1,114 research outputs found
Cooling nonlinear lattices toward localisation
We describe the energy relaxation process produced by surface damping on
lattices of classical anharmonic oscillators. Spontaneous emergence of
localised vibrations dramatically slows down dissipation and gives rise to
quasi-stationary states where energy is trapped in the form of a gas of weakly
interacting discrete breathers. In one dimension (1D), strong enough on--site
coupling may yield stretched--exponential relaxation which is reminiscent of
glassy dynamics. We illustrate the mechanism generating localised structures
and discuss the crucial role of the boundary conditions. For two--dimensional
(2D) lattices, the existence of a gap in the breather spectrum causes the
localisation process to become activated. A statistical analysis of the
resulting quasi-stationary state through the distribution of breathers'
energies yield information on their effective interactions.Comment: 10 pages, 11 figure
Slow energy relaxation and localization in 1D lattices
We investigate the energy relaxation process produced by thermal baths at
zero temperature acting on the boundary atoms of chains of classical anharmonic
oscillators. Time-dependent perturbation theory allows us to obtain an explicit
solution of the harmonic problem: even in such a simple system nontrivial
features emerge from the interplay of the different decay rates of Fourier
modes. In particular, a crossover from an exponential to an inverse-square-root
law occurs on a time scale proportional to the system size . A further
crossover back to an exponential law is observed only at much longer times (of
the order ). In the nonlinear chain, the relaxation process is initially
equivalent to the harmonic case over a wide time span, as illustrated by
simulations of the Fermi-Pasta-Ulam model. The distinctive feature is
that the second crossover is not observed due to the spontaneous appearance of
breathers, i.e. space-localized time-periodic solutions, that keep a finite
residual energy in the lattice. We discuss the mechanism yielding such
solutions and also explain why it crucially depends on the boundary conditions.Comment: 16 pages, 6 figure
Emergence of chaotic behaviour in linearly stable systems
Strong nonlinear effects combined with diffusive coupling may give rise to
unpredictable evolution in spatially extended deterministic dynamical systems
even in the presence of a fully negative spectrum of Lyapunov exponents. This
regime, denoted as ``stable chaos'', has been so far mainly characterized by
numerical studies. In this manuscript we investigate the mechanisms that are at
the basis of this form of unpredictable evolution generated by a nonlinear
information flow through the boundaries. In order to clarify how linear
stability can coexist with nonlinear instability, we construct a suitable
stochastic model. In the absence of spatial coupling, the model does not reveal
the existence of any self-sustained chaotic phase. Nevertheless, already this
simple regime reveals peculiar differences between the behaviour of finite-size
and that of infinitesimal perturbations. A mean-field analysis of the truly
spatially extended case clarifies that the onset of chaotic behaviour can be
traced back to the diffusion process that tends to shift the growth rate of
finite perturbations from the quenched to the annealed average. The possible
characterization of the transition as the onset of directed percolation is also
briefly discussed as well as the connections with a synchronization transition.Comment: 30 pages, 8 figures, Submitted to Journal of Physics
Boltzmann-Gibbs thermal equilibrium distribution for classical systems and Newton law: A computational discussion
We implement a general numerical calculation that allows for a direct
comparison between nonlinear Hamiltonian dynamics and the Boltzmann-Gibbs
canonical distribution in Gibbs -space. Using paradigmatic
first-neighbor models, namely, the inertial XY ferromagnet and the
Fermi-Pasta-Ulam -model, we show that at intermediate energies the
Boltzmann-Gibbs equilibrium distribution is a consequence of Newton second law
(). At higher energies we discuss partial agreement
between time and ensemble averages.Comment: New title, revision of the text. EPJ latex, 4 figure
Characterizing dynamics with covariant Lyapunov vectors
A general method to determine covariant Lyapunov vectors in both discrete-
and continuous-time dynamical systems is introduced. This allows to address
fundamental questions such as the degree of hyperbolicity, which can be
quantified in terms of the transversality of these intrinsic vectors. For
spatially extended systems, the covariant Lyapunov vectors have localization
properties and spatial Fourier spectra qualitatively different from those
composing the orthonormalized basis obtained in the standard procedure used to
calculate the Lyapunov exponents.Comment: 4 pages, 3 figures, submitted to Physical Review letter
Chronic neural probe for simultaneous recording of single-unit, multi-unit, and local field potential activity from multiple brain sites
Drug resistant focal epilepsy can be treated by resecting the epileptic focus
requiring a precise focus localization using stereoelectroencephalography
(SEEG) probes. As commercial SEEG probes offer only a limited spatial
resolution, probes of higher channel count and design freedom enabling the
incorporation of macro and microelectrodes would help increasing spatial
resolution and thus open new perspectives for investigating mechanisms
underlying focal epilepsy and its treatment. This work describes a new
fabrication process for SEEG probes with materials and dimensions similar to
clinical probes enabling recording single neuron activity at high spatial
resolution. Polyimide is used as a biocompatible flexible substrate into which
platinum electrodes and leads are...
The resulting probe features match those of clinically approved devices.
Tests in saline solution confirmed the probe stability and functionality.
Probes were implanted into the brain of one monkey (Macaca mulatta), trained to
perform different motor tasks. Suitable configurations including up to 128
electrode sites allow the recording of task-related neuronal signals. Probes
with 32 and 64 electrode sites were implanted in the posterior parietal cortex.
Local field potentials and multi-unit activity were recorded as early as one
hour after implantation. Stable single-unit activity was achieved for up to 26
days after implantation of a 64-channel probe. All recorded signals showed
modulation during task execution. With the novel probes it is possible to
record stable biologically relevant data over a time span exceeding the usual
time needed for epileptic focus localization in human patients. This is the
first time that single units are recorded along cylindrical polyimide probes
chronically implanted 22 mm deep into the brain of a monkey, which suggests the
potential usefulness of this probe for human applications
Nonequilibrium dynamics of a stochastic model of anomalous heat transport: numerical analysis
We study heat transport in a chain of harmonic oscillators with random
elastic collisions between nearest-neighbours. The equations of motion of the
covariance matrix are numerically solved for free and fixed boundary
conditions. In the thermodynamic limit, the shape of the temperature profile
and the value of the stationary heat flux depend on the choice of boundary
conditions. For free boundary conditions, they also depend on the coupling
strength with the heat baths. Moreover, we find a strong violation of local
equilibrium at the chain edges that determine two boundary layers of size
(where is the chain length), that are characterized by a
different scaling behaviour from the bulk. Finally, we investigate the
relaxation towards the stationary state, finding two long time scales: the
first corresponds to the relaxation of the hydrodynamic modes; the second is a
manifestation of the finiteness of the system.Comment: Submitted to Journal of Physics A, Mathematical and Theoretica
Values, ideological attitudes and patriotism
We tested a series of discriminant associations, investigating how dimensions of patriotism (i.e. blind and constructive) differently relate to value orientations, and to ideological attitudes such as Right-Wing Authoritarianism (RWA) and Social Dominance Orientation (SDO). Using an Italian student sample (N = 146) we found that blind patriotism correlated positively with tradition and negatively with univer- salism, whilst constructive patriotism correlated negatively with tradition and positively with universal- ism. Both RWA and SDO correlated negatively with universalism, whilst only RWA was associated with security and tradition and only SDO related positively to power and self-direction. Mediation analyses revealed that most of the effects of value orientations on patriotism were mediated by SDO and RWA
Coherent Manipulation of Orbital Feshbach Molecules of Two-Electron Atoms
Ultracold molecules have experienced increasing attention in recent years.
Compared to ultracold atoms, they possess several unique properties that make
them perfect candidates for the implementation of new quantum-technological
applications in several fields, from quantum simulation to quantum sensing and
metrology. In particular, ultracold molecules of two-electron atoms (such as
strontium or ytterbium) also inherit the peculiar properties of these atomic
species, above all the possibility to access metastable electronic states via
direct excitation on optical clock transitions with ultimate sensitivity and
accuracy. In this paper we report on the production and coherent manipulation
of molecular bound states of two fermionic Yb atoms in different
electronic (orbital) states S and P in proximity of a
scattering resonance involving atoms in different spin and electronic states,
called orbital Feshbach resonance. We demonstrate that orbital molecules can be
coherently photoassociated starting from a gas of ground-state atoms in a
three-dimensional optical lattices by observing several photoassociation and
photodissociation cycles. We also show the possibility to coherently control
the molecular internal state by using Raman-assisted transfer to swap the
nuclear spin of one of the atoms forming the molecule, thus demonstrating a
powerful manipulation and detection tool of these molecular bound states.
Finally, by exploiting this peculiar detection technique we provide first
information on the lifetime of the molecular states in a many-body setting,
paving the way towards future investigations of strongly interacting Fermi
gases in a still unexplored regime.Comment: 11 pages, 8 figure
Synthetic dimensions and spin-orbit coupling with an optical clock transition
We demonstrate a novel way of synthesizing spin-orbit interactions in
ultracold quantum gases, based on a single-photon optical clock transition
coupling two long-lived electronic states of two-electron Yb atoms. By
mapping the electronic states onto effective sites along a synthetic
"electronic" dimension, we have engineered synthetic fermionic ladders with
tunable magnetic fluxes. We have detected the spin-orbit coupling with
fiber-link-enhanced clock spectroscopy and directly measured the emergence of
chiral edge currents, probing them as a function of the magnetic field flux.
These results open new directions for the investigation of topological states
of matter with ultracold atomic gases.Comment: Minor changes with respect to v1 (we have corrected some typos, fixed
the use of some mathematical symbols, added one reference
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