Formation of Polymorphic Cluster Phases for Purely Repulsive Soft Spheres

We present results from density functional theory and computer simulations that unambiguously predict the occurrence of first-order freezing transitions for a large class of ultrasoft model systems into cluster crystals. The clusters consist of fully overlapping particles and arise without the existence of attractive forces. The number of particles participating in a cluster scales linearly with density, therefore the crystals feature density-independent lattice constants. Clustering is accompanied by polymorphic bcc-fcc transitions, with fcc being the stable phase at high densities.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let

Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory

We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive-definite pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio L = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor changes in structure of pape

Genetic algorithms in condensed matter theory

In dieser Arbeit wurde ein neuer Ansatz zur Vorhersage von stabilen Kristallstrukturen, die beim Gefrieren auftreten, untersucht. Diese Methode basiert auf der Verwendung von genetischen Algorithmen, die eine Suche nach dem globalen Minimum der freien Energie ermöglichen.Mit Hilfe dieser Methode war es möglich, das aus der Literatur bekannte Phasendigramm von neutralen Sternpolymeren zu verbessern und das Phasendiagramm für ionische Mikrogele neu zu berechnen, das ungwöhnliche Kristallstrukturen wie hexagonale und trigonale Gitter enthält.Zusätzlich wurden parallel geschichtete zweidimensionale Gitter, sowie Systeme, die bei hohen Dichten sog.cluster bilden, studiert.Das neue Verfahren erwies sich als effizient und flexibel und könnte mit entsprechenden Modifikationen für eine Vielzahl von ähnlichen Problemen in der Theorie der kondensierten Materie verwenden werden.In this work a new concept to predict equilibrium crystal structures in freezing processes has been introduced.This approach is based on a genetic algorithm that allows an unbiased search through the parameter space of the lattices.It was possible to improve the already published phase diagram of neutral star polymers, and to calculate the phase diagram of ionic microgels that includes unusual structures such as hexagonal and trigonal lattices. Additionally, layers of two-dimensional lattices and system that freeze into clustered crystals were studied.The new method proved its versatility and power for all cases it was applied to and could be used for numerous similar problems in condensed matter physics.13

Investigation of phase separation in multiblock copolymers consisting of polysulfone and a liquid crystalline polymer

Multiblock copolymers (MBCPs) consisting of polysulfone (PSU) segments and segments of the liquid crystalline poly(ethyleneterephthalate-co-oxybenzoate) (PET/HBA) form rather complex morphologies. Scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) investigations of two block copolymers with significantly different block molecular weights proved the existence of both macro-and microphase separation. These morphologies, existing on different length scales, were found to be superimposed in the samples. A suitable fractionation procedure was used to suppress macrophase separation. Then, it was found that microphase separation is controlled by the segment molecular weights

Cluster-forming systems of ultrasoft repulsive particles: statics and dynamics

6 pages.-- PACS nrs.: 64.70.Dv; 82.30.Nr; 61.20.Ja; 82.70.Dd.-- Special issue of Computer Physics Communications based on the Conference on Computational Physics 2007 - CCP 2007.In this paper, we present a short review as well as novel results on a recently established counterintuitive phenomenon of cluster aggregation of particles that interact via purely repulsive interactions. We demonstrate how repulsion can lead to clustering provided that the interaction allows full particle overlaps and also displays negative Fourier components. The formation of crystals with average site occupancy that scales linearly with density, and which is in general a noninteger number, is demonstrated by means of density functional theory and Monte Carlo simulations. Noninteger average occupancy of lattice sites is sustained by incessant hopping processes in the crystal, which we observe and quantitatively analyze by means of Molecular Dynamics simulations. The simultaneous presence of relaxative dynamics on the single-particle and arrested dynamics on the collective level leads to an unusual interplay between the two and to novel scenarios of dynamical arrest in the cluster crystal.C.N.L. acknowledges partial support from the DFG through the SFB-TR6, Project Section C3. B.M.M., D.G., and G.K. acknowledge financial support by the Österreichische Forschungsfond (FWF) under Project Nos. P17823-N08 and P19890-N16, and the Hochschuljubiläumsstiftung der Stadt Wien under Project No. 1080/2002. Further, B.M.M. thanks the Österreichische Forschungsgemeinschaft for financial support. This work was partly carried out under the HPC-EUROPA Project (RII3-CT-2003-506079), with the support of the European Community—Research Infrastructure Action under the FP6 "Structuring the European Research Area" Programme. A.J.M. acknowledges financial support from SoftComp (NMP3-CT-2004-502235) and DIPC-Spain.Peer reviewe