Monoclinic and triclinic phases in higher-order Devonshire theory

Devonshire theory provides a successful phenomenological description of many cubic perovskite ferroelectrics such as BaTiO3 via a sixth-order expansion of the free energy in the polar order parameter. However, the recent discovery of a novel monoclinic ferroelectric phase in the PZT system by Noheda et al. (Appl. Phys. Lett. 74, 2059 (1999)) poses a challenge to this theory. Here, we confirm that the sixth-order Devonshire theory cannot support a monoclinic phase, and consider extensions of the theory to higher orders. We show that an eighth-order theory allows for three kinds of equilibrium phases in which the polarization is confined not to a symmetry axis but to a symmetry plane. One of these phases provides a natural description of the newly observed monoclinic phase. Moreover, the theory makes testable predictions about the nature of the phase boundaries between monoclinic, tetragonal, and rhombohedral phases. A ferroelectric phase of the lowest (triclinic) symmetry type, in which the polarization is not constrained by symmetry, does not emerge until the Devonshire theory is carried to twelfth order. A topological analysis of the critical points of the free-energy surface facilitates the discussion of the phase transition sequences.Comment: 10 pages, with 5 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/dv_pzt/index.htm

Nonuniqueness in a minimal model for cell motility

Two–phase flow models have been used previously to model cell motility, however these have rapidly become very complicated, including many physical processes, and are opaque. Here we demonstrate that even the simplest one–dimensional, two–phase, poroviscous, reactive flow model displays a number of behaviours relevant to cell crawling. We present stability analyses that show that an asymmetric perturbation is required to cause a spatially uniform, stationary strip of cytoplasm to move, which is relevant to cell polarization. Our numerical simulations identify qualitatively distinct families of travelling–wave solution that co–exist at certain parameter values. Within each family, the crawling speed of the strip has a bell–shaped dependence on the adhesion strength. The model captures the experimentally observed behaviour that cells crawl quickest at intermediate adhesion strengths, when the substrate is neither too sticky nor too slippy

Obscuring and feeding supermassive black holes with evolving nuclear star clusters

Recently, high resolution observations with the help of the near-infrared adaptive optics integral field spectrograph SINFONI at the VLT proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies. With the help of high resolution hydrodynamical simulations with the PLUTO-code, we follow the evolution of such clusters, especially focusing on mass and energy feedback from young stars. This leads to a filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale. Here, we concentrate on the long-term evolution of the nuclear disc in NGC 1068 with the help of an effective viscous disc model, using the mass input from the large scale simulations and accounting for star formation in the disc. This two-stage modelling enables us to connect the tens of parsec scale region (observable with SINFONI) with the parsec scale environment (MIDI observations). At the current age of the nuclear star cluster, our simulations predict disc sizes of the order of 0.8 to 0.9 pc, gas masses of 1.0e6 Msun and mass transfer rates through the inner boundary of 0.025 Msun/yr in good agreement with values derived from observations.Comment: 6 pages, 3 figures, to appear in the proceedings of the IAU General Assembly 2009, Rio de Janeiro, S267 Co-evolution of Central Black Holes and Galaxie

Cooperating Agents for 3D Scientific Data Interpretation

Many organizations collect vast quantities of three-dimensional (3-D) scientific data in volumetric form for a range of purposes, including resource exploration, market forecasting, and process modelling. Traditionally, these data have been interpreted by human experts with only minimal software assistance. However, such manual interpretation is a painstakingly slow and tedious process. Moreover, since interpretation involves subjective judgements and each interpreter has different scientific knowledge and experience, formulation of an effective interpretation often requires the cooperation of numerous such experts. Hence, there is a pressing need for a software system in which individual interpretations can be generated automatically and then refined through the use of cooperative reasoning and information sharing. To this end, a prototype system, SurfaceMapper, has been developed in which a community of cooperating software agents automatically locate and display interpretations in a volume of 3-D scientific data. The challenges and experiences in designing and building such a system are discussed. Particular emphasis is given to the agents' interactions and an empirical evaluation of the effectiveness of different cooperation strategies is presented

Neutral density map of Hall thruster plume expansion in a vacuum chamber

A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09 mg/s14.09mg∕s. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46 mg/s10.46mg∕s for discharge voltages of 300 and 500 V500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5 to 5.0 kW1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140 000 and 240 000 l/s240000l∕s. The pressure map is performed with a rake consisting of five calibrated Bayard–Alpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTF’s centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2 m2m downstream of the exit plane. Comparison of axial pressure profiles on the LVTF’s centerline shows that the neutral density is nearly the same for cold flow and hot flow. The study shows that a cold flow neutral density background map accurately characterizes the neutral density in an operating Hall thruster plume.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87890/2/053509_1.pd

Expanded Thruster Mass Model Incorporating Nested Hall Thrusters

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143089/1/6.2017-4729.pd

Neutral Flow Evolution in a Six-Kilowatt Hall Thruster

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90645/1/AIAA-54141-803.pd

Strongly Non-Equilibrium Bose-Einstein Condensation in a Trapped Gas

We present a qualitative (and quantitative, at the level of estimates) analysis of the ordering kinetics in a strongly non-equilibrium state of a weakly interacting Bose gas, trapped with an external potential. At certain conditions, the ordering process is predicted to be even more rich than in the homogeneous case. Like in the homogeneous case, the most characteristic feature of the full-scale non-equilibrium process is the formation of superfluid turbulence.Comment: 4 pages, revtex, no figures. Submitted to PR

Comparing placentas from normal and abnormal pregnancies

This report describes work carried out at a Mathematics-in-Medicine Study Group. It is believed that placenta shape villous network characteristics are strongly linked to the placenta’s efficiency, and hence to pregnancy outcome. We were asked to consider mathematical ways to describe the shape and other characteristics of a placenta, as well as forming mathematical models for placenta development. In this report we propose a number of possible measure of placental shape, form, and efficiency, which can be computed from images already obtained. We also consider various models for the early development of placentas and the growth of the villous tree

Computation of Neutral Gas Flow From a Hall Thruster Into a Vacuum Chamber

The finite pressure present in vacuum chamber testing of space propulsion systems such as Hall thrusters can have a number of undesirable effects. For example, the thrust generated by the thruster is higher and the plume divergence angle larger in ground tests in comparison to space operation. To try to quantify these effects, the direct simulation Monte Carlo method is applied to model a cold flow of xenon gas expanding from a Hall thruster into a vacuum chamber. The simulations are performed for the P5 Hall thruster operating in a large vacuum tank at the University of Michigan. Comparison of the simulation results is made with experimental measurements of pressure obtained with a series of ion gauges. The mass flow rate through the thruster and the total pumping speed of the vacuum chamber are varied. A key physical parameter in the simulations concerns the probability that a xenon atom incident on a cryogenic pumping panel actually sticks to the panel. For a reasonable range of values for the sticking coefficient, excellent agreement between simulation and experiment is obtained for several different conditions. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87929/2/541_1.pd