674 research outputs found
Effects of disorder in location and size of fence barriers on molecular motion in cell membranes
The effect of disorder in the energetic heights and in the physical locations
of fence barriers encountered by transmembrane molecules such as proteins and
lipids in their motion in cell membranes is studied theoretically. The
investigation takes as its starting point a recent analysis of a periodic
system with constant distances between barriers and constant values of barrier
heights, and employs effective medium theory to treat the disorder. The
calculations make possible, in principle, the extraction of confinement
parameters such as mean compartment sizes and mean intercompartmental
transition rates from experimentally reported published observations. The
analysis should be helpful both as an unusual application of effective medium
theory and as an investigation of observed molecular movements in cell
membranes.Comment: 9 pages, 5 figure
Traversal Times for Random Walks on Small-World Networks
We study the mean traversal time for a class of random walks on Newman-Watts
small-world networks, in which steps around the edge of the network occur with
a transition rate F that is different from the rate f for steps across
small-world connections. When f >> F, the mean time to traverse the network
exhibits a transition associated with percolation of the random graph (i.e.,
small-world) part of the network, and a collapse of the data onto a universal
curve. This transition was not observed in earlier studies in which equal
transition rates were assumed for all allowed steps. We develop a simple
self-consistent effective medium theory and show that it gives a quantitatively
correct description of the traversal time in all parameter regimes except the
immediate neighborhood of the transition, as is characteristic of most
effective medium theories.Comment: 9 pages, 5 figure
Equilibration, generalized equipartition, and diffusion in dynamical Lorentz gases
We prove approach to thermal equilibrium for the fully Hamiltonian dynamics
of a dynamical Lorentz gas, by which we mean an ensemble of particles moving
through a -dimensional array of fixed soft scatterers that each possess an
internal harmonic or anharmonic degree of freedom to which moving particles
locally couple. We establish that the momentum distribution of the moving
particles approaches a Maxwell-Boltzmann distribution at a certain temperature
, provided that they are initially fast and the scatterers are in a
sufficiently energetic but otherwise arbitrary stationary state of their free
dynamics--they need not be in a state of thermal equilibrium. The temperature
to which the particles equilibrate obeys a generalized equipartition
relation, in which the associated thermal energy is equal to
an appropriately defined average of the scatterers' kinetic energy. In the
equilibrated state, particle motion is diffusive
Transport Properties of Random Walks on Scale-Free/Regular-Lattice Hybrid Networks
We study numerically the mean access times for random walks on hybrid
disordered structures formed by embedding scale-free networks into regular
lattices, considering different transition rates for steps across lattice bonds
() and across network shortcuts (). For fast shortcuts () and
low shortcut densities, traversal time data collapse onto an universal curve,
while a crossover behavior that can be related to the percolation threshold of
the scale-free network component is identified at higher shortcut densities, in
analogy to similar observations reported recently in Newman-Watts small-world
networks. Furthermore, we observe that random walk traversal times are larger
for networks with a higher degree of inhomogeneity in their shortcut
distribution, and we discuss access time distributions as functions of the
initial and final node degrees. These findings are relevant, in particular,
when considering the optimization of existing information networks by the
addition of a small number of fast shortcut connections.Comment: 8 pages, 6 figures; expanded discussions, added figures and
references. To appear in J Stat Phy
Adiabatic-Nonadiabatic Transition in the Diffusive Hamiltonian Dynamics of a Classical Holstein Polaron
We study the Hamiltonian dynamics of a free particle injected onto a chain
containing a periodic array of harmonic oscillators in thermal equilibrium. The
particle interacts locally with each oscillator, with an interaction that is
linear in the oscillator coordinate and independent of the particle's position
when it is within a finite interaction range. At long times the particle
exhibits diffusive motion, with an ensemble averaged mean-squared displacement
that is linear in time. The diffusion constant at high temperatures follows a
power law D ~ T^{5/2} for all parameter values studied. At low temperatures
particle motion changes to a hopping process in which the particle is bound for
considerable periods of time to a single oscillator before it is able to escape
and explore the rest of the chain. A different power law, D ~ T^{3/4}, emerges
in this limit. A thermal distribution of particles exhibits thermally activated
diffusion at low temperatures as a result of classically self-trapped polaronic
states.Comment: 15 pages, 4 figures Submitted to Physical Review
The relationship between core symptoms of ADHD and the Cognitive Reflection Test in a non-clinical sample.
INTRODUCTION: Attention Deficit and Hyperactivity Disorder (ADHD) symptoms are frequently linked to executive function deficits. There is reason to believe that these deficits may give rise to problems with complex reasoning and problem solving. METHODS: Eighty-six men (N = 45) and women (N = 41) completed a self-report measure to assess ADHD symptoms, along with a complex reasoning task; the Cognitive Reflection Test (CRT). IQ was also tested due to its covariance with reasoning ability. RESULTS: Analysis suggested that all three symptoms of ADHD (inattention, hyperactivity, and impulsivity) are negatively related to performance on the CRT, however, only inattention significantly contributed to a model that predicted CRT performance. CONCLUSIONS: Of the three core symptoms of ADHD, inattention is the most important for reasoning ability. Results are discussed with reference to an executive function model of ADHD, with particular emphasis on the role of working memory in inattention
Fluorescence decay in aperiodic Frenkel lattices
We study motion and capture of excitons in self-similar linear systems in
which interstitial traps are arranged according to an aperiodic sequence,
focusing our attention on Fibonacci and Thue-Morse systems as canonical
examples. The decay of the fluorescence intensity following a broadband pulse
excitation is evaluated by solving the microscopic equations of motion of the
Frenkel exciton problem. We find that the average decay is exponential and
depends only on the concentration of traps and the trapping rate. In addition,
we observe small-amplitude oscillations coming from the coupling between the
low-lying mode and a few high-lying modes through the topology of the lattice.
These oscillations are characteristic of each particular arrangement of traps
and they are directly related to the Fourier transform of the underlying
lattice. Our predictions can be then used to determine experimentally the
ordering of traps.Comment: REVTeX 3.0 + 3PostScript Figures + epsf.sty (uuencoded). To appear in
Physical Review
Urbanization alters plastic responses in the common dandelion Taraxacum officinale
Urban environments expose species to contrasting selection pressures relative to rural areas due to altered microclimatic conditions, habitat fragmentation, and changes in species interactions. To improve our understanding on how urbanization impacts selection through biotic interactions, we assessed differences in plant defense and tolerance, dispersal, and flowering phenology of a common plant species (Taraxacum officinale) along an urbanization gradient and their reaction norms in response to a biotic stressor (i.e., herbivory). We raised plants from 45 lines collected along an urbanization gradient under common garden conditions and assessed the impact of herbivory on plant growth (i.e., aboveground biomass), dispersal capacity (i.e., seed morphology), and plant phenology (i.e., early seed production) by exposing half of our plants to two events of herbivory (i.e., grazing by locusts). Independent from their genetic background, all plants consistently increased their resistance to herbivores by which the second exposure to locusts resulted in lower levels of damage suffered. Herbivory had consistent effects on seed pappus length, with seeds showing a longer pappus (and, hence, increased dispersal capacities) regardless of urbanization level. Aboveground plant biomass was neither affected by urbanization nor herbivore presence. In contrast to consistent responses in plant defenses and pappus length, plant fitness did vary between lines. Urban lines had a reduced early seed production following herbivory while rural and suburban lines did not show any plastic response. Our results show that herbivory affects plant phenotypes but more importantly that differences in herbivory reaction norms exist between urban and rural populations
Structure, magnetic and transport properties of Ti-substituted La0.7Sr0.3MnO3
Ti-substituted perovskites, La0.7Sr0.3Mn1-xTixO3, with x between 0 to 0.20,
were investigated by neutron diffraction, magnetization, electric resistivity,
and magnetoresistance (MR) measurements. All samples show a rhombohedral
structure (space group R3c) from 10 K to room temperature. At room temperature,
the cell parameters a, c and the unit cell volume increase with increasing Ti
content. However, at 10 K, the cell parameter a has a maximum value for x =
0.10, and decreases for x greater than 0.10, while the unit cell volume remains
nearly constant for x greater than 0.10. The average (Mn,Ti)-O bond length
increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with
increasing Ti content to its minimum value at x=0.15 at room temperature. Below
the Curie temperature T_C, the resistance exhibits metallic behavior for the x
_ 0.05 samples. A metal (semiconductor) to insulator transition is observed for
the x_ 0.10 samples. A peak in resistivity appears below T_C for all samples,
and shifts to a lower temperature as x increases. The substitution of Mn by Ti
decreases the 2p-3d hybridization between O and Mn ions, reduces the bandwidth
W, and increases the electron-phonon coupling. Therefore, the TC shifts to a
lower temperature and the resistivity increases with increasing Ti content. A
field-induced shift of the resistivity maximum occurs at x less than or equal
to 0.10. The maximum MR effect is about 70% for La0.7Sr0.3Mn0.8Ti0.2O3. The
separation of TC and the resistivity maximum temperature Tmax enhances the MR
effect in these compounds due to the weak coupling between the magnetic
ordering and the resistivity as compared with La0.7Sr0.3MnO3.Comment: zip fil
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