767 research outputs found
How to manage the post pandemic opening? A Pontryagin Maximum Principle approach
The COVID-19 pandemic has completely disrupted the operation of our
societies. Its elusive transmission process, characterized by an unusually long
incubation period, as well as a high contagion capacity, has forced many
countries to take quarantine and social isolation measures that conspire
against the performance of national economies. This situation confronts
decision makers in different countries with the alternative of reopening the
economies, thus facing the unpredictable cost of a rebound of the infection.
This work tries to offer an initial theoretical framework to handle this
alternative.Comment: 10 page
A Solvable Sequence Evolution Model and Genomic Correlations
We study a minimal model for genome evolution whose elementary processes are
single site mutation, duplication and deletion of sequence regions and
insertion of random segments. These processes are found to generate long-range
correlations in the composition of letters as long as the sequence length is
growing, i.e., the combined rates of duplications and insertions are higher
than the deletion rate. For constant sequence length, on the other hand, all
initial correlations decay exponentially. These results are obtained
analytically and by simulations. They are compared with the long-range
correlations observed in genomic DNA, and the implications for genome evolution
are discussed.Comment: 4 pages, 4 figure
Millikelvin magnetic relaxation measurements of alpha-Fe2O3 antiferromagnetic particles
In this paper we report magnetic relaxation data for antiferromagnetic
alpha-Fe2O3 particles of 5 nm mean diameter in the temperature range 0.1 K to
25 K. The average spin value of these particles S=124 and the uniaxial
anisotropy constant D=1.6x10^-2 K have been estimated from the experimental
values of the blocking temperature and anisotropy field. The observed plateau
in the magnetic viscosity from 3 K down to 100 mK agrees with the occurrence of
spin tunneling from the ground state Sz = S. However, the scaling M vs Tln(nu
t) is broken below 5 K, suggesting the occurrence of tunneling from excited
states below this temperature.Comment: 4 pages (two columns), 4 figure
Correlation between magnetic interactions and domain structure in A1 FePt ferromagnetic thin films
We have investigated the relationship between the domain structure and the
magnetic interactions in a series of FePt ferromagnetic thin films of varying
thickness. As-made films grow in the magnetically soft and chemically
disordered A1 phase that may have two distinct domain structures. Above a
critical thickness nm the presence of an out of plane
anisotropy induces the formation of stripes, while for planar
domains occur.
Magnetic interactions have been characterized using the well known DCD-IRM
remanence protocols, plots, and magnetic viscosity measurements. We
have observed a strong correlation between the domain configuration and the
sign of the magnetic interactions. Planar domains are associated with positive
exchange-like interactions, while stripe domains have a strong negative
dipolar-like contribution. In this last case we have found a close correlation
between the interaction parameter and the surface dipolar energy of the stripe
domain structure. Using time dependent magnetic viscosity measurements, we have
also estimated an average activation volume for magnetic reversal, nm which is approximately
independent of the film thickness or the stripe period.Comment: 25 pages, 11 figure
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