Attractive Coulomb Interaction Research Articles

Intramolecular Diamagnetic Currents in Microscopic Sized Neutral Polyacetylenes

Intramolecular diamagnetic currents in neutral polyacetylene molecules with microscopic sizes, the mechanism of which has not been elucidated, can be explained by electron pairing originating from attractive Coulomb interactions between two electrons with opposite spins occupying the π molecular orbitals, mediated through nuclei with positive charge, in neutral polyacetylene molecules at 298 K, according to our calculated results. That is, vibronic interactions cannot play a role in electron pairing in the neutral polyacetylene molecule with microscopic size. The transition temperature Tc,Nchainπ value at which intramolecular diamagnetic currents can disappear in each polyacetylene molecule is estimated. The Tc,Nchainπ values for the neutral polyacetylene molecules with microscopic sizes are estimated to be much larger than the superconducting transition temperatures Tc,BCS values for the conventional superconductors with macroscopic sizes. This result can be understood from continuous energy levels of el...

About Two-Component Physics of HTSC

We present a first-principles, microscopic calculation of the ground state of cuprates indicating the presence of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to a charge density wave instability in the free component. The instability occurs in underdoped cuprates where the attractive Coulomb interaction between dopant impurities and charge carriers becomes strongly over-screened at low carrier densities and divides charge carriers into two orthogonal states. Within this new two-component state a novel quasi-particle entity, a microscopic Coulomb clump (CC), emerges.

Assembly of sol–gel-grown Li x CoO2 nanocrystals through electromagnetic irradiation

We report the fabrication of assembled nanostructures from the pre-synthesized nanocrystals building blocks through optical means of exciton formation and dissociation. We demonstrate that LixCoO2 nanocrystals assemble to an acicular architecture, upon prolonged exposure to ultraviolet–visible radiation emitted from a 125 W mercury vapor lamp, through intermediate excitation of excitons. The results obtained in the present study clearly show how nanocrystals of various materials with band gaps appropriate for excitations of excitons at given optical wavelengths can be assembled to unusual nanoarchitectures through illumination with incoherent light sources. The disappearance of exciton bands due to LixCoO2 phase in the optical spectrum of the irradiated film comprising acicular structure is consistent with the proposed mechanism of exciton dissociation in the observed light-induced assembly process. The assembly process occurs through attractive Coulomb interactions between charged dots created upon exciton dissociation. Our work presents a new type of nanocrystal assembly process that is driven by light and exciton directed.

Porous Organo-Functionalized Silica/Clay Hybrids

Two negatively charged species of silica nanoparticles and exfoliated clay nanosheets have been successfully assembled into highly porous frameworks through a direct hybridization reaction with the help of a surface modification technique. In order to create attractive Coulombic interaction between two components, the surface of silica was modified with positively chargeable aminosilane coupling agents. Without any additional post-calcination, as-prepared hybrid materials are highly porous with the BET specific surface area of 480 m2/g.

Molecular Dynamics Study of Oxygen Vacancy Diffusion in BaTiO<sub>3</sub> Doped with Rare Earth Ions

Atomistic mechanisms that restrain diffusion of oxygen vacancies in BaTiO3 doped with rare earth ions as donors were analyzed using molecular dynamics simulation. It was confirmed that formation of cation vacancies and reduction of lattice volume are sources of resistance for the diffusion. The cation vacancies trap the oxygen vacancies at the nearby O2- sites by an attractive force associated with Coulombic interaction. In contrast, the rare earth ions repel the oxygen vacancies, which migrate via O2- sites, and accelerate the diffusion. This is one of the factors that determine the restraint behavior of the diffusion, which depends on the type of rare earth ion in BaTiO3-based materials.

Diamagnetic currents in the closed-shell electronic structures in sp 3-type hydrocarbons

The mechanism of the occurrence of diamagnetic currents in sp 3-type electronic states in alkanes and pure diamonds is suggested, from analogy with the nondissipative diamagnetic currents in closed-shell electronic structures in aromatic hydrocarbons such as polyacenes and annulenes with microscopic sizes. It is found that most of all electrons can form electron pairs originating from attractive Coulomb interactions between two electrons with opposite momenta and spins occupying the same orbitals in pure diamonds with macroscopic sizes at 298 K. In principle, diamagnetic currents in alkanes and pure diamonds can be expected to occur by such electron pairing. The diamagnetic conducting transition temperature T c neutral N sp 3 values for macroscopic sized diamonds with large valence–conduction band gaps are estimated to be much larger than the superconducting transition temperatures T c,BCS values for the conventional superconductors with small valence–conduction band gaps. Even very small ratio of impurities or dopants would destroy possible nondissipative diamagnetic current states in the diamonds with macroscopic sizes. Therefore, very pure diamonds exhibiting nondissipative diamagnetic currents are very difficult to be produced up to now, and diamonds with impurities would become insulator or semiconductor. The technology which enables us to produce very pure diamonds in which no impurities exist is awaited to investigate whether such pure diamonds become nondissipative diamagnetic current states or not at 298 K.

Selective breakdown of quantum Hall edge states and non-monotonic Coulomb drag in a GaAs–AlGaAs electron–hole bilayer

The attractive Coulomb interaction between a two-dimensional electron gas (2DEG) and hole gas (2DHG) confined to quantum wells separated by a thin insulating barrier ( ∼ 20 nm ) may give rise to several interesting phenomena. The possible bosonic behaviour of these bilayer excitons have been anticipated for many years. We have developed a technique of fabricating closely spaced electron–hole bilayers in GaAs–AlGaAs heterostructures. Our device incorporates a novel method of making negative Schottky barrier based ohmic contacts, to a low density( < 5 × 10 10 cm - 2 ) 2-DEG, using MBE grown doped InAs. These contacts do not require annealing and are completely free from spiking. Electrons and holes accumulate on opposite sides of a 25 nm Al 0.3 Ga 0.7 As barrier, when suitably biased. Only in devices with very low barrier leakage, we find that at electron densities N e > 10 11 cm - 2 , instabilities start appearing in the quantum Hall plateaux in the electron layer. The effect first appears in the ν = 2 plateau. At higher densities, lower temperatures and tilted magnetic fields the effect starts appearing in the odd integer plateaus as well. Hall drag measurements strongly indicate that the instabilities result from interlayer interaction. At the lowest densities measured so far, the Coulomb drag in zero magnetic field shows an upturn at T < 1 K , which is a possible indication of electron–hole pairing fluctuations.

Control of the Oscillator Strength of the Exciton in a Single InGaN-GaN Quantum Dot

We report direct evidence for the control of the oscillator strength of the exciton state in a single quantum dot by the application of a vertical electric field. This is achieved through the study of the radiative lifetime of a single InGaN-GaN quantum dot in a p-i-n diode structure. Our results are in good quantitative agreement with theoretical predictions from an atomistic tight-binding model. Furthermore, the increase of the overlap between the electron and hole wave functions due to the applied field is shown experimentally to increase the attractive Coulomb interaction leading to a change in the sign of the biexcitonic binding energy.

Intramolecular ring current in (4 n + 2)π electronic states in the neutral acenes

The mechanism of the intramolecular diamagnetic ring currents in the (4 n + 2)π electronic states of neutral acenes with microscopic sizes such as naphthalene ( 10ac), anthracene ( 14ac), and tetracene ( 18ac) is discussed, and compared with that of electrical conductivity in normal metals and conventional superconductors with macroscopic sizes. Diamagnetic intramolecular ring current in the (4 n + 2)π electronic states of neutral acenes with microscopic sizes can be explained in terms of the electron pairing caused by attractive Coulomb interactions between two electrons with opposite spins occupying the same nondegenerate π molecular orbitals. The critical temperature T c neutral acenes at which diamagnetic intramolecular ring current states are destroyed for the (4 n + 2)π electronic states of each neutral acene with microscopic sizes such as 10ac, 14ac, and 18ac are estimated to be much larger than the critical temperature ( T c,BCS) for the conventional superconductors with macroscopic sizes. Energy lowering in the exponential factor in the equation for T c,BCS, originating from continuous energy levels of electronic states in the conventional superconductors with macroscopic sizes, and the energy lowering factor which is proportional to the T c neutral acenes values, originating from discrete energy levels of electronic states in acene molecules with small sizes such as 10ac, 14ac, and 18ac, are the main reason that the T c neutral acenes values for the (4 n + 2)π electronic states of neutral acenes with microscopic sizes are estimated to be much larger than the T c,BCS values for the conventional superconductors with macroscopic sizes. In bulk system, electrical conductivity in the (4 n + 1)π electronic states of the monocation crystal is compared with that in the (4 n + 2)π electronic states of neutral acene crystals formed by small acene molecules.

Excitonic effects in a time-dependent density functional theory

Excited state properties of one-dimensional molecular materials are dominated by many-body interactions resulting in strongly bound confined excitons. These effects cannot be neglected or treated as a small perturbation and should be appropriately accounted for by electronic structure methodologies. We use adiabatic time-dependent density functional theory to investigate the electronic structure of one-dimensional organic semiconductors, conjugated polymers. Various commonly used functionals are applied to calculate the lowest singlet and triplet state energies and oscillator strengths of the poly(phenylenevinylene) and ladder-type (poly)(para-phenylene) oligomers. Local density approximations and gradient-corrected functionals cannot describe bound excitonic states due to lack of an effective attractive Coulomb interaction between photoexcited electrons and holes. In contrast, hybrid density functionals, which include long-range nonlocal and nonadiabatic corrections in a form of a fraction of Hartree-Fock exchange, are able to reproduce the excitonic effects. The resulting finite exciton sizes are strongly dependent on the amount of the orbital exchange included in the functional.

Diamagnetic Currents in the Neutral He Atoms

The mechanism of the occurrence of intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes is investigated. It is found that most of all electrons can form electron pairs originating from attractive Coulomb interactions between two electrons with opposite spins occupying the 1s atomic orbital in the neutral He atom at 298 K. Intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes can be explained by such electron pairing. The transition temperature Tc(He),(1s) value at which intraatomic diamagnetic currents can disappear in each He atom is estimated. The Tc(He),(1s) values for the neutral He atoms with microscopic sizes are estimated to be much larger than the superconducting transition temperatures Tc,BCS values for the conventional superconductors with macroscopic sizes. This result can be understood from continuous energy levels of electronic states in conventional superconductivity with macroscopic sizes, and from discrete energy levels of electronic states in the neutral He atoms with microscopic sizes. The energy difference between the occupied and unoccupied orbitals decreases with an increase in material size and thus the second-order perturbation effect becomes more important with an increase in material size. Therefore, the mechanism of the occurrence of intraatomic diamagnetic current in the neutral He atoms suggested in this research would not be true for materials with large sizes. The dependence of electronic properties on temperature in the diamagnetic currents in the neutral He atoms with microscopic sizes is studied and compared with that in the conventional superconductivity with macroscopic sizes.

Adsorption of Copper Clusters in TS-1 Pores: Ti versus Si and Gold versus Copper

We present a quantum mechanics/molecular mechanics (QM/MM) analysis of interaction of Cu1-5 clusters with nondefect and metal-vacancy defect versions of T6-Ti and T6-Si sites in TS-1. The clusters inside the TS-1 pores are significantly stabilized due to long-range interactions such as van der Waals forces. We predict stronger adsorption of Cu clusters on Ti sites versus that on Si sites and on defect sites versus that on nondefect sites. A graph of adsorption energy (ΔEads) versus cluster size (n) shows a peak around n = 3. While these findings are consistent with previous results on Au clusters, we have identified significant differences in the Au/TS-1 and Cu/TS-1 systems, especially in the charge-transfer interactions. Moreover, the binding configurations and cluster−support interactions are different in Cu/Ti and Cu/Si systems. While Cu1-5 clusters adsorbed on Si sites are negatively charged, the Cu3 and Cu5 clusters adsorbed on Ti sites are significantly positively charged. Due to relatively low ionization potential, these clusters lose electron density to cationic Ti sites through negatively charged lattice oxygens. The attractive coulomb interactions between positively charged Cu atoms and negatively charged lattice oxygens are consistent with the X2O2 and X3O3 (XmOn: m metal atoms and n oxygen atoms) binding configurations of Cu3/Cu5 clusters adsorbed on Ti sites, which make the lattice-oxygen-mediated charge-transfer energetically favorable. The X1On (n = 1 or 2) binding configurations are preferred by the Cu1-5 clusters adsorbed on Si sites and also by the Au1-5 clusters adsorbed on both Ti and Si sites. Because the Au−O interactions are weaker than the Cu−O interactions, the X2O2 and X3O3 binding configurations are not preferred by the Au3/Au5 clusters adsorbed on Ti sites. The differences in cluster−support interactions in Cu/Ti versus Cu/Si and Au/Ti versus Cu/Ti systems can be explained on the basis of geometric and electrostatic effects.

Phosphorylation‐Induced Conformational Changes in Short Peptides Probed by Fluorescence Resonance Energy Transfer in the 10 Å Domain

Phosphorylation-induced conformational changes in short polypeptides were probed by a fluorescence resonance energy transfer (FRET) method by employing a short-distance FRET pair (R(0) approximately 10 A) based on tryptophan as natural donor and a 2,3-diazabicyclo[2.2.2]oct-2-ene-labeled asparagine (Dbo) as synthetic acceptor. Two substrates for kinases, LeuArgArgTrpSerLeuGly-Dbo (peptide I) and TrpLysArgThrLeuArgArg-Dbo (peptide II), were investigated, with serine and threonine, respectively, as phosphorylation sites. Steady-state and time-resolved fluorescence experiments in H(2)O revealed a decrease in FRET efficiency for peptide I and an increase for peptide II; this suggested that the effective distances between donor and acceptor increased and decreased, respectively. The same trends and similar absolute variations in effective donor-acceptor distances were observed in propylene glycol, a less polar and highly viscous solvent; this suggested that the variations are due to intrinsic structural preferences. Fitting of the time-resolved decay traces according to a distribution function model (Gaussian distribution) provided the mean donor-acceptor distances, which showed an increase upon phosphorylation for peptide I (from 9.7 to 10.5 A) and a decrease for peptide II (from 10.9 to 9.3 A) in H(2)O. The broadness (half-width) of the distributions, which provides a measure of the rigidity of the peptides, remained similar upon phosphorylation of peptide I (3.0 versus 3.1 A), but decreased for peptide II (from 3.1 to 0.73 A in H(2)O); this suggests a more compact, structured conformation upon phosphorylation of the latter peptide. The elongation of the peptide backbone (by ca. 0.7 A) for peptide I is attributed to an increase in steric demand upon phosphorylation, which favors an extended conformation. The contraction (by ca. 1.4 A) and structural rigidification of peptide II is attributed to attractive Coulombic interactions and hydrogen bonding between the phosphate group and the arginine residues.

Correct Interpretation of Hund&amp;rsquo;s Rule and Chemical Bonding Based on the Virial Theorem

We have investigated Hund’s spin-multiplicity rule for the second and third row atoms (C, N, O, Si, P, and S) and the methylene molecule (CH2) by means of diffusion Monte Carlo method and complete active space self-consistent field method, respectively. It is found that Hund’s rule is interpreted to be ascribed to a lowering in the electron-nucleus attractive Coulomb interaction energy which is realized at the cost of increasing the electron-electron repulsive Coulomb interaction energy as well as the kinetic energy. We have also studied correlation in the hydrogen molecule H2. Correlation in H2 gives an increase of the electron density distribution nðrÞ in the left and right anti-binding regions, a reduction of nðrÞ in the binding region, and an increase in the equilibrium internuclear separation. The importance of the virial theorem is stressed in the evaluation of correlation effects on both Hund’s rule and chemical bonding in H2. [doi:10.2320/matertrans.48.662]

Environment-Controlled Interchromophore Charge Transfer Transitions in Dipeptides Probed by UV Absorption and Electronic Circular Dichroism Spectroscopy

Charge transfer (CT) transitions between the C-terminal carboxylate and peptide group have been investigated for alanyl-X and X-alanine dipeptides by far-UV absorption and electronic circular dichroism (ECD) spectroscopy (where X represents different amino acid residues). The spectra used in the present study were obtained by subtracting the spectrum of the cationic species from that of the corresponding zwitterionic peptide spectrum. These spectra displayed three bands, e.g., band I between 44 and 50 kK (kK = 10(3) cm(-1)), band II at 53 kK, and band III above 55 kK, which were, respectively, assigned to a n(COO-) --> pi* CT transition, a pi(COO-) --> pi* CT transition, and a carboxylate pi --> pi* (NV1) transition, respectively By comparison of the intensity, bandwidth, and wavenumber position of band I of some of the investigated dipeptides, we found that positive charges on the N-terminal side chain (for X = K), and to a minor extent also the N-terminal proton, reduce its intensity. This can be understood in terms of attractive Coulomb interactions that stabilize the ground state over the charge transfer state. For alanylphenylalanine, we assigned band I to a n(COO-) --> pi* CT transition into the aromatic side chain, indicating that aromatic side chains interact electronically with the backbone. We also performed ECD measurements at different pH values (pH 1-6) for a selected subset of XA and AX peptides. By subtraction of the pH 1 spectrum from that observed at pH 6, the ECD spectrum of the CT transition was obtained. A titration curve of their spectra reveals a substantial dependence on the protonation state of the aspartic acid side chain of AD, which is absent in DA and AE. This most likely reflects a conformational transition of the C-terminus into a less extended state, though the involvement of a side chain --> peptide CT transition cannot be completely ruled out.

Front Cover: Macromol. Rapid Commun. 10/2006

Front Cover: Water‐soluble cationic ammonium‐functionalized PPV possessing photoluminescence with high quantum efficiency interact with an anionic iron‐sulfur protein rubredoxin by attractive Coulomb interaction. Such polymer‐based biosensor exhibits high sensitivity and promising application in biodetection. Further details can be found in the article by F. Cheng, G.‐W. Zhang, X.‐M. Lu, Y.‐Q. Huang, Y. Chen, Y. Zhou, Q.‐L. Fan,* and W. Huang* on page 799.

Lysozyme and bovine serum albumin adsorption on uncoated silica and AlOOH-coated silica particles: the influence of positively and negatively charged oxide surface coatings

The adsorption of lysozyme and bovine serum albumin on silica and AlOOH-coated silica particles—representing negatively and positively charged oxide surfaces—was investigated. The protein-treated uncoated and completely AlOOH-coated silica particles were characterized by zeta potential analysis and UV/VIS spectroscopy. It was found that at pH 7 a protein oppositely charged to the oxide surface adsorbs in significantly higher amounts. In contrast, proteins of the same charge did not or only in very low amounts adsorbed on an oxide surface. As both oxide surfaces were measured to be very hydrophilic it can be concluded that electrostatic interactions dominate the adsorption process at the investigated experimental conditions. The pH regions where the proteins interact via attractive and repulsive coulomb interaction with the particular oxide surfaces were calculated and outlined.

Exciton sizes of conducting polymers predicted by time-dependent density functional theory

The electronic structure and size scaling of spectroscopic observables in conjugated polymers are investigated using time-dependent density functional theory. We show that local density approximations and gradient-corrected functionals do not have an effective attractive Coulomb interaction between photoexcited electron-hole pairs to form bound states and therefore do not reproduce finite exciton sizes. Long-range nonlocal and nonadiabatic density functional corrections (such as hybrid mixing with an exact Hartree-Fock exchange) are necessary to capture correct delocalization of photoexcitations in one-dimensional polymeric chains.

Localization and entanglement of an exciton in a quantum dot molecule by an ac electric field

We investigate the dynamical behaviors of an interacting electron–hole pair in a double coupled quantum dot molecule under an ac electric field. We theoretically analyze the phenomenon of localization of an exciton in the quantum system, and indicate the conditions that dynamical localization occurs. The attractive Coulomb interaction plays an important role in forming localization and entangled states of the system. It is found that they move between the two coupled dots as a whole even if the strength of the electric field is much stronger than the Coulomb interaction. The dynamics of the exciton are mainly confined in a low-energy subspace, which is constructed by two localized states. Quantum entanglement of the exciton in this system can be manipulated by using the ac field. The same results are obtained numerically within Floquet theory.

Nature of interaction energy anisotropy in the Li(2S)–HF (˜X1Σ+) van der Waals complex. A theoretical study

The potential-energy surface for the Li(2S)–HF (˜ X1Σ+ interaction, where HF is kept rigid, is calculated using the supermolecular unrestricted fourth-order Moller–Plesset perturbation theory. The basis set superposition error corrected potential indicates two minima. The global minimum occurs for the bent Li...FH structure at R=1.95 A and θ=70° with a relatively deep well of De=1,706 cm−1 and the secondary minimum is found for the linear Li...HF configuration at R=4.11 A with a well depth ofDe=288 cm−1. A barrier of 177 cm−1 (with respect to the secondary linear minimum) separates these two minima. In this study 27 bound states of the bent Li...FH minimum and eight bound states of the linear Li...HF minimum up to the Li+HF dissociation threshold are calculated. The energy partitioning using the intermolecular perturbation theory scheme shows that the origins of the stability of the structures studied are entirely different. The global minimum is stabilised using the attractive Coulombic interaction and unrestricted Hartree–Fock deformation energy. The latter term originates from the mutual electric polarisation effects. The secondary linear minimum is mostly determined by the anisotropy of the repulsive Heitler–London exchange-penetration and attractive dispersion energies.