Repulsive Coupling Research Articles

A thermodynamic model for the monoclinic (ripple) phase of hydrated phospholipid bilayers

A thermodynamic model for the formation of the ripple phase of hydrated phospholipid bilayers is proposed based on the idea of a spontaneous local curvature free energy. It is suggested that this arises from electrostatic coupling between water dipoles and the dipolar head groups of the lipid molecules. A bilayer profile for the ripple phase is calculated using the model and agrees qualitatively with x-ray structure measurements. The premelting transition is discussed in terms of a repulsive coupling between the spontaneous curvature free energy and the free energy of ordering of the head group array in the Lβ′ phase. This leads to a mechanism for the premelting transition based on the competition between head group ordering, which softens with increasing temperature, and the free energy of ripple formation, which is favored by an increase in the bilayer polarisability when the head groups become disordered.

The effects of normalizing and disruptive selection on genes for recombination.

SUMMARYDeterministic simulations have been carried out of populations under normalizing and disruptive selection for a trait determined by genes with additive effects at six loci. In some simulations a pair of alleles at a seventh locus determined the rate of recombination between the seven loci. Normalizing selection with a single optimum, fixed or fluctuating, invariably led to genetic homozygosity. If the optimum fluctuates widely, the approach to homozygosity may be accompanied by a large decline in the mean fitness of the population. Disruptive selection was simulated by having two ‘niches’ with separate optima and separate density-dependent regulation, but with the adult population mating randomly. If the optima are widely separated, this leads to stable polymorphism. Selection produced linkage disequilibrium, normalizing selection causing repulsion and disruptive selection coupling between + and − alleles. This linkage disequilibrium accelerates the phenotypic response to selection, but delays changes in gene frequency. Selection always favoured alleles for low recombination at the expense of alleles for high recombination.

Theory of the quasi-one-dimensional electron gas with strong "on-site" interactions

It is shown that a quasi-one-dimensional system with strong attractive or repulsive coupling between electrons on the same site is equivalent to a Bose gas with hard core and longer-range interactions and to a Heisenberg-Ising magnet. Interchain and intrachain hopping and coupling are included and the equivalences are derived by elementary degenerate perturbation theory. Electronic correlation functions are derived from known pseudomagnetic ones and are used to discuss the circumstances in which superconductivity, charge- or spin-density wave transitions occur in the coupled chains.

Disorder and ionic polarons in solid electrolytes

The role of ion-ion repulsion and ion-phonon coupling in superionic conduction is explored. It is argued that the order-disorder phase transition is not associated with the conductivity discontinuity, but with a higher temperature second order phase transition which has been seen in some superionic conductors and which we predict for others. The specific heat, ion distribution, and conductivity are calculated.

Possible Phases and Some Properties of the One-Dimensional Metal with the Half-Filled Band

A one-dimensional metal with a half-filled band is studied by the model of Dzyaloshinskii and Larkin. The .next-order renormalization group theory is applied. The invariant charges become smooth functions over all temperature. The possible phases-the singlet and the triplet Cooper pairing states, and SDW and CDW ·states at T ~0 are discussed. The criterion for these phases depends on th~ bare coupling constants via their signs and relative magni­ tudes. The phase diagram differs from that of the first-order theory. The Green's function, the vertex function, and 2pF and uniform response functions are found in the asymptotic power form. The exponents are- univ~rsal except for the case of the repulsive couplings. The results are compared with the exact theorems abo~t the Hubbard and Tomonaga models.

Analysis of Raman scattering from two-phonon bound states

The possible formation of a bound state of two phonons by repulsive anharmonic coupling has been previously discussed in terms of a simple model Hamiltonian. These theoretical efforts yield a reasonable estimate for the bound-state energy, and provide a physical interpretation for the observed anomalous peak in the two-phonon Raman spectrum of diamond. In the present paper we consider the Raman intensity attributed to scattering from two-phonon bound states. The present theory is based on a two-phonon deformation-potential approach. The calculated intensity for diamond is in good agreement with available experimental data and thus provides further evidence for the existence of bound phonon pairs.

Effect of a semiconducting interface on the transition temperature of a superconductor: The image term

The problem of superconductivity near a metal-semiconductor interface has been studied, using the dielectric formulation of the problem. Assuming a sharp boundary, the bulk regions of the two media are described by simple model dielectric functions, and the effective electron-electron interaction in the metallic region near the interface is obtained within the framework of classical electrostatics. The strength of the effective coupling constant for the classical image term relevant to superconductivity depends strongly on ${q}_{s}L$, where ${q}_{s}$ is the static screening wave vector of the metal and $L$ is its thickness. It is appreciable only for very small ${q}_{s}L$ (lsim; 10). However, this is always smaller in magnitude than the repulsive Coulomb coupling constant, so that by itself it cannot lead to superconductivity. Nevertheless, it can appreciably alter the superconducting transition temperature ${T}_{c}$ of a metal, which becomes superconducting due to some other mechanism. This has been calculated as a function of ${q}_{s}L$, $\frac{{q}_{s}}{2{k}_{F}}$, and ${\ensuremath{\epsilon}}_{0}$, where ${k}_{F}$ is the Fermi wave vector of the metal and ${\ensuremath{\epsilon}}_{0}$ is the static dielectric constant of the semiconductor.

The magnetic properties of transition metal pseudohalide complexes. The significance of the orbital reduction factor k

The temperature dependence of the magnetic susceptibilities of some pseudohalide (SCN −, SeCN −, N 3 −) complexes of transition metals has been measured. The data together with others reported in the recent literature are interpreted by the theory of Lewis and Figgis. The physical significance of the orbital reduction factor is discussed. The variation of this factor due to chemical bonding and its relation to electron repulsion and spin-orbit coupling parameters is investigated.

The vibrational spectra of methanethiol

The infrared spectra of CH 3SH and CH 3SD have been investigated in the gas, liquid and solid state with the purpose of resolving some inconsistencies in the fundamental frequency assignment of CH 3SH. A study of the Raman spectra of liquid CH 3SH and CH 3SD to complement the infrared results was also carried out. A modified valence force field which included the diagonal symmetrized force constants plus a single added repulsive interaction constant coupling the methyl wagging with the SH bending vibrations was used in a normal coordinate treatment. Excellent agreement between the observed and calculated frequencies was obtained.

Absolute Infrared Intensities in Crystalline C2H4 and C2D4

Absolute infrared intensities of the ungerade fundamentals of crystalline C2H4 and C2D4 have been measured under high spectral resolution at 63°K and compared with the available intensity data in the gas phase. The intensity changes from gas to solid vary considerably among modes of the same symmetry species. Attempts to predict the transfer of intensity among certain vibrational modes in the crystal by an intermolecular hydrogen repulsion coupling mechanism have met with limited success. Intensity sum rules are discussed.