4,051 research outputs found
Tradeoff between short-term and long-term adaptation in a changing environment
We investigate the competition dynamics of two microbial or viral strains
that live in an environment that switches periodically between two states. One
of the strains is adapted to the long-term environment, but pays a short-term
cost, while the other is adapted to the short-term environment and pays a cost
in the long term. We explore the tradeoff between these alternative strategies
in extensive numerical simulations, and present a simple analytic model that
can predict the outcome of these competitions as a function of the mutation
rate and the time scale of the environmental changes. Our model is relevant for
arboviruses, which alternate between different host species on a regular basis.Comment: 9 pages, 3 figures, PRE in pres
Strong coupling isotropization of non-Abelian plasmas simplified
We study the isotropization of a homogeneous, strongly coupled, non-Abelian
plasma by means of its gravity dual. We compare the time evolution of a large
number of initially anisotropic states as determined, on the one hand, by the
full non-linear Einstein's equations and, on the other, by the Einstein's
equations linearized around the final equilibrium state. The linear
approximation works remarkably well even for states that exhibit large
anisotropies. For example, it predicts with a 20% accuracy the isotropization
time, which is of the order of t_iso \lesssim 1/T, with T the final equilibrium
temperature. We comment on possible extensions to less symmetric situations.Comment: 4 pages, 4 figures; v2: minor changes, matches PRL versio
Strong magnetoresistance induced by long-range disorder
We calculate the semiclassical magnetoresistivity of
non-interacting fermions in two dimensions moving in a weak and smoothly
varying random potential or random magnetic field. We demonstrate that in a
broad range of magnetic fields the non-Markovian character of the transport
leads to a strong positive magnetoresistance. The effect is especially
pronounced in the case of a random magnetic field where becomes
parametrically much larger than its B=0 value.Comment: REVTEX, 4 pages, 2 eps figure
Longitudinal Coherence in a Holographic Model of p-Pb Collisions
As a model of the longitudinal structure in heavy ion collisions, we simulate
gravitational shock wave collisions in anti-de Sitter space in which each shock
is composed of multiple constituents. We find that all constituents act
coherently, and their separation leaves no imprint on the resulting plasma,
when this separation is , with the
temperature of the plasma at the time when hydrodynamics first becomes
applicable. In particular, the center-of-mass of the plasma coincides with the
center-of-mass of all the constituents participating in the collision, as
opposed to the center-of-mass of the individual collisions. We discuss the
implications for nucleus-nucleus and proton-nucleus collisions.Comment: 5 pages, 3 figures. v2 matches published versio
Zero-frequency anomaly in quasiclassical ac transport: Memory effects in a two-dimensional metal with a long-range random potential or random magnetic field
We study the low-frequency behavior of the {\it ac} conductivity
of a two-dimensional fermion gas subject to a smooth random
potential (RP) or random magnetic field (RMF). We find a non-analytic
correction to , which corresponds to a
long-time tail in the velocity correlation function. This contribution
is induced by return processes neglected in Boltzmann transport theory. The
prefactor of this -term is positive and proportional to for
RP, while it is of opposite sign and proportional to in the weak RMF
case, where is the mean free path and the disorder correlation length.
This non-analytic correction also exists in the strong RMF regime, when the
transport is of a percolating nature. The analytical results are supported and
complemented by numerical simulations.Comment: 12 pages, RevTeX, 7 figure
Solution of the Quasispecies Model for an Arbitrary Gene Network
In this paper, we study the equilibrium behavior of Eigen's quasispecies
equations for an arbitrary gene network. We consider a genome consisting of genes, so that each gene sequence may be written as . We assume a single fitness peak (SFP) model
for each gene, so that gene has some ``master'' sequence for which it is functioning. The fitness landscape is then determined by
which genes in the genome are functioning, and which are not. The equilibrium
behavior of this model may be solved in the limit of infinite sequence length.
The central result is that, instead of a single error catastrophe, the model
exhibits a series of localization to delocalization transitions, which we term
an ``error cascade.'' As the mutation rate is increased, the selective
advantage for maintaining functional copies of certain genes in the network
disappears, and the population distribution delocalizes over the corresponding
sequence spaces. The network goes through a series of such transitions, as more
and more genes become inactivated, until eventually delocalization occurs over
the entire genome space, resulting in a final error catastrophe. This model
provides a criterion for determining the conditions under which certain genes
in a genome will lose functionality due to genetic drift. It also provides
insight into the response of gene networks to mutagens. In particular, it
suggests an approach for determining the relative importance of various genes
to the fitness of an organism, in a more accurate manner than the standard
``deletion set'' method. The results in this paper also have implications for
mutational robustness and what C.O. Wilke termed ``survival of the flattest.''Comment: 29 pages, 5 figures, to be submitted to Physical Review
The Error and Repair Catastrophes: A Two-Dimensional Phase Diagram in the Quasispecies Model
This paper develops a two gene, single fitness peak model for determining the
equilibrium distribution of genotypes in a unicellular population which is
capable of genetic damage repair. The first gene, denoted by ,
yields a viable organism with first order growth rate constant if it
is equal to some target ``master'' sequence . The second
gene, denoted by , yields an organism capable of genetic repair
if it is equal to some target ``master'' sequence . This
model is analytically solvable in the limit of infinite sequence length, and
gives an equilibrium distribution which depends on \mu \equiv L\eps , the
product of sequence length and per base pair replication error probability, and
\eps_r , the probability of repair failure per base pair. The equilibrium
distribution is shown to exist in one of three possible ``phases.'' In the
first phase, the population is localized about the viability and repairing
master sequences. As \eps_r exceeds the fraction of deleterious mutations,
the population undergoes a ``repair'' catastrophe, in which the equilibrium
distribution is still localized about the viability master sequence, but is
spread ergodically over the sequence subspace defined by the repair gene. Below
the repair catastrophe, the distribution undergoes the error catastrophe when exceeds \ln k/\eps_r , while above the repair catastrophe, the
distribution undergoes the error catastrophe when exceeds , where denotes the fraction of deleterious mutations.Comment: 14 pages, 3 figures. Submitted to Physical Review
Systematic study of the two band/two gap superconductivity in carbon-substituted MgB2 by point-contact spectroscopy
Point-contact measurements on the carbon-substituted Mg(BC)
filament/powder samples directly reveal a retention of the two superconducting
energy gaps in the whole doping range from to . The
large gap on the -band is decreased in an essentially linear fashion
with increasing the carbon concentrations. The changes in the the small gap
up to 3.8 % C are proportionally smaller and are more difficult
to detect but for the heavily doped sample with and
K both gaps are still present, and significantly reduced, consistent with a
strong essentially linear, reduction of each gap with the transition
temperature.Comment: 5 eps figure
Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments
We have studied the evolution of superconducting and normal state properties
of neutron irradiated Mg(BC) wire segments as a function
of post exposure annealing time and temperature. The initial fluence fully
suppressed superconductivity and resulted in an anisotropic expansion of the
unit cell. Superconductivity was restored by post-exposure annealing. The upper
critical field, H(T=0), approximately scales with T starting with an
undamaged T near 37 K and H(T=0) near 32 T. Up to an annealing
temperature of 400 C the recovery of T tends to coincide with a
decrease in the normal state resistivity and a systematic recovery of the
lattice parameters. Above 400 C a decrease in order along the c- direction
coincides with an increase in resistivity, but no apparent change in the
evolution of T and H. To first order, it appears that carbon doping
and neutron damaging effect the superconducting properties of MgB
independently
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