2,352 research outputs found
Phase Separation of Saturated and Mono-unsaturated Lipids as determined from a Microscopic Model
A molecular model is proposed of a bilayer consisting of fully saturated DPPC
and mono unsaturated DOPC. The model not only encompasses the constant density
within the hydrophobic core of the bilayer, but also the tendency of chain
segments to align. It is solved within self-consistent field theory. A model
bilayer of DPPC undergoes a main chain transition to a gel phase, while a
bilayer of DOPC does not do so above zero degrees centigrade because of the
double bond which disrupts order. We examine structural and thermodynamic
properties of these membranes and find our results in reasonable accord with
experiment. In particular, order-parameter profiles are in good agreement with
NMR experiments. A phase diagram is obtained for mixtures of these lipids in a
membrane at zero tension. The system undergoes phase separation below the
main-chain transition temperature of the saturated lipid. Extensions to the
ternary DPPC, DOPC, and cholesterol system are outlined.Comment: 29 pages, 4 figures, 1 table. revised versio
The strong Atiyah conjecture for right-angled Artin and Coxeter groups
We prove the strong Atiyah conjecture for right-angled Artin groups and
right-angled Coxeter groups. More generally, we prove it for groups which are
certain finite extensions or elementary amenable extensions of such groups.Comment: Minor change
New mechanism of membrane fusion
We have carried out Monte Carlo simulation of the fusion of bilayers of
single chain amphiphiles which show phase behavior similar to that of
biological lipids. The fusion mechanism we observe is very different from the
``stalk'' hypothesis. Stalks do form on the first stage of fusion, but they do
not grow radially to form a hemifused state. Instead, stalk formation
destabilizes the membranes and results in hole formation in the vicinity of the
stalks. When holes in each bilayer nucleate spontaneously next to the same
stalk, an incomplete fusion pore is formed. The fusion process is completed by
propagation of the initial connection, the stalk, along the edges of the
aligned holes.Comment: 4 pages, 3 figure
Wheat forecast economics effect study
A model to assess the value of improved information regarding the inventories, productions, exports, and imports of crop on a worldwide basis is discussed. A previously proposed model is interpreted in a stochastic control setting and the underlying assumptions of the model are revealed. In solving the stochastic optimization problem, the Markov programming approach is much more powerful and exact as compared to the dynamic programming-simulation approach of the original model. The convergence of a dual variable Markov programming algorithm is shown to be fast and efficient. A computer program for the general model of multicountry-multiperiod is developed. As an example, the case of one country-two periods is treated and the results are presented in detail. A comparison with the original model results reveals certain interesting aspects of the algorithms and the dependence of the value of information on the incremental cost function
Quantum interface unbinding transitions
We consider interfacial phenomena accompanying bulk quantum phase transitions
in presence of surface fields. On general grounds we argue that the surface
contribution to the system free energy involves a line of singularities
characteristic of an interfacial phase transition, occurring below the bulk
transition temperature T_c down to T=0. This implies the occurrence of an
interfacial quantum critical regime extending into finite temperatures and
located within the portion of the phase diagram where the bulk is ordered. Even
in situations, where the bulk order sets in discontinuously at T=0, the
system's behavior at the boundary may be controlled by a divergent length scale
if the tricritical temperature is sufficiently low. Relying on an effective
interfacial model we compute the surface phase diagram in bulk spatial
dimensionality and extract the values of the exponents describing the
interfacial singularities in
Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy
Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems. Optically driven spin transfer is the fastest process to manipulate magnetism. Here, the authors show that this process emerges as the dominant mechanism in femtosecond spin dynamics enabling to the engineering of functional magnetic systems for future all optical technologies
Kinetics of Phase Separation in Thin Films: Simulations for the Diffusive Case
We study the diffusion-driven kinetics of phase separation of a symmetric
binary mixture (AB), confined in a thin-film geometry between two parallel
walls. We consider cases where (a) both walls preferentially attract the same
component (A), and (b) one wall attracts A and the other wall attracts B (with
the same strength). We focus on the interplay of phase separation and wetting
at the walls, which is referred to as {\it surface-directed spinodal
decomposition} (SDSD). The formation of SDSD waves at the two surfaces, with
wave-vectors oriented perpendicular to them, often results in a metastable
layered state (also referred to as ``stratified morphology''). This state is
reminiscent of the situation where the thin film is still in the one-phase
region but the surfaces are completely wet, and hence coated with thick wetting
layers. This metastable state decays by spinodal fluctuations and crosses over
to an asymptotic growth regime characterized by the lateral coarsening of
pancake-like domains. These pancakes may or may not be coated by precursors of
wetting layers. We use Langevin simulations to study this crossover and the
growth kinetics in the asymptotic coarsening regime.Comment: 39 pages, 19 figures, submitted to Phys.Rev.
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