3,191 research outputs found
Tuning of tetrahedrality in a silicon potential yields a series of monatomic (metal-like) glassformers of very high fragility
We obtain monatomic glass formers in simulations by modifying the tetrahedral
character in a silicon potential to explore a triple point zone between
potentials favoring diamond (dc) and bcc crystals. dc crystallization is always
preceded by a polyamorphic transformation of the liquid, and is frustrated when
the Kauzmann temperature of the high temperature liquid intersects the
liquid-liquid coexistence line. The glass forming liquids are extraordinarily
fragile. Our results suggest that Si and Ge liquids may be vitrified at a
pressure close to the diamond-beta-tin-liquid triple point.Comment: 12 pages, including 3 figures. This revised version covers the same
as the original submission plus a discussion of the effect of the
polyamorphic transformation on the glass formation ability of the tetrahedral
liquids studie
Crossover to Potential Energy Landscape Dominated Dynamics in a Model Glass-forming Liquid
An equilibrated model glass-forming liquid is studied by mapping successive
configurations produced by molecular dynamics simulation onto a time series of
inherent structures (local minima in the potential energy). Using this
``inherent dynamics'' approach we find direct numerical evidence for the long
held view that below a crossover temperature, , the liquid's dynamics can
be separated into (i) vibrations around inherent structures and (ii)
transitions between inherent structures (M. Goldstein, J. Chem. Phys. {\bf 51},
3728 (1969)), i.e., the dynamics become ``dominated'' by the potential energy
landscape. In agreement with previous proposals, we find that is within
the vicinity of the mode-coupling critical temperature . We further find
that at the lowest temperature simulated (close to ), transitions between
inherent structures involve cooperative, string like rearrangements of groups
of particles moving distances substantially smaller than the average
interparticle distance.Comment: Expanded from 4 to 7 page
Sparse Recovery Analysis of Preconditioned Frames via Convex Optimization
Orthogonal Matching Pursuit and Basis Pursuit are popular reconstruction
algorithms for recovery of sparse signals. The exact recovery property of both
the methods has a relation with the coherence of the underlying redundant
dictionary, i.e. a frame. A frame with low coherence provides better guarantees
for exact recovery. An equivalent formulation of the associated linear system
is obtained via premultiplication by a non-singular matrix. In view of bounds
that guarantee sparse recovery, it is very useful to generate the
preconditioner in such way that the preconditioned frame has low coherence as
compared to the original. In this paper, we discuss the impact of
preconditioning on sparse recovery. Further, we formulate a convex optimization
problem for designing the preconditioner that yields a frame with improved
coherence. In addition to reducing coherence, we focus on designing well
conditioned frames and numerically study the relationship between the condition
number of the preconditioner and the coherence of the new frame. Alongside
theoretical justifications, we demonstrate through simulations the efficacy of
the preconditioner in reducing coherence as well as recovering sparse signals.Comment: 9 pages, 5 Figure
Effect of Heavy Metal Uptake by E. coli and Bacillus Sps
Over the past century, unrestricted mining, extensive industrialization, modern agricultural practices and faulty waste disposal methods have resulted in the release of unprecedented levels of toxic heavy metals like Cd, Hg, Ag, Sn, Pb, Cu, Co, Mn, Zn, etc into the environment. Many metals are essential for microbial growth in less concentration, yet are toxic in higher concentrations. Biosorption is an attractive alternative approach which involves the binding or adsorption of heavy metals to living or dead cells. Many microbes have the ability to selectively accumulate metals. The present study is intended to analyze the uptake systems of Bacillus and E. coli against different conc. of heavy metals like Zn, Cu, Cd, and Hg in their salt form incorporated into nutrient broth medium observed over a regular interval of time. Analysis was based on how much of the metal from the original conc. Used was left behind in the media after the rest being up taken by the organism. This was done using AAS which was indirectly the representation of percent uptake of heavy metal by the respective organism. The study showed that Gram –ve organisms like E. coli exhibited more resistance to metals like Zn, Cu and Hg in relative comparison with Gram +ve organisms like Bacillus. Bacillus sps was less sensitive to effect of Cd than in E. coli
Social Game for Building Energy Efficiency: Utility Learning, Simulation, and Analysis
We describe a social game that we designed for encouraging energy efficient
behavior amongst building occupants with the aim of reducing overall energy
consumption in the building. Occupants vote for their desired lighting level
and win points which are used in a lottery based on how far their vote is from
the maximum setting. We assume that the occupants are utility maximizers and
that their utility functions capture the tradeoff between winning points and
their comfort level. We model the occupants as non-cooperative agents in a
continuous game and we characterize their play using the Nash equilibrium
concept. Using occupant voting data, we parameterize their utility functions
and use a convex optimization problem to estimate the parameters. We simulate
the game defined by the estimated utility functions and show that the estimated
model for occupant behavior is a good predictor of their actual behavior. In
addition, we show that due to the social game, there is a significant reduction
in energy consumption
Memory formation in matter
Memory formation in matter is a theme of broad intellectual relevance; it
sits at the interdisciplinary crossroads of physics, biology, chemistry, and
computer science. Memory connotes the ability to encode, access, and erase
signatures of past history in the state of a system. Once the system has
completely relaxed to thermal equilibrium, it is no longer able to recall
aspects of its evolution. Memory of initial conditions or previous training
protocols will be lost. Thus many forms of memory are intrinsically tied to
far-from-equilibrium behavior and to transient response to a perturbation. This
general behavior arises in diverse contexts in condensed matter physics and
materials: phase change memory, shape memory, echoes, memory effects in
glasses, return-point memory in disordered magnets, as well as related contexts
in computer science. Yet, as opposed to the situation in biology, there is
currently no common categorization and description of the memory behavior that
appears to be prevalent throughout condensed-matter systems. Here we focus on
material memories. We will describe the basic phenomenology of a few of the
known behaviors that can be understood as constituting a memory. We hope that
this will be a guide towards developing the unifying conceptual underpinnings
for a broad understanding of memory effects that appear in materials
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