Direct Repulsion Research Articles

Mechanism for high-temperature superconductivity.

An explanation of the mechanism for high-temperature superconductivity is given, based upon a strong-coupling analysis of the extended Hubbard model previously introduced by one of us. The basic carriers are oxygen-hole quasiparticles in p\ensuremath{\sigma} orbitals, whose spin is strongly correlated with that of adjacent copper holes. These quasiparticles interact through the enhanced superexchange of the associated spins on the Cu sites, and an enhanced zero-point motion of the surrounding Cu holes. These are nonretarded attractive interactions whose strength increases as the oxygen-copper Coulomb repulsion increases and can be strong enough, for realistic parameters, to overcome the direct oxygen-oxygen Coulomb repulsion. The superconducting transition temperature that results is proportional to the Fermi energy of the oxygen holes.

Measurement of chemiluminescence polarization as a function of collision velocity by time-of-flight spectroscopy: Reactions of Xe* with HCl, HBr, Cl2, Br2, and I2

A new method is described for obtaining chemiluminescence polarization as a function of reactant relative velocity, employing a beam-gas time-of-flight technique. These experiments provide information about product alignment as a function of velocity. Results are reported for the formation of xenon halide excimers from the reactions of Xe(3P2,0) with HCl, HBr, Cl2, Br2, and I2. The HX reactions give strong excimer polarization, primarily due to kinematic constraints, whereas with the X2 reactions the excimers are polarized due to the collision dynamics. In this work we have made the first observation of cases in which the products rotate preferentially in planes normal to the initial relative velocity vector. This is observed for the Xe*+Cl2 and Br2 reactions at low velocities, and is attributed to a mechanism which gives preferential repulsive energy release perpendicular to the reactant relative velocity vector. The direct interaction product repulsion (DIPR) model does not give agreement with the data, showing that a more realistic potential surface model will be necessary to explain the present results.

Inter-Polaron Interaction and Bipolaron Formation. I

The system of two electrons coupling with phonon fields is studied with the use of the path integral method. In particular, the condition of the formation of a bipolaron and its physical properties are investigated in detail. The deformation potential interaction with acoustic phonons (the coupling constant S ac ) and the Frohlich interaction with optical phonons ( S op ) act to help the bipolaron formation against the direct Coulomb repulsion which prevents it. Usually, S ac plays a dominant role. With e ∞ / e 0 ≪1 and the optical phonon energy not so large compared with the electron band width, however, the bipolaron can be formed at large values of S op even when S ac =0. The bipolaron always has an enormous effective mass; thus it is practically localized. The mean distance between two electrons in the bipolaron is usually of the order of the interatomic distance.

Phonon-mediated electron-electron interaction in real space

We have calculated the phonon-mediated interaction between two $s$-band electrons as a function of their relative separation in real space. We consider the two cases of acoustic phonons in a nonpolar crystal and of optic phonons in an ionic crystal, and present specific results for electrons at the bottom of the band, using parameters of aluminum and lithium chloride, respectively. In the acoustic-phonon case, even when the Coulomb repulsion between the two electrons is not included, the interaction has a wide repulsive core, which extends to a separation of about 200 \AA{} and afterwards oscillates between attractive and repulsive regions as the distance between the electrons increases. Classically, i.e., when the electrons are infinitely heavy, this potential is attractive when the electrons are less than a few angstroms apart and oscillates at greater distances. For ionic crystals one anticipates that the phonon-mediated interaction causes merely the optic-phonon screening of the Coulomb repulsion; i.e., $\frac{{e}^{2}}{{\ensuremath{\epsilon}}_{\ensuremath{\infty}}r}$ is converted to $\frac{{e}^{2}}{{\ensuremath{\epsilon}}_{0}r}$. However, we find that this mechanism produces a striking oscillation (versus $r$) about the expected result. Consequently, in LiCl, for example, the total potential---the direct Coulomb repulsion plus the phonon-mediated interaction---has deep, attractive potential wells, the first and largest of which has a depth of 28 meV, with its minimum occurring when the electrons are 33 \AA{} apart.

Evidence for the existence of directional repulsion effects by lone valence electron pairs and .pi. bonds in trigonal-bipyramidal molecules

Evidence for the existence of directional repulsion effects by lone valence electron pairs and .pi. bonds in trigonal-bipyramidal molecules

Binding of thallium and other cations to the gramicidin A channel: Equilibrium dialysis study of gramicidin in phosphatidylcholine vesicles

Gramicidin A is a linear peptide antibiotic which forms dimer transmembrane channels selective for small monovalent cations, including thallium ions (Tl +) which are strongly bound. While there is great interest in the number of ion-binding sites per channel and the affinities of the sites for the various cations, measurements of the kinetics of ion permeation yield these equilibrium parameters only as indirect estimates dependent on the model assumed for the channel. Sonicated lipid vesicles. containing 1 mole of gramicidin per 30 moles of dimyristoylphosphatidylcholine. can be prepared with 5 m m-gramicidin. Evidence from our previous spectroscopic studies strongly supports the belief that this gramicidin is in the form of symmetrical dimer channels. Lipid vesicles containing gramicidin were dialyzed against control vesicles without gramicidin in the presence of a constant amount of radioactive 201Tl + and increasing amounts of non-radioactive Tl +. The ratio of 201Tl + free in solution to 201Tl + bound to the channel was measured after equilibrium (≥ 48 h) at 23 °C, and this ratio was plotted as a function of the free Tl + concentration. The inverse of the slope yielded 0.8 to 1.1 for the maximum number of simultaneously occupied highest affinity sites per channel, and the inverse of the intercept yielded a highest affinity constant of 500 to 1000 m −1 for each site. It appears that direct electrostatic repulsion prevents ions from binding simultaneously to the identical channel ends for thallium ion concentrations up to 20 m m. Estimates of the highest affinity constants for Rb + and Na + were also obtained.

Measured work of deformation and repulsion of lecithin bilayers.

We used three complementary techniques to vary the chemical potential of water in lipid/water mixtures; we measured the work of removing water from the multilayer lattice formed in water by the zwitterionic phospholipid egg lecithin. By x-ray diffraction, we observed the structural consequences of water removal. There are no discrete classes of "bound water" in this system; the work of removal is a continuous function of water content and lattice repeat spacing. From 30 to 3 A separation between bilayers there exists an exponential "hydration force" repulsion with a 2.6 A decay length. This interaction translates into a very large force to prevent contact between vesicles and planar membranes. It may be an important feature in controlling vesicle-to-cell fusion. As water is removed, bilayers not only move closer, but thicken as the lipid polar groups on the same bilayer move closer together. It is possible to divide the applied work into that of direct bilayer repulsion and that of bilayer deformation. We thus obtained a first determination of the lateral pressure required to create large increases in bilayer thickness and concomitant decreases in bilayer area. The lateral pressure reaches 25 dynes/cm for a 25% decrease in bilayer area. Systematic measurements of the mechanical properties of bilayers suffering such large deformation will allow critical tests of theories on bilayer stability and phase transition.

The Importance of Social Interactions and Habitat in Competition Between Microtus Agrestis and M. Arvalis

The Importance of Social Interactions and Habitat in Competition Between Microtus Agrestis and M. Arvalis

The chemisorption of carbon monoxide on palladium single crystal surfaces: IR spectroscopic evidence for localised site adsorption

Combined IR reflection-absorption and LEED studies of the room temperature adsorption of carbon monoxide on the (100), (111) and (210) surfaces of palladium are reported. The CO stretch frequency on the (100) surface varies from 1895 cm −1 at low coverages to 1949 cm −1 at θ = 0.5. A further strong shift to higher frequency is observed during the compression stage for θ > 0.5. On the (111) surface the CO stretch frequency is found at 1823 cm −1 at very low coverages, rising to 1936 cm −1 at θ = 0.5. In the compression stage above θ = 0.5 a second peak is observed above 2000 cm −1. On the much more open (210) surface, where surface atoms with the next nearest neighbour distance of 2.73 Å do not exist, the behaviour of the IR band resembles surprisingly that of the (100) surface. The infrared reflection studies provide detailed information on the CO/Pd system which other surface techniques have not as yet revealed, namely, evidence for surface specificity and localised site adsorption at low CO coverage. The frequency shifts observed in the spectra are discussed in terms of the three important intermolecular interaction mechanisms, namely, dipole-dipole coupling, direct intermolecular repulsion and indirect effects via the metal atoms.

Total Energy of Copper, Silver, and Gold

The pseudopotential theory of $d$-band metals discussed by the author in a previous paper is used to consider the total energy of the noble metals. The theoretical total-energy calculation is completed by adding to the total electronic energy the direct electrostatic repulsion between ions and subtracting from it an energy equal to the electron-electron interaction. The result is expressed as a sum of four quantities: a free-electron energy, a band-structure energy, an electrostatic (or Ewald) energy, and an overlap energy. The first three are directly analogous to the usual quantities found in the simple-metal total energy. The fourth contribution enters as a result of overlapping atomic $d$ states and is most conveniently expressed in terms of a repulsive pair potential. Energy-wave-number characteristics are evaluated for copper, silver, and gold by the numerical procedures previously used to calculate the form factors of these metals. Two improvements relating to exchange approximations are introduced, however. The most important of these involves a modification of the Kohn-Sham conduction-core, $d$ exchange potential in a manner suggested by Lindgren. This removes the spurious behavior otherwise obtained at long wavelengths in both the form factor and the energy-wave-number characteristic. The overlap potential for each metal is evaluated as a function of the separation between two ions in the metal and then fitted to a simple analytic form. Applications to the calculation of the binding energies, the low-temperature stable phases, and the phonon spectra of the noble metals are described. All three metals are calculated to be most stable in an hcp structure, rather than in the observed fcc structure. Reasonable phonon spectra are obtained, although large Kohn anomalies, which have not been reported experimentally, are seen in copper.

Molecular orbital calculations of lone pair interactions in hexahydropyrimidine

Explicit inclusion of directional electron repulsion integrals within CNDO types of MO methods for atoms containing lone pairs allows effects due to these lone pairs to be calculated.

Atoms and Ether

ATTEMPTS to dispense, in physics, with the ideas of direct attraction and repulsion, however interesting, lead generally to a petitio principii, and I fear Prof. Challis's view, to which attention is called in NATURE of August 7, cannot be received as an exception.