54,965 research outputs found
Gamow shell-model calculations of drip-line oxygen isotopes
We employ the Gamow shell model (GSM) to describe low-lying states of the
oxygen isotopes 24O and 25O. The many-body Schrodinger equation is solved
starting from a two-body Hamiltonian defined by a renormalized low-momentum
nucleon-nucleon (NN) interaction, and a spherical Berggren basis. The Berggren
basis treats bound, resonant, and continuum states on an equal footing, and is
therefore an appropriate representation of loosely bound and unbound nuclear
states near threshold. We show that such a basis is necessary in order to
obtain a detailed and correct description of the low-lying 1+ and 2+ excited
states in 24O. On the other hand, we find that a correct description of binding
energy systematics of the ground states is driven by proper treatment and
inclusion of many-body correlation effects. This is supported by the fact that
we get 25O unstable with respect to 24O in both oscillator and Berggren
representations starting from a 22O core. Furthermore, we show that the
structure of these loosely bound or unbound isotopes are strongly influenced by
the 1S0 component of the NN interaction. This has important consequences for
our understanding of nuclear stability.Comment: 5 pages, 3 figure
Time delay as a key to Apoptosis Induction in the p53 Network
A feedback mechanism that involves the proteins p53 and mdm2, induces cell
death as a controled response to severe DNA damage. A minimal model for this
mechanism demonstrates that the respone may be dynamic and connected with the
time needed to translate the mdm2 protein. The response takes place if the
dissociation constant k between p53 and mdm2 varies from its normal value.
Although it is widely believed that it is an increase in k that triggers the
response, we show that the experimental behaviour is better described by a
decrease in the dissociation constant. The response is quite robust upon
changes in the parameters of the system, as required by any control mechanism,
except for few weak points, which could be connected with the onset of cancer
1/z-renormalization of the mean-field behavior of the dipole-coupled singlet-singlet system HoF_3
The two main characteristics of the holmium ions in HoF_3 are that their
local electronic properties are dominated by two singlet states lying well
below the remaining 4f-levels, and that the classical dipole-coupling is an
order of magnitude larger than any other two-ion interactions between the
Ho-moments. This combination makes the system particularly suitable for testing
refinements of the mean-field theory. There are four Ho-ions per unit cell and
the hyperfine coupled electronic and nuclear moments on the Ho-ions order in a
ferrimagnetic structure at T_C=0.53 K. The corrections to the mean-field
behavior of holmium triflouride, both in the paramagnetic and ferrimagnetic
phase, have been calculated to first order in the high-density 1/z-expansion.
The effective medium theory, which includes the effects of the single-site
fluctuations, leads to a substantially improved description of the magnetic
properties of HoF_3, in comparison with that based on the mean-field
approximation.Comment: 26pp, plain-TeX, JJ
Priorities for sustainable turfgrass management: a research and industry perspective
This paper provides a brief review and assessment of the key environmental, regulatory and technical issues facing the turfgrass sector with specific reference to the European context. It considers the range of externalities or ‘drivers for change' facing the industry, and the challenges and opportunities available for promoting and achieving more sustainable turfgrass management within the sports, landscape and amenity sectors. The analysis confirms that there are a number of key areas where a concerted research and industrial effort is required. These include responding to the pressures from government demands for greater environmental regulation, the increasing pressure on natural resources (notably water, energy and land), the emerging role of turf management in supporting ecosystem services and enhancing biodiversity, the continued need to promote integrated pest management, and the looming challenges posed by a changing climate, and urgent need to adapt. Whilst many of these externalities appear to be risks to the sports turf industry, there will also be significant opportunities, for those where the labour, energy and agronomic costs are minimized and where the drive to adopt a multifunctional approach to sportsturf management is embraced
Fear and its implications for stock markets
The value of stocks, indices and other assets, are examples of stochastic
processes with unpredictable dynamics. In this paper, we discuss asymmetries in
short term price movements that can not be associated with a long term positive
trend. These empirical asymmetries predict that stock index drops are more
common on a relatively short time scale than the corresponding raises. We
present several empirical examples of such asymmetries. Furthermore, a simple
model featuring occasional short periods of synchronized dropping prices for
all stocks constituting the index is introduced with the aim of explaining
these facts. The collective negative price movements are imagined triggered by
external factors in our society, as well as internal to the economy, that
create fear of the future among investors. This is parameterized by a ``fear
factor'' defining the frequency of synchronized events. It is demonstrated that
such a simple fear factor model can reproduce several empirical facts
concerning index asymmetries. It is also pointed out that in its simplest form,
the model has certain shortcomings.Comment: 5 pages, 5 figures. Submitted to the Proceedings of Applications of
Physics in Financial Analysis 5, Turin 200
A stochastic theory for temporal fluctuations in self-organized critical systems
A stochastic theory for the toppling activity in sandpile models is
developed, based on a simple mean-field assumption about the toppling process.
The theory describes the process as an anti-persistent Gaussian walk, where the
diffusion coefficient is proportional to the activity. It is formulated as a
generalization of the It\^{o} stochastic differential equation with an
anti-persistent fractional Gaussian noise source. An essential element of the
theory is re-scaling to obtain a proper thermodynamic limit, and it captures
all temporal features of the toppling process obtained by numerical simulation
of the Bak-Tang-Wiesenfeld sandpile in this limit.Comment: 9 pages, 4 figure
Bursts and Shocks in a Continuum Shell Model
We study a "burst" event, i. e. the evolution of an initial condition having
support only in a finite interval of k-space, in the continuum shell model due
to Parisi. We show that the continuum equation without forcing or dissipation
can be explicitly written in characteristic form and that the right and left
moving parts can be solved exactly. When this is supplemented by the
appropriate shock condition it is possible to find the asymptotic form of the
burst.Comment: 15 pages, 2 eps figures included, Latex 2e. Contribution to the
proceedings of the conference: Disorder and Chaos, in honour of Giovanni
Paladin, September 22-24, 1997, in Rom
Vortices Clustering: The Origin of the Second Peak in the Magnetisation Loops of High Temperature Superconductors
We study vortex clustering in type II Superconductors. We demonstrate that
the ``second peak'' observed in magnetisation loops may be a dynamical effect
associated with a density driven instability of the vortex system. At the
microscopic level the instability shows up as the clustering of individual
vortices at (rare) preferential regions of the pinning potential. In the limit
of quasi-static ramping the instability is related to a phase transition in the
equilibrium vortex system.Comment: 11 pages + 3 figure
Oscillations and temporal signalling in cells
The development of new techniques to quantitatively measure gene expression
in cells has shed light on a number of systems that display oscillations in
protein concentration. Here we review the different mechanisms which can
produce oscillations in gene expression or protein concentration, using a
framework of simple mathematical models. We focus on three eukaryotic genetic
regulatory networks which show "ultradian" oscillations, with time period of
the order of hours, and involve, respectively, proteins important for
development (Hes1), apoptosis (p53) and immune response (NFkB). We argue that
underlying all three is a common design consisting of a negative feedback loop
with time delay which is responsible for the oscillatory behaviour
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