongly correlated electronic systems. Time-reversed electron pairs on alternant Cuprate and the iron-based pnictide and chalcogenide lattices may have a weak long-range attractive tail and much stronger short-range repulsive Coulomb interaction. The long-range attractive tail may find its origin in one of the many suggested proposals for high-Tc superconductivity and thus has an uncertain origin. A phenomenological Hamiltonian is invoked whose model parameters are obtained by fitting to experimental data. A detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time-reversed pairs of electrons effectively avoid the repulsive hard-core of the Coulomb interaction but reside on average in the attractive well of the long-range potential. Thus, the pairing of electrons itself provides an enhanced screening mechanism. The alternant lattice structure is the key to achieving robust high-temperature superconductivity with dx2-y2 or sign alternating s-wave or s± condensate symmetries in cuprates and iron-based compounds. Some attention is also given to the question first raised by Leggett as to where the Coulombic energy is saved in the superconducting transition in cuprates. A mean-field-type model in which the condensate density serves as an order parameter is discussed. Many of the observed trends in the thermal properties of cuprate superconductors are reproduced giving strong support for the proposed model for high-temperature superconductivity in such strongly correlated electronic systems. © 2015 Wiley Periodicals, Inc.

Exciton quasicondensation in one-dimensional systems

A quasi-exciton condensate is a phase characterized by quasi-long range order of an exciton (electron-hole pair) order parameter. Such a phase can arise naturally in a system of two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that the quasi-exciton condensate phase can be stabilized in an extended range of parameters, in both spinless and spinful systems. For spinful electrons, the exciton phase is shown to be distinct from the usual quasi-long range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their inter-wire tunneling current-voltage characteristics. In the quasi-exciton condensate phase the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both phases are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.

Trends in Atomic Parameters for Crystals and Free Ions across the Lanthanide Series: The Case of LaCl3:Ln(3+).

Analyses of the crystal field energy levels of the series LaCl3:Ln(3+) using a semiempirical Hamiltonian shows that only five ions (Pr, Nd, Pm, Dy, Ho) meet the criteria to avoid overfitting of the atomic part. A new parameter (SNES) has been introduced to represent the strength of the normalized electrostatic repulsion for these ions. This parameter varies linearly (R(2)adj = 0.9994, N = 5) with the reciprocal of the radius of the tripositive lanthanide ion, as expected from the form of repulsive Coulomb interaction. The Slater parameters from the crystal field analyses, F(k)(corr) (i.e., corrected for the effects of the two-particle component of the three-body operator associated with the T(2) parameter), exhibit an exponential variation with the number of electrons, n, in 4f(n). This is explained by reference to the radial part of a hydrogen-like wave function. The ratio of F(k)(corr) with the ab initio free ion Slater parameter F(k)(ab initio) varies linearly with n. Fitted parameters F(k)(corr: free ion) from the free ion data for Pr(3+) and Nd(3+) show that the corresponding ab initio values are between 14 and 27% too high. The spin-orbit coupling constant from crystal field analyses (ζ4f) exhibits a quartic variation with atomic number, and the ratio ζ4f/ζ4f(ab initio) follows an exponential growth model with n. The results serve to confirm the hypothesis that smooth trends can be observed across the Ln(3+) series for the fitted parameters despite the fact that the majority of experimental data is lacking.

Analysis of the superconductivity in perovskite oxides using three-square-well BCS formalism

Superconductivity in perovskite, BaKBiO, is studied in the Bardeen–Cooper–Schrieffer (BCS) model, with three-square-well potentials. Components of the new coupling are: the attractive acoustic phonon–electron, optical phonon–electron and repulsive Coulomb interactions. With these in the BCS pairing Hamiltonian, expressions for the superconducting transition temperature and isotope effect exponent are obtained. Results of our analysis are consistent with experiments. Contributions of interactions to system properties are exhibited and analysed. Acoustic phonon–electron and optical phonon–electron interactions have near-identical elevation of transition temperature, holding out possible explanations for high- Tc. Contrastingly, optical phonon–electron and Coulomb couplings cause debilitation of isotope exponent, a possible explanation for low isotope exponent in the cuprates and other high- Tc systems. It is found that BCS electron–phonon coupling appears synonymous with acoustic phonon–electron coupling.

First principles study of structural, magnetic and electronic properties of C-doped monoclinic ZrO2

Structural, magnetic and electronic properties of C-doped monoclinic ZrO2 have been studied by using the first principles projector augmented wave potential within the generalized gradient approximation as well as taking into account on site Coulomb repulsive interaction. The optimized structures show the lattice constants a, b, c and monoclinic angle β of C-doped monoclinic ZrO2 change slightly. While the lattice constants a, b, c and monoclinic angle β of C1-doped monoclinic ZrO2 are smaller than that of C2-doped monoclinic ZrO2. The total magnetic moment of C-doped monoclinic ZrO2 is mainly contributed by the atomic magnetic moment of C atom. For C atom substituting O atom, the systems are found to be half-metallic material and usable in magnetoelectronic and spintronic devices.

First principles study of structural, magnetic and electronic properties of N-doped monoclinic ZrO2

Using the first principle projector augmented wave potential within the generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction, the structure, magnetic and electronic properties of N-doped monoclinic ZrO2 have been studied. For N doping in different oxygen sites, the lattice constants a, b, c and monoclinic β angle are almost equal. The N-doped m-ZrO2 is a ferromagnetic half-metallic material. The total magnetic moment is mostly contributed by the N atom. Spin splitting occurs between the partial DOS of the up-spin and down-spin channels for N 2s and 2p states, as well as the hybridization appears between N 2p and O 2p states. The strong ionic bonding character in N-doped m-ZrO2 is revealed in the substantial charges transfer from Zr atoms to N and O atoms.

Dipolar origin of tilting of rod-like molecules in the smectic C phase

We review various proposed molecular mechanisms for tilting of rod-like molecules in the smectic C phase. We analyse some experimental results, viz. (1) unusual odd–even effects in smectic A to nematic (TAN) and smectic A to smectic C (TAC) transition temperatures and/or maxima in TAC in some homologous series of compounds, (2) occurrence of re-entrant SmA and/or hexatic SmB as lower temperature phases, (3) SmC replaced by SmA when a halogen atom is substituted at the end of one of the chains, (4) suppression of SmC phase at high pressures, and in binary mixtures with dissimilar aromatic cores or chain lengths, (5) induction of SmC phase in mixtures of some structural isomers and (6) some recent all-atom simulations. The results can be understood on the basis of our molecular theory in which the tilting occurs due to the repulsive coulomb interaction between neighbouring molecules having off-axis dipoles with transverse components and freely rotating about their long axes in smectic layers. The model also takes into account the entropic cost of tilting which depends on the length to width ratio of the molecules. We have included some simple calculations of the induced dipoles in the chains to account for the odd–even effects.

Singlet levels of the NV− centre in diamond

The characteristic transition of the centre at 637 nm is between and triplet states. There are also intermediate and singlet states, and the infrared transition at 1042 nm between these singlets is studied here using uniaxial stress. The stress shift and splitting parameters are determined, and the physical interaction giving rise to the parameters is considered within the accepted electronic model of the centre. It is established that this interaction for the infrared transition is due to a modification of electron–electron Coulomb repulsion interaction. This is in contrast to the visible 637 nm transition where shifts and splittings arise from modification to the one-electron Coulomb interaction. It is also established that a dynamic Jahn–Teller interaction is associated with the singlet state, which gives rise to a vibronic level 115 cm−1 above the electronic state. Arguments associated with this level are used to provide experimental confirmation that the is the upper singlet level and is the lower singlet level.

Spin Fluctuations and High-Temperature Superconductivity in Cuprates

A microscopic theory of spin excitations and superconductivity in strongly correlated electronic systems as cuprates is discussed. We argue that in the Mott-Hubbard regime with two Hubbard subbands, there emerges a new energy scale induced by kinematical interaction of electrons with spin fluctuations of the order of electronic kinetic energy. The spectrum of magnetic excitations reveals a small damping caused by the involvement of spin excitations in the decay process besides the particle-hole continuum. This results in the magnetic resonance mode (RM) with weak temperature dependence as observed in experiments. Electronic spectrum and superconductivity are studied within the extended Hubbard model which includes intersite Coulomb repulsion and electron-phonon interaction. We found the d-wave pairing with high- T c mediated by strong kinematical interaction of electrons with spin fluctuations. Contributions to the d-wave pairing coming from intersite Coulomb repulsion and phonons turned out to be small.

Constraining low-energy proton capture on beryllium-7 through charge radius measurements

In this paper, we point out that a measurement of the charge radius of boron-8 provides indirect access to the S-factor for radiative proton capture on beryllium-7 at low energies. We use leading-order halo effective field theory to explore this correlation and we give a relation between the charge radius and the S-factor. Furthermore, we present important technical aspects relevant to the renormalization of point-like P -wave interactions in the presence of a repulsive Coulomb interaction.

Spin thermopower in interacting Rashba dots

Spintronics or spin electronics has been an active area of research based on the active control and manipulation of spin degrees of freedom. In this work we study the thermoelectric properties of a Rashba dot using the Anderson model in presence of the repulsive Coulomb interaction within the mean-field formalism. The temperature difference applied across the dot drives a spin current which depends on the Rashba coupling and chemical potential. We demonstrate that the Rashba dot in presence of the Coulomb interaction behaves as a spin filter for selected values of the chemical potential and is able to filter electrons by their spin orientation. The spin thermopower has also been studied where the effects of the impurity energy level, temperature and also the Rashba term have been observed.

Valence-skipping and negative-Uin thed-band from repulsive local Coulomb interaction

We show that repulsive local Coulomb interaction alone can drive valence-skipping charge disproportionation in the degenerate $d$-band, resulting in effective negative-$U$. This effect is shown to originate from anisotropic orbital-multipole scattering, and it occurs only for ${d}^{1},{d}^{4},{d}^{6}$, and ${d}^{9}$ fillings (and their immediate surroundings). Explicit boundaries for valence-skipping are derived, and the paramagnetic phase diagram for ${d}^{4}$ and ${d}^{6}$ is calculated. We also establish that the valence-skipping metal is very different, in terms of its local valence distribution, compared to the atomiclike Hund's metal. These findings explain why transition-metal compounds with the aforementioned $d$-band fillings are more prone to valence-skipping charge order and anomalous superconductivity.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Competing salt effects on phase behavior of protein solutions: tailoring of protein interaction by the binding of multivalent ions and charge screening.

The phase behavior of protein solutions is affected by additives such as crowder molecules or salts. In particular, upon addition of multivalent counterions, a reentrant condensation can occur; i.e., protein solutions are stable for low and high multivalent ion concentrations but aggregating at intermediate salt concentrations. The addition of monovalent ions shifts the phase boundaries to higher multivalent ion concentrations. This effect is found to be reflected in the protein interactions, as accessed via small-angle X-ray scattering. Two simulation schemes (a Monte Carlo sampling of the counterion binding configurations using the detailed protein structure and an analytical coarse-grained binding model) reproduce the shifts of the experimental phase boundaries. The results support a consistent picture of the protein interactions responsible for the phase behavior. The repulsive Coulomb interaction is varied by the binding of multivalent counterions and additionally screened by any increase of the ionic strength. The attractive interaction is induced by the binding of multivalent ions, most likely due to ion bridging between protein molecules. The overall picture of these competing interactions provides interesting insight into possible mechanisms for tailoring interactions in solutions via salt effects.