272 research outputs found
DNA denaturation and wetting in the presence of disorder
We present a precise equivalence of the Lifson-Poland-Scheraga model with
wetting models. Making use of a representation of the former model in terms of
random matrices, we obtain, in the limit of weak disorder, a mean--field
approximation, that shows a change of the critical behavior due to disorder.Comment: 4 page
Manipulating energy and spin currents in nonequilibrium systems of interacting qubits
We consider generic interacting chain of qubits, which are coupled at the
edges to baths of fixed polarizations. We can determine the nonequilibrium
steady states, described by the fixed point of the Lindblad Master Equation.
Under rather general assumptions about local pumping and interactions,
symmetries of the reduced density matrix are revealed. The symmetries
drastically restrict the form of the steady density matrices in such a way that
an exponentially large subset of one--point and many--point correlation
functions are found to vanish. As an example we show how in a Heisenberg spin
chain a suitable choice of the baths can completely switch off either the spin
or the energy current, or both of them, despite the presence of large boundary
gradients.Comment: 8 pages, 3 Figure
Thermal conduction in classical low-dimensional lattices
Deriving macroscopic phenomenological laws of irreversible thermodynamics
from simple microscopic models is one of the tasks of non-equilibrium
statistical mechanics. We consider stationary energy transport in crystals with
reference to simple mathematical models consisting of coupled oscillators on a
lattice. The role of lattice dimensionality on the breakdown of the Fourier's
law is discussed and some universal quantitative aspects are emphasized: the
divergence of the finite-size thermal conductivity is characterized by
universal laws in one and two dimensions. Equilibrium and non-equilibrium
molecular dynamics methods are presented along with a critical survey of
previous numerical results. Analytical results for the non-equilibrium dynamics
can be obtained in the harmonic chain where the role of disorder and
localization can be also understood. The traditional kinetic approach, based on
the Boltzmann-Peierls equation is also briefly sketched with reference to
one-dimensional chains. Simple toy models can be defined in which the
conductivity is finite. Anomalous transport in integrable nonlinear systems is
briefly discussed. Finally, possible future research themes are outlined.Comment: 90 pages, revised versio
Computational analysis of folding and mutation properties of C5 domain from Myosin binding protein C
Thermal folding Molecular Dynamics simulations of the domain C5 from Myosin
Binding Protein C were performed using a native-centric model to study the role
of three mutations related to Familial Hypertrophic Cardiomyopathy. Mutation of
Asn755 causes the largest shift of the folding temperature, and the residue is
located in the CFGA' beta-sheet featuring the highest Phi-values. The mutation
thus appears to reduce the thermodynamic stability in agreement with
experimental data. The mutations on Arg654 and Arg668, conversely, cause a
little change in the folding temperature and they reside in the low Phi-value
BDE beta-sheet, so that their pathologic role cannot be related to impairment
of the folding process but possibly to the binding with target molecules. As
the typical signature of Domain C5 is the presence of a longer and
destabilizing CD-loop with respect to the other Ig-like domains we completed
the work with a bioinformatic analysis of this loop showing a high density of
negative charge and low hydrophobicity. This indicates the CD-loop as a
natively unfolded sequence with a likely coupling between folding and ligand
binding.Comment: RevTeX, 10 pages, 9 eps-figure
Stability of the splay state in pulse--coupled networks
The stability of the dynamical states characterized by a uniform firing rate
({\it splay states}) is analyzed in a network of globally coupled leaky
integrate-and-fire neurons. This is done by reducing the set of differential
equations to a map that is investigated in the limit of large network size. We
show that the stability of the splay state depends crucially on the ratio
between the pulse--width and the inter-spike interval. More precisely, the
spectrum of Floquet exponents turns out to consist of three components: (i) one
that coincides with the predictions of the mean-field analysis [Abbott-van
Vreesvijk, 1993]; (ii) a component measuring the instability of
"finite-frequency" modes; (iii) a number of "isolated" eigenvalues that are
connected to the characteristics of the single pulse and may give rise to
strong instabilities (the Floquet exponent being proportional to the network
size). Finally, as a side result, we find that the splay state can be stable
even for inhibitory coupling.Comment: 13 pages, 10 figures, submitted for pubblication to Physical Review
Studies of thermal conductivity in Fermi-Pasta-Ulam like lattices
The pioneering computer simulations of the energy relaxation mechanisms
performed by Fermi, Pasta and Ulam can be considered as the first attempt of
understanding energy relaxation and thus heat conduction in lattices of
nonlinear oscillators. In this paper we describe the most recent achievements
about the divergence of heat conductivity with the system size in 1d and 2d
FPU-like lattices. The anomalous behavior is particularly evident at low
energies, where it is enhanced by the quasi-harmonic character of the lattice
dynamics. Remakably, anomalies persist also in the strongly chaotic region
where long--time tails develop in the current autocorrelation function. A modal
analysis of the 1d case is also presented in order to gain further insight
about the role played by boundary conditions.Comment: Invited article to appear in the Chaos focus issue on "Studies of
Nonlinear Problems. I" by Enrico Fermi, John Pasta, and Stanislaw Ulam
Boundary-induced instabilities in coupled oscillators
Published version, minor changesPeer reviewedPublisher PD
Intertangled stochastic motifs in networks of excitatory-inhibitory units
We have benefited from discussions with A. Politi. The authors acknowledge financial support from H2020- MSCA-ITN-2015 project COSMOS 642563.Peer reviewedPostprin
Covariant Lyapunov vectors
The recent years have witnessed a growing interest for covariant Lyapunov
vectors (CLVs) which span local intrinsic directions in the phase space of
chaotic systems. Here we review the basic results of ergodic theory, with a
specific reference to the implications of Oseledets' theorem for the properties
of the CLVs. We then present a detailed description of a "dynamical" algorithm
to compute the CLVs and show that it generically converges exponentially in
time. We also discuss its numerical performance and compare it with other
algorithms presented in literature. We finally illustrate how CLVs can be used
to quantify deviations from hyperbolicity with reference to a dissipative
system (a chain of H\'enon maps) and a Hamiltonian model (a Fermi-Pasta-Ulam
chain)
- …