Critical packing fraction at the phase separation transition in hard-core mixtures

In this paper, I relate the phase separation transition in binary hard-core mixtures in the limit of small size ratio to a bond percolation transition. It allows to estimate the critical packing fraction at the transition as a function of the size ratio and the composition of the mixture for different shapes of objects. The theoretical predictions are in excellent quantitative agreement with numerical simulations of binary parallel hard squares mixtures.Comment: 4 pages and 1 figur

Cluster algorithm for non-additive hard-core mixtures

In this paper, we present a cluster algorithm for the numerical simulations of non-additive hard-core mixtures. This algorithm allows one to simulate and equilibrate systems with a number of particles two orders of magnitude larger than previous simulations. The phase separation for symmetric binary mixtures is studied for different non-additvities as well as for the Widom-Rowlinson model (B. Widom and J. S. Rowlinson, J. Chem. Phys. 52, 1670 (1970)) in two and three dimensions. The critical densities are determined from finite size scaling. The critical exponents for all the non-additivities are consistent with the Ising universality class.Comment: 11 pages and 9 figures, to be published in J. Chem. Phys. Minor corrections and some references adde

On the Extension Behavior of Helicogenic Polypeptides

The force laws governing the extension behavior of homopolypeptides are obtained from a phenomenological free energy capable of describing the helix-coil transition. Just above the melting temperature of the free chains, T*, the plot of force, f, vs. end-to-end distance, R, exhibits two plateaus associated with coexistence of helical and coil domains. The lower plateau is due to tension induced onset of helix-coil transition. The higher plateau corresponds to the melting of the helices by overextension. Just below T* the f-R plot exhibits only the upper plateau. The f-R plots, the helical fraction, the number of domains and their polydispersity are calculated for two models: In one the helical domains are viewed as rigid rods while in the second they are treated as worm like chains.Comment: 18 pages, 10 figures, to be published in Macromolecule

On the Helix-Coil transition in grafted chains

The helix-coil transition is modified by grafting to a surface. This modification is studied for short peptides capable of forming $\alpha$-helices. Three factors are involved: (i) the grafting can induced change of the boundary free energy of the helical domain (ii) the van der Waals attraction between the helices and (iii) the crowding induced stretching of the coils. As a result the helix-coil transition acquires ``all or nothing'' characteristics. In addition the transition temperature is elevated and the transition itself sharpens as the grafting density increases.Comment: 6 pages, 1 figures, europhys.sty and euromacro.sty Submitted to Europhys. Let

Storage Capacity of the Tilinglike Learning Algorithm

The storage capacity of an incremental learning algorithm for the parity machine, the Tilinglike Learning Algorithm, is analytically determined in the limit of a large number of hidden perceptrons. Different learning rules for the simple perceptron are investigated. The usual Gardner-Derrida one leads to a storage capacity close to the upper bound, which is independent of the learning algorithm considered.Comment: Proceedings of the Conference Disordered and Complex Systems, King's College, London, July 2000. 6 pages, 1 figure, uses aipproc.st

Reply to A. Louis Comment

Reply to A. Louis Comment: Fluid-solid phase-separation in hard-sphere mixtures is unrelated to bond-percolationComment: Reply to a comment of PRL 82 p960 To be published in PR

Rigorous Bounds to Retarded Learning

We show that the lower bound to the critical fraction of data needed to infer (learn) the orientation of the anisotropy axis of a probability distribution, determined by Herschkowitz and Opper [Phys.Rev.Lett. 86, 2174 (2001)], is not always valid. If there is some structure in the data along the anisotropy axis, their analysis is incorrect, and learning is possible with much less data points.Comment: 1 page, 1 figure. Comment accepted for publication in Physical Review Letter

Polymer translocation through nano-pores in vibrating thin membranes

Polymer translocation is a promising strategy for the next-generation DNA sequencing technologies. The use of biological and synthetic nano-pores, however, still suffers from serious drawbacks. In particular, the width of the membrane layer can accommodate several bases at the same time, making difficult accurate sequencing applications. More recently, the use of graphene membranes has paved the way to new sequencing capabilities, with the possibility to measure transverse currents, among other advances. The reduced thickness of these new membranes poses new questions on the effect of deformability and vibrations of the membrane on the translocation process, two features which are not taken into account in the well-established theoretical frameworks. Here, we make a first step forward in this direction. We report numerical simulation work on a model system simple enough to allow gathering significant insight on the effect of these features on the average translocation time, with appropriate statistical significance. We have found that the interplay between thermal fluctuations and the deformability properties of the nano-pore play a crucial role in determining the process. We conclude by discussing new directions for further work

Analysis of the history of landscape changes over a period of 200 years. How can we predict past landscape pattern scenario and the impact on habitat diversity?

Popular Science The decline of plant species biodiversity is closely linked with changes in rural landscape both in time and space. During the last 200 years rural landscape has undergone dramatic changes. Before 1900 rural landscape was mainly composed of grassland and wooded area with grazing cattle. Arable lands at this time were a small part of the landscape and were located around villages. After 1900 most of the grasslands were transformed into arable land or forest. Unfortunately our knowledge of the rural landscape around 1800 is not very good because of the lack of spatial information. However there is in Sweden a good coverage of historical maps but they only covered small area around villages. In rural landscape there are three key habitats: semi-natural grassland, wetlands and deciduous forest. The aim of study was to create a model predicting the landscape as it was in 1800. We used land use historical maps from 1800 as well as land use maps from 1900, soil map and elevation map. In order to create the model we tested the relationship between soil, the slope of the terrain, the distance to settlement and the distance to water and the three habitats. This has been done the Geographical Information System software ArcGIS and the statistical open source software R. The results showed that between 1800 and 1900 a high diversity of habitats became lost especially for semi-natural grasslands. The small amount of data and the high diversity of habitats make it difficult for the construction of the model. However the accuracy for the 1800 was good and the accuracy for the accuracy for the 1900 model was very good. The model for 1900 can be used in a larger extent than the study area. With such a model it is possible to have a better understanding of rural landscape dynamics in time and space. Conservationist could use this model for pinpointing areas of interest for conservation or restoration.Biodiversity, and in particular plant species diversity decline is linked to landscape fragmentation and habitats loss due to land use transformations but also to decreasing dispersal vectors in the landscape. During the last 200 years the landscape has undergone dramatic changes in its structure and its habitats diversity. Among the different habitats that constitute rural landscape there are three key habitats that are important for plant species biodiversity: Semi-natural grasslands, deciduous forests and wetlands. The aim of this study is first to create a model that predict the landscape as it was in 1800 using land cover data from 1900 and variables such as soil properties, distance to settlements, distance to water and slope percentage using logistic regression and GIS technology. Then land cover changes are also tested with soil properties, distance to settlement, distance to water and slope percentage. Thus it is possible to have a better understanding of the factor that influence changes from one land cover to another. The results show that between 1800 and 1900, a diversity of habitats has become lost, especially semi-natural grasslands (semi-natural grasslands on fine soil) and wetlands (fen and wooded mire). The small amount of data and the high diversity of habitats made difficult the construction of an accurate model for 1800. In 1800 it was likely too find any type of habitats on any type of soil or terrain conditions. The important finding of this study is that before 1850 there were a high diversity of semi-natural grasslands compare with today as well as a high diversity of wetlands. The model for 1900 has a very good accuracy and could be used for modelling the landscape in a larger extent than the study area. Such a model can give more opportunities for studying landscape history and therefore get more data for studying the fragmentation of the landscape, which play an important role in the maintenance of a high plant species diversity. Conservationists could use this model for decision-making regarding the restoration of habita