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B Groh

Publications and source records attributed to B Groh.

12 recordsLinked to original sources
PubMed2001

Crystal structures and freezing of dipolar fluids.

We investigate the crystal structure of classical systems of spherical particles with an embedded point dipole at T=0. The ferroelectric ground state energy is calculated using generalizations of the Ewald summation technique. Due to the reduced symmetry compared to the nonpolar case the crystals are never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar) interaction three phases are found upon increasing the dipole moment: hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even richer phase diagram arises for dipolar soft spheres with a purely repulsive inverse power law potential approximately r(-n). A crossover between qualitatively different sequences of phases occurs near the exponent n=12. The results are applicable to electro- and magnetorheological fluids. In addition to the exact ground state analysis we study freezing of the Stockmayer fluid by density-functional theory.

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PubMed2000

Hard-sphere solids near close packing: testing theories for crystallization

The freezing transition of hard spheres has been well described by various versions of density-functional theory (DFT). These theories should possess the close-packed crystal as a special limit, which represents an extreme testing ground for the quality of such liquid-state based theories. We therefore study the predictions of DFT for the structure and thermodynamics of the hard-sphere crystal in this limit. We examine the Ramakrishnan-Yussouff (RY) approximation and two variants of the fundamental-measure theory (FMT) developed by Rosenfeld and co-workers. We allow for general shapes of the density peaks, going beyond the common Gaussian approximation. In all cases we find that upon approaching close packing, the peak width vanishes proportionally to the free distance a between the particles and the free energy depends logarithmically on a. However, different peak shapes and next-to-leading contributions to the free energy result from the different approximate functionals. For the RY theory, within the Gaussian approximation, we establish that the crystalline solutions form a closed loop with a stable and an unstable branch both connected to the close-packing point at a=0, consistent with the absence of a liquid-solid spinodal. That version of FMT that has previously been applied to freezing, predicts asymptotically steplike density profiles confined to the cells of self-consistent cell theory. But a recently suggested improved version which employs tensor weighted densities yields wider and almost Gaussian peaks that are shown to be in very good agreement with computer simulations.

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PubMed2000

Density-functional theory for vacancies in hard-sphere crystals

The equilibrium vacancy concentration in solids can be computed from density-functional theory (DFT) if allowance is made for density profiles with less than one particle per lattice site. For the fundamental-measure theory (FMT), this approach predicts reasonably small vacancy concentrations in hard sphere crystals, in contrast to earlier DFTs. Using an asymptotic analysis of the FMT functional, it is shown that the number of vacancies depends exponentially on the distance to the close packing density, as expected from heuristic arguments. The prefactor of the exponential is calculated for three recently suggested variants of the theory, using density profiles obtained from a quasifree minimization. Extrapolation of the asymptotic behavior to the melting density yields good agreement with other estimates and computer simulation results.

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PubMed1999

Vesicles in solutions of hard rods.

The surface free energy of ideal hard rods near curved hard surfaces is determined to second order in curvature for surfaces of general shapes. In accordance with previous results for spherical and cylindrical surfaces it is found that this quantity is nonanalytical when one of the principal curvatures changes signs. This prohibits writing it in the common Helfrich form. It is shown that the nonanalytical terms are the same for any aspect ratio of the rods. These results are used to find the equilibrium shape of vesicles immersed in solutions of rodlike (colloidal) particles. The presence of the particles induces a change in the equilibrium shape and a shift of the prolate-oblate transition in the vesicle phase diagram, which are calculated within the framework of the spontaneous curvature model. As a consequence of the special form of the energy contribution due to the rods, these changes cannot be accounted for by a simple rescaling of the elastic constants of the vesicle as for solutions of spherical colloids or polymers.

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PubMed1999

Why all crystals need not be bcc: symmetry breaking at the liquid-solid transition revisited.

Alexander and McTague [Phys.Rev. Lett. 41, 702 (1978)] argued that if there is a spinodal point associated with the liquid-solid transition in a fluid of spherically symmetric particles, the bcc phase will be uniquely favored as the only accessible symmetry breaking structure that forms a regular three-dimensional lattice. By reconsidering their analysis in the framework of density-functional theory, we show that at a liquid-solid spinodal in fact many other solid stuctures also are simultaneously accessible, among them the fcc structure. Nevertheless, the bcc structure is still shown to be special, as, independent of the details of the interaction, the free energy of the unstable bcc phase close to the spinodal is always lower than that of the other solidlike structures. We illustrate our general results by explicit calculations on a toy model, the "Onsager solid." This simple model also indicates that the ultimately stable crystal phase, which, as usual for sufficiently steep repulsive forces, turns out to be fcc, is dictated by properties of the free energy that cannot be obtained perturbatively starting from the spinodal point.

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