Glass transition in Ultrathin Polymer Films : A Thermal Expansion Study

Glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of glass transition temperature (Tg) as the thickness of the polymer film reduces below the radius of gyration but the change in the average Tg was found to be very small. Existence of reversible negative and positive thermal expansion below and above Tg increased the sensitivity of our thickness measurements performed using energy dispersive x-ray reflectivity. A simple model of Tg variation as a function of depth expected from sliding motion could explain the results. We observe clear glass transition even for 4 nm polystyrene film that was predicted to be absent from ellipsometry measurements of thicker films.Comment: 11 pages, 5 figure

Fluid-fluid phase separation in hard spheres with a bimodal size distribution

The effect of polydispersity on the phase behaviour of hard spheres is examined using a moment projection method. It is found that the Boublik-Mansoori-Carnahan-Starling-Leland equation of state shows a spinodal instability for a bimodal distribution if the large spheres are sufficiently polydisperse, and if there is sufficient disparity in mean size between the small and large spheres. The spinodal instability direction points to the appearance of a very dense phase of large spheres.Comment: 7 pages, 3 figures, moderately REVISED following referees' comments (original was 4 pages, 3 postscript figures

Viscous instabilities in flowing foams: A Cellular Potts Model approach

The Cellular Potts Model (CPM) succesfully simulates drainage and shear in foams. Here we use the CPM to investigate instabilities due to the flow of a single large bubble in a dry, monodisperse two-dimensional flowing foam. As in experiments in a Hele-Shaw cell, above a threshold velocity the large bubble moves faster than the mean flow. Our simulations reproduce analytical and experimental predictions for the velocity threshold and the relative velocity of the large bubble, demonstrating the utility of the CPM in foam rheology studies.Comment: 10 pages, 3 figures. Replaced with revised version accepted for publication in JSTA

Quantum cosmology with a curvature squared action

The correct quantum description for a curvature squared term in the action can be obtained by casting the action in the canonical form with the introduction of a variable which is the negative of the first derivative of the field variable appearing in the action, only after removing the total derivative terms from the action. We present the Wheeler-DeWitt equation and obtain the expression for the probability density and current density from the equation of continuity. Furthermore, in the weak energy limit we obtain the classical Einstein equation. Finally we present a solution of the wave equation.Comment: 8 pages, revte

Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study

The dynamical behavior of the magnetism of diluted magnetic semiconductors (DMS) has been investigated by means of atomistic spin dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low concentration limit, although all simulations are done for 5% Mn-doped GaAs with various concentrations of As antisite defects. The magnetization curve, $M(T)$, and the Curie temperature $T_C$ have been calculated, and are found to be in good correspondence to results from Monte Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium behavior of the magnetic pair correlation function have been extracted. The dynamics of DMS systems reveals a substantial short ranged magnetic order even at temperatures at or above the ordering temperature, with a non-vanishing pair correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short ranged anti-ferromagnetic coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin-correlation develops very fast ($\sim$ 1ps) extending up to 15 atomic shells. Above $\sim$ 10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets like bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behaviour, for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet like bcc Fe.Comment: 24 pages, 15 figure

Extraction of density profile for near perfect multilayers

A simple inversion scheme, based on Born approximation, to determine the electron density profile of near perfect multilayers from specular X-ray reflectivity data has been presented. This scheme is useful for semiconductor multilayers and other thin films, which are grown almost according to the designed parameters. We also indicate the possibility of separating out the contribution of interdiffusion and roughness in electron density profiles of interfaces by utilizing information obtained from the analysis of diffuse scattering data. The extracted compositional profile was used to calculate structural details of epitaxial films along the growth direction. Simulated and metal organic vapor phase epitaxy grown InP/InxGa1−xAs/InP quantum-well systems have been used to demonstrate this scheme

Magnetic properties of 3d-impurities substituted in GaAs

We have calculated the magnetic properties of substituted 3d-impurities (Cr-Ni) in a GaAs host by means of first principles electronic structure calculations. We provide a novel model explaining the ferromagnetic long rang order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism is shown to be due to delocalized spin-uncompensated As dangling bond electrons. Besides the quantitative prediction of the magnetic moments, our model provides an understanding of the halfmetallicity, and the raise of the critical temperature with the impurity concentration

The evolution of rotating very massive stars with LMC composition

We present a dense model grid with tailored input chemical composition appropriate for the Large Magellanic Cloud. We use a one-dimensional hydrodynamic stellar evolution code, which accounts for rotation, transport of angular momentum by magnetic fields, and stellar wind mass loss to compute our detailed models. We calculate stellar evolution models with initial masses of 70-500 Msun and with initial surface rotational velocities of 0-550 km/s, covering the core-hydrogen burning phase of evolution. We find our rapid rotators to be strongly influenced by rotationally induced mixing of helium, with quasi-chemically homogeneous evolution occurring for the fastest rotating models. Above 160 Msun, homogeneous evolution is also established through mass loss, producing pure helium stars at core hydrogen exhaustion independent of the initial rotation rate. Surface nitrogen enrichment is also found for slower rotators, even for stars that lose only a small fraction of their initial mass. For models above 150 MZAMS, and for models in the whole considered mass range later on, we find a considerable envelope inflation due to the proximity of these models to their Eddington limit. This leads to a maximum zero-age main sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of stars in the mass range 50-100 Msun to the regime of luminous blue variables in the HR diagram with high internal Eddington factors. Inflation also leads to decreasing surface temperatures during the chemically homogeneous evolution of stars above 180 Msun. The cool surface temperatures due to the envelope inflation in our models lead to an enhanced mass loss, which prevents stars at LMC metallicity from evolving into pair-instability supernovae. The corresponding spin-down will also prevent very massive LMC stars to produce long-duration gamma-ray bursts, which might, however, originate from lower masses.Comment: 21 pages, 25 figure

Stability and Stabilization of Relative Equilibria of Dumbbell Bodies in Central Gravity

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77368/1/AIAA-10546-792.pd

Evidence for Hysteretic Substrate Channeling in the Proline Dehydrogenaseand ∆\u3csup\u3e1\u3c/sup\u3e-Pyrroline-5-carboxylate Dehydrogenase Coupled Reaction of Proline UtilizationA(PutA)

Background: PutA from Escherichia coli is a bifunctional enzyme and transcriptional repressor in proline catabolism. Results: Steady-state and transient kinetic data revealed a mechanism in which the two enzymatic reactions are coupled by an activation step. Conclusion: Substrate channeling in PutA exhibits hysteretic behavior. Significance: This is the first kinetic model of bi-enzyme activity in PutA and reveals a novel mechanism of channeling activation