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We study the adsorption-desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces-are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye-Huckel approximation is often not feasible and the nonlinear Poisson-Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson-Boltzmann equation is smaller than the Debye-Huckel result, such that the required critical surface charge density for polyelectrolyte adsorption sigma(c) increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical-chemical and biophysical systems.
We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption σc increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems.
We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption σc increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems.
Since more than 15 years, the Cluster mission passes through Earth's radiation belts at least once every 2 days for several hours, measuring the electron intensity at energies from 30 to 400 keV. These data have previously been considered not usable due to contamination caused by penetrating energetic particles (protons at >100 keV and electrons at >400 keV). In this study, we assess the level of distortion of energetic electron spectra from the Research with Adaptive Particle Imaging Detector (RAPID)/Imaging Electron Spectrometer (IES) detector, determining the efficiency of its shielding. We base our assessment on the analysis of experimental data and a radiation transport code (Geant4). In simulations, we use the incident particle energy distribution of the AE9/AP9 radiation belt models. We identify the Roederer L values, L⋆, and energy channels that should be used with caution: at 3≤L⋆≤4, all energy channels (40–400 keV) are contaminated by protons (≃230 to 630 keV and >600 MeV); at L⋆≃1 and 4–6, the energy channels at 95–400 keV are contaminated by high-energy electrons (>400 keV). Comparison of the data with electron and proton observations from RBSP/MagEIS indicates that the subtraction of proton fluxes at energies ≃ 230–630 keV from the IES electron data adequately removes the proton contamination. We demonstrate the usefulness of the corrected data for scientific applications.
Consequences of twenty-first-century policy for multi-millennial climate
and sea-level change
(2016)
Most of the policy debate surrounding the actions needed to mitigate and adapt to anthropogenic climate change has been framed by observations of the past 150 years as well as climate and sea-level projections for the twenty-first century. The focus on this 250-year window, however, obscures some of the most profound problems associated with climate change. Here, we argue that the twentieth and twenty-first centuries, a period during which the overwhelming majority of human-caused carbon emissions are likely to occur, need to be placed into a long-term context that includes the past 20 millennia, when the last Ice Age ended and human civilization developed, and the next ten millennia, over which time the projected impacts of anthropogenic climate change will grow and persist. This long-term perspective illustrates that policy decisions made in the next few years to decades will have profound impacts on global climate, ecosystems and human societies - not just for this century, but for the next ten millennia and beyond.
Traditional economic theory could not explain, much less predict, the near collapse of the financial system and its long-lasting effects on the global economy. Since the 2008 crisis, there has been increasing interest in using ideas from complexity theory to make sense of economic and financial markets. Concepts, such as tipping points, networks, contagion, feedback, and resilience have entered the financial and regulatory lexicon, but actual use of complexity models and results remains at an early stage. Recent insights and techniques offer potential for better monitoring and management of highly interconnected economic and financial systems and, thus, may help anticipate and manage future crises.
We perform experiments and phase model simulations with a ring network of oscillatory electrochemical reactions to explore the effect of random connections and nonisochronicity of the interactions on the pattern formation. A few additional links facilitate the emergence of the fully synchronized state. With larger nonisochronicity, complex rotating waves or persistent irregular phase dynamics can derail the convergence to global synchronization. The observed long transients of irregular phase dynamics exemplify the possibility of a sudden onset of hypersynchronous behavior without any external stimulus or network reorganization.
In this Comment, we review the results of pattern formation in a reaction-diffusion-advection system following the kinetics of the Gray-Scott model. A recent paper by Das [Phys. Rev. E 92, 052914 (2015)] shows that spatiotemporal chaos of the intermittency type can disappear as the advective flow is increased. This study, however, refers to a single point in the space of kinetic parameters of the original Gray-Scott model. Here we show that the wealth of patterns increases substantially as some of these parameters are changed. In addition to spatiotemporal intermittency, defect-mediated turbulence can also be found. In all cases, however, the chaotic behavior is seen to disappear as the advective flow is increased, following a scenario similar to what was reported in our earlier work [I. Berenstein and C. Beta, Phys. Rev. E 86, 056205 (2012)] as well as by Das. We also point out that a similar phenomenon can be found in other reaction-diffusion-advection models, such as the Oregonator model for the Belousov-Zhabotinsky reaction under flow conditions.
We consider a generalized diffusion equation in two dimensions for modeling diffusion on a comb-like structures. We analyze the probability distribution functions and we derive the mean squared displacement in x and y directions. Different forms of the memory kernels (Dirac delta, power-law, and distributed order) are considered. It is shown that anomalous diffusion may occur along both x and y directions. Ultraslow diffusion and some more general diffusive processes are observed as well. We give the corresponding continuous time random walk model for the considered two dimensional diffusion-like equation on a comb, and we derive the probability distribution functions which subordinate the process governed by this equation to the Wiener process.
Large-scale transitions in societies are associated with both individual behavioural change and restructuring of the social network. These two factors have often been considered independently, yet recent advances in social network research challenge this view. Here we show that common features of societal marginalization and clustering emerge naturally during transitions in a co-evolutionary adaptive network model. This is achieved by explicitly considering the interplay between individual interaction and a dynamic network structure in behavioural selection. We exemplify this mechanism by simulating how smoking behaviour and the network structure get reconfigured by changing social norms. Our results are consistent with empirical findings: The prevalence of smoking was reduced, remaining smokers were preferentially connected among each other and formed increasingly marginalized clusters. We propose that self-amplifying feedbacks between individual behaviour and dynamic restructuring of the network are main drivers of the transition. This generative mechanism for co-evolution of individual behaviour and social network structure may apply to a wide range of examples beyond smoking.
The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.
Recent advances in high-throughput sequencing experiments and their theoretical descriptions have determined fast dynamics of the "chromatin and epigenetics" field, with new concepts appearing at high rate. This field includes but is not limited to the study of DNA-protein-RNA interactions, chromatin packing properties at different scales, regulation of gene expression and protein trafficking in the cell nucleus, binding site search in the crowded chromatin environment and modulation of physical interactions by covalent chemical modifications of the binding partners. The current special issue does not pretend for the full coverage of the field, but it rather aims to capture its development and provide a snapshot of the most recent concepts and approaches. Eighteen open-access articles comprising this issue provide a delicate balance between current theoretical and experimental biophysical approaches to uncover chromatin structure and understand epigenetic regulation, allowing free flow of new ideas and preliminary results.
The Milky Way is only one out of billions of galaxies in the universe. However, it is a special galaxy because it allows to explore the main mechanisms involved in its evolution and formation history by unpicking the system star-by-star. Especially, the chemical fingerprints of its stars provide clues and evidence of past events in the Galaxy’s lifetime. These information help not only to decipher the current structure and building blocks of the Milky Way, but to learn more about the general formation process of galaxies.
In the past decade a multitude of stellar spectroscopic Galactic surveys have scanned millions of stars far beyond the rim of the solar neighbourhood. The obtained spectroscopic information provide unprecedented insights to the chemo-dynamics of the Milky Way. In addition analytic models and numerical simulations of the Milky Way provide necessary descriptions and predictions suited for comparison with observations in order to decode the physical properties that underlie the complex system of the Galaxy.
In the thesis various approaches are taken to connect modern theoretical modelling of galaxy formation and evolution with observations from Galactic stellar surveys. With its focus on the chemo-kinematics of the Galactic disk this work aims to determine new observational constraints on the formation of the Milky Way providing also proper comparisons with two different models. These are the population synthesis model TRILEGAL based on analytical distribution functions, which aims to simulate the number and distribution of stars in the Milky Way and its different components, and a hybrid model (MCM) that combines an N-body simulation of a Milky Way like galaxy in the cosmological framework with a semi-analytic chemical evolution model for the Milky Way. The major observational data sets in use come from two surveys, namely the “Radial Velocity Experiment” (RAVE) and the “Sloan Extension for Galactic Understanding and Exploration” (SEGUE).
In the first approach the chemo-kinematic properties of the thin and thick disk of the Galaxy as traced by a selection of about 20000 SEGUE G-dwarf stars are directly compared to the predictions by the MCM model. As a necessary condition for this, SEGUE's selection function and its survey volume are evaluated in detail to correct the spectroscopic observations for their survey specific selection biases. Also, based on a Bayesian method spectro-photometric distances with uncertainties below 15% are computed for the selection of SEGUE G-dwarfs that are studied up to a distance of 3 kpc from the Sun.
For the second approach two synthetic versions of the SEGUE survey are generated based on the above models. The obtained synthetic stellar catalogues are then used to create mock samples best resembling the compiled sample of observed SEGUE G-dwarfs. Generally, mock samples are not only ideal to compare predictions from various models. They also allow validation of the models' quality and improvement as with this work could be especially achieved for TRILEGAL. While TRILEGAL reproduces the statistical properties of the thin and thick disk as seen in the observations, the MCM model has shown to be more suitable in reproducing many chemo-kinematic correlations as revealed by the SEGUE stars. However, evidence has been found that the MCM model may be missing a stellar component with the properties of the thick disk that the observations clearly show. While the SEGUE stars do indicate a thin-thick dichotomy of the stellar Galactic disk in agreement with other spectroscopic stellar studies, no sign for a distinct metal-poor disk is seen in the MCM model.
Usually stellar spectroscopic surveys are limited to a certain volume around the Sun covering different regions of the Galaxy’s disk. This often prevents to obtain a global view on the chemo-dynamics of the Galactic disk. Hence, a suitable combination of stellar samples from independent surveys is not only useful for the verification of results but it also helps to complete the picture of the Milky Way. Therefore, the thesis closes with a comparison of the SEGUE G-dwarfs and a sample of RAVE giants. The comparison reveals that the chemo-kinematic relations agree in disk regions where the samples of both surveys show a similar number of stars. For those parts of the survey volumes where one of the surveys lacks statistics they beautifully complement each other. This demonstrates that the comparison of theoretical models on the one side, and the combined observational data gathered by multiple surveys on the other side, are key ingredients to understand and disentangle the structure and formation history of the Milky Way.
The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.
Context. Most theoretical investigations of dust charging processes in space have treated the current balance condition as independent of grain size. However, for small grains, since they are often observed in space environments, a dependence on grain size is expected owing to secondary electron emission (SEE). Here, by the term "small" we mean a particle size comparable to the typical penetration depth for given primary electron energy. The results are relevant for the dynamics of small, charged dust particles emitted by the volcanic moon Io, which forms the Jovian dust streams. Aims. We revise the theory of charging of small (submicron sized) micrometeoroids to take into account a high production of secondary electrons for small grains immersed in an isotropic flux of electrons. We apply our model to obtain an improved estimate for the charge of the dust streams leaving the Jovian system, detected by several spacecraft. Methods. We apply a continuum model to describe the penetration of primary electrons in a grain and the emission of secondary electrons along the path. Averaging over an isotropic flux of primaries, we derive a new expression for the secondary electron yield, which can be used to express the secondary electron current on a grain. Results. For the Jupiter plasma environment we derive the surface potential of grains composed of NaCl (believed to be the major constituent of Jovian dust stream particles) or silicates. For small particles, the potential depends on grain size and the secondary electron current induces a sensitivity to material properties. As a result of the small particle effect, the estimates for the charging times and for the fractional charge fluctuations of NaCl grains obtained using our general approach to SEE give results qualitatively different from the analogous estimates derived from the traditional approach to SEE. We find that for the charging environment considered in this paper field emission does not limit the charging of NaCl grains.
Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced pi-pi-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.
Time-delayed collection field experiments are applied to planar organometal halide perovskite (CH3NH3PbI3) based solar cells to investigate charge carrier recombination in a fully working solar cell at the nanosecond to microsecond time scale. Recombination of mobile (extractable) charges is shown to follow second-order recombination dynamics for all fluences and time scales tested. Most importantly, the bimolecular recombination coefficient is found to be time-dependent, with an initial value of ca. 10(-9) cm(3)/s and a progressive reduction within the first tens of nanoseconds. Comparison to the prototypical organic bulk heterojunction device PTB7:PC71BM yields important differences with regard to the mechanism and time scale of free carrier recombination. (C) 2016 AIP Publishing LLC.
We present a nanostructured device that functions as photoacoustic hard x-ray switch. The device is triggered by femtosecond laser pulses and allows for temporal gating of hard x-rays on picosecond (ps) timescales. It may be used for pulse picking or even pulse shortening in 3rd generation synchrotron sources. Previous approaches mainly suffered from insufficient switching contrasts due to excitation-induced thermal distortions. We present a new approach where thermal distortions are spatially separated from the functional switching layers in the structure. Our measurements yield a switching contrast of 14, which is sufficient for efficient hard x-ray pulse shortening. The optimized structure also allows for utilizing the switch at high repetition rates of up to 208 kHz. Published by AIP Publishing.