Some observations on the renormalization of membrane rigidity by long-range interactions

We consider the renormalization of the bending and Gaussian rigidity of model membranes induced by long-range interactions between the components making up the membrane. In particular we analyze the effect of a finite membrane thickness on the renormalization of the bending and Gaussian rigidity by long-range interactions. Particular attention is paid to the case where the interactions are of a van der Waals type.Comment: 11 pages RexTex, no figure

Effective diffusion constant in a two dimensional medium of charged point scatterers

We obtain exact results for the effective diffusion constant of a two dimensional Langevin tracer particle in the force field generated by charged point scatterers with quenched positions. We show that if the point scatterers have a screened Coulomb (Yukawa) potential and are uniformly and independently distributed then the effective diffusion constant obeys the Volgel-Fulcher-Tammann law where it vanishes. Exact results are also obtained for pure Coulomb scatterers frozen in an equilibrium configuration of the same temperature as that of the tracer.Comment: 9 pages IOP LaTex, no figure

Spin-Dependent Neutralino-Nucleus Scattering for A∼127A \sim 127 Nuclei

We perform nuclear shell model calculations of the neutralino-nucleus cross section for several nuclei in the A = 127 region. Each of the four nuclei considered is a primary target in a direct dark matter detection experiment. The calculations are valid for all relevant values of the momentum transfer. Our calculations are performed in the $3s 2d 1g_{7/2} 1h_{11/2}$ model space using extremely large bases, allowing us to include all relevant correlations. We also study the dependence of the nuclear response upon the assumed nuclear Hamiltonian and find it to be small. We find good agreement with the observed magnetic moment as well as other obervables for the four nuclei considered: ^{127}I, ^{129,131}Xe, and ^{125}Te.Comment: 23 pages + 7 postscript figures. LaTeX uses RevTe

Aging on Parisi's tree

We present a detailed study of simple `tree' models for off equilibrium dynamics and aging in glassy systems. The simplest tree describes the landscape of a random energy model, whereas multifurcating trees occur in the solution of the Sherrington-Kirkpatrick model. An important ingredient taken from these models is the exponential distribution of deep free-energies, which translate into a power-law distribution of the residence time within metastable `valleys'. These power law distributions have infinite mean in the spin-glass phase and this leads to the aging phenomenon. To each level of the tree are associated an overlap and the exponent of the time distribution. We solve these models for a finite (but arbitrary) number of levels and show that a two level tree accounts very well for many experimental observations (thermoremanent magnetisation, a.c susceptibility, second noise spectrum....). We introduce the idea that the deepest levels of the tree correspond to equilibrium dynamics whereas the upper levels correspond to aging. Temperature cycling experiments suggest that the borderline between the two is temperature dependent. The spin-glass transition corresponds to the temperature at which the uppermost level is put out of equilibrium but is subsequently followed by a sequence of (dynamical) phase transitions corresponding to non equilibrium dynamics within deeper and deeper levels. We tentatively try to relate this `tree' picture to the real space `droplet' model, and speculate on how the final description of spin-glasses might look like.Comment: 30 pages, RevTeX, 9 figures, available on request, report # 077 / SPEC / 199

Monte Carlo methods and applications for the nuclear shell model

The shell-model Monte Carlo (SMMC) technique transforms the traditional nuclear shell-model problem into a path-integral over auxiliary fields. We describe below the method and its applications to four physics issues: calculations of sdpf- shell nuclei, a discussion of electron-capture rates in pf-shell nuclei, exploration of pairing correlations in unstable nuclei, and level densities in rare earth systems.Comment: Proceedings of the Nuclear Structure '98 conference, Gatlinburg, TN, 10-15 August 199

Weak non-linear surface charging effects in electrolytic films

A simple model of soap films with nonionic surfactants stabilized by added electrolyte is studied. The model exhibits charge regularization due to the incorporation of a physical mechanism responsible for the formation of a surface charge. We use a Gaussian field theory in the film but the full non-linear surface terms which are then treated at a one-loop level by calculating the mean-field Poisson-Boltzmann solution and then the fluctuations about this solution. We carefully analyze the renormalization of the theory and apply it to a triple layer model for a thin film with Stern layer of thickness $h$. For this model we give expressions for the surface charge $\sigma(L)$ and the disjoining pressure $P_d(L)$ and show their dependence on the parameters. The influence of image charges naturally arise in the formalism and we show that predictions depend strongly on $h$ because of their effects. In particular, we show that the surface charge vanishes as the film thickness $L \to 0$. The fluctuation terms about this class of theories exhibit a Casimir-like attraction across the film and although this attraction is well known to be negligible compared with the mean-field component for thick films in the presence of electrolyte, in the model studied here these fluctuations also affect the surface charge regulation leading to a fluctuation component in the disjoining pressure which has the same behavior as the mean-field component even for large film thickness.Comment: 17 pages, 12 figures, latex sourc

Boundary Effects in the One Dimensional Coulomb Gas

We use the functional integral technique of Edwards and Lenard to solve the statistical mechanics of a one dimensional Coulomb gas with boundary interactions leading to surface charging. The theory examined is a one dimensional model for a soap film. Finite size effects and the phenomenon of charge regulation are studied. We also discuss the pressure of disjunction for such a film. Even in the absence of boundary potentials we find that the presence of a surface affects the physics in finite systems. In general we find that in the presence of a boundary potential the long distance disjoining pressure is positive but may become negative at closer interplane separations. This is in accordance with the attractive forces seen at close separations in colloidal and soap film experiments and with three dimensional calculations beyond mean field. Finally our exact results are compared with the predictions of the corresponding Poisson-Boltzmann theory which is often used in the context of colloidal and thin liquid film systems.Comment: 28 pages, LATEX2e, 11 figures, uses styles[12pt] resubmission because of minor corrections to tex

Strange Carers

The present comment focuses on the distinction between attachment as bond formation and expectations of availability and responsiveness (security) within attachment relationships. We enumerate key components of bonding and functions of carer secure base support. Our analysis has implications for design and suggests that robots are unlikely to serve effectively as sole carers. Even with robots as part-time carers, attachment-like bonds would likely focus on human carers. Similarly, although infants and children would certainly build expectations regarding the availability and responsiveness of robot carers, the quality of human care would probably be the determining influence on later development and competence. Notwithstanding their limitations of robots as attachment figures they have considerable potential to extend parental care and enrich infant exploration. The Sharkey’s paper and further consideration of robots as carers for infants, children, older adults, an

Renormalization of Drift and Diffusivity in Random Gradient Flows

We investigate the relationship between the effective diffusivity and effective drift of a particle moving in a random medium. The velocity of the particle combines a white noise diffusion process with a local drift term that depends linearly on the gradient of a gaussian random field with homogeneous statistics. The theoretical analysis is confirmed by numerical simulation. For the purely isotropic case the simulation, which measures the effective drift directly in a constant gradient background field, confirms the result previously obtained theoretically, that the effective diffusivity and effective drift are renormalized by the same factor from their local values. For this isotropic case we provide an intuitive explanation, based on a {\it spatial} average of local drift, for the renormalization of the effective drift parameter relative to its local value. We also investigate situations in which the isotropy is broken by the tensorial relationship of the local drift to the gradient of the random field. We find that the numerical simulation confirms a relatively simple renormalization group calculation for the effective diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep