1,714 research outputs found
Mechanisms of kinetic trapping in self-assembly and phase transformation
In self-assembly processes, kinetic trapping effects often hinder the
formation of thermodynamically stable ordered states. In a model of viral
capsid assembly and in the phase transformation of a lattice gas, we show how
simulations in a self-assembling steady state can be used to identify two
distinct mechanisms of kinetic trapping. We argue that one of these mechanisms
can be adequately captured by kinetic rate equations, while the other involves
a breakdown of theories that rely on cluster size as a reaction coordinate. We
discuss how these observations might be useful in designing and optimising
self-assembly reactions
A Generalized Q-operator for U_q(\hat(sl_2)) Vertex Models
In this paper, we construct a Q-operator as a trace of a representation of
the universal R-matrix of over an infinite-dimensional
auxiliary space. This auxiliary space is a four-parameter generalization of the
q-oscillator representations used previously. We derive generalized T-Q
relations in which 3 of these parameters shift. After a suitable restriction of
parameters, we give an explicit expression for the Q-operator of the 6-vertex
model and show the connection with Baxter's expression for the central block of
his corresponding operator.Comment: 22 pages, Latex2e. This replacement is a revised version that
includes a simple explicit expression for the Q matrix for the 6-vertex mode
Glassy behaviour in an exactly solved spin system with a ferromagnetic transition
We show that applying simple dynamical rules to Baxter's eight-vertex model
leads to a system which resembles a glass-forming liquid. There are analogies
with liquid, supercooled liquid, glassy and crystalline states. The disordered
phases exhibit strong dynamical heterogeneity at low temperatures, which may be
described in terms of an emergent mobility field. Their dynamics are
well-described by a simple model with trivial thermodynamics, but an emergent
kinetic constraint. We show that the (second order) thermodynamic transition to
the ordered phase may be interpreted in terms of confinement of the excitations
in the mobility field. We also describe the aging of disordered states towards
the ordered phase, in terms of simple rate equations.Comment: 11 page
Analyzing mechanisms and microscopic reversibility of self-assembly
We use computer simulations to investigate self-assembly in a system of model
chaperonin proteins, and in an Ising lattice gas. We discuss the mechanisms
responsible for rapid and efficient assembly in these systems, and we use
measurements of dynamical activity and assembly progress to compare their
propensities for kinetic trapping. We use the analytic solution of a simple
minimal model to illustrate the key features associated with such trapping,
paying particular attention to the number of ways that particles can misbind.
We discuss the relevance of our results for the design and control of
self-assembly in general.Comment: 11 pages, 8 figures. Discussion clarified in response to referee
coment
Quantifying reversibility in a phase-separating lattice gas: an analogy with self-assembly
We present dynamic measurements of a lattice gas during phase separation,
which we use as an analogy for self-assembly of equilibrium ordered structures.
We use two approaches to quantify the degree of 'reversibility' of this
process: firstly, we count events in which bonds are made and broken; secondly,
we use correlation-response measurements and fluctuation-dissipation ratios to
probe reversibility during different time intervals. We show how correlation
and response functions can be related directly to microscopic (ir)reversibility
and we discuss time-dependence and observable- dependence of these
measurements, including the role of fast and slow degrees of freedom during
assembly.Comment: 10 pages, 8 figure
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
Seasonal phosphate activity in three characteristic soils of the English uplands polluted by long-term atmospheric nitrogen deposition
Phosphomonoesterase activities were determined monthly during a seasonal cycle in three characteristic soil types of the English
uplands that have been subject to long-term atmospheric nitrogen deposition. Activities (moll para-nitrophenol ri soil dry wt.
h-1) ranged between 83.9 and 307 in a blanket peat (total carbon 318 mg g-1, pH 3.9), 45.2-86.4 in an acid organic grassland soil
(total carbon 354 mg g-1, pH 3.7) and 10.4-21.1 in a calcareous grassland soil (total carbon 140 mg g-1, pH 7.3). These are
amongst the highest reported soil phosphomonoesterase activities and confirm the strong biological phosphorus limitation in this
environmen
Asymptotic Limits and Zeros of Chromatic Polynomials and Ground State Entropy of Potts Antiferromagnets
We study the asymptotic limiting function , where is the chromatic polynomial for a graph
with vertices. We first discuss a subtlety in the definition of
resulting from the fact that at certain special points , the
following limits do not commute: . We then
present exact calculations of and determine the corresponding
analytic structure in the complex plane for a number of families of graphs
, including circuits, wheels, biwheels, bipyramids, and (cyclic and
twisted) ladders. We study the zeros of the corresponding chromatic polynomials
and prove a theorem that for certain families of graphs, all but a finite
number of the zeros lie exactly on a unit circle, whose position depends on the
family. Using the connection of with the zero-temperature Potts
antiferromagnet, we derive a theorem concerning the maximal finite real point
of non-analyticity in , denoted and apply this theorem to
deduce that and for the square and
honeycomb lattices. Finally, numerical calculations of and
are presented and compared with series expansions and bounds.Comment: 33 pages, Latex, 5 postscript figures, published version; includes
further comments on large-q serie
Impact Exercise Increases BMC During Growth: An 8-Year Longitudinal Study
Our aim was to assess BMC of the hip over 8 yr in prepubertal children who participated in a 7-mo jumping intervention compared with controls who participated in a stretching program of equal duration. We hypothesized that jumpers would gain more BMC than control subjects. The data reported come from two cohorts of children who participated in separate, but identical, randomized, controlled, school-based impact exercise interventions and reflect those subjects who agreed to long-term follow-up (N = 57; jumpers = 33, controls = 24; 47% of the original participants). BMC was assessed by DXA at baseline, 7 and 19 mo after intervention, and annually thereafter for 5 yr (eight visits over 8 yr). Multilevel random effects models were constructed and used to predict change in BMC from baseline at each measurement occasion. After 7 mo, those children that completed high-impact jumping exercises had 3.6% more BMC at the hip than control subjects whom completed nonimpact stretching activities (p \u3c 0.05) and 1.4% more BMC at the hip after nearly 8 yr (BMC adjusted for change in age, height, weight, and physical activity; p \u3c 0.05). This provides the first evidence of a sustained effect on total hip BMC from short-term high-impact exercise undertaken in early childhood. If the benefits are sustained into young adulthood, effectively increasing peak bone mass, fracture risk in the later years could be reduced
Ground State Entropy of the Potts Antiferromagnet on Cyclic Strip Graphs
We present exact calculations of the zero-temperature partition function
(chromatic polynomial) and the (exponent of the) ground-state entropy for
the -state Potts antiferromagnet on families of cyclic and twisted cyclic
(M\"obius) strip graphs composed of -sided polygons. Our results suggest a
general rule concerning the maximal region in the complex plane to which
one can analytically continue from the physical interval where . The
chromatic zeros and their accumulation set exhibit the rather
unusual property of including support for and provide further
evidence for a relevant conjecture.Comment: 7 pages, Latex, 4 figs., J. Phys. A Lett., in pres
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