30 research outputs found
Nonlinear localized flatband modes with spin-orbit coupling
We report the coexistence and properties of stable compact localized states
(CLSs) and discrete solitons (DSs) for nonlinear spinor waves on a flatband
network with spin-orbit coupling (SOC). The system can be implemented by means
of a binary Bose-Einstein condensate loaded in the corresponding optical
lattice. In the linear limit, the SOC opens a minigap between flat and
dispersive bands in the system's bandgap structure, and preserves the existence
of CLSs at the flatband frequency, simultaneously lowering their symmetry.
Adding onsite cubic nonlinearity, the CLSs persist and remain available in an
exact analytical form, with frequencies which are smoothly tuned into the
minigap. Inside of the minigap, the CLS and DS families are stable in narrow
areas adjacent to the FB. Deep inside the semi-infinite gap, both the CLSs and
DSs are stable too.Comment: 10 figures, Physical Review B, in pres
Dynamic polarization of graphene by moving external charges: random phase approximation
We evaluate the stopping and image forces on a charged particle moving
parallel to a doped sheet of graphene by using the dielectric response
formalism for graphene's -electron bands in the random phase approximation
(RPA). The forces are presented as functions of the particle speed and the
particle distance for a broad range of charge-carrier densities in graphene. A
detailed comparison with the results from a kinetic equation model reveal the
importance of inter-band single-particle excitations in the RPA model for high
particle speeds. We also consider the effects of a finite gap between graphene
and a supporting substrate, as well as the effects of a finite damping rate
that is included through the use of Mermin's procedure. The damping rate is
estimated from a tentative comparison of the Mermin loss function with a HREELS
experiment. In the limit of low particle speeds, several analytical results are
obtained for the friction coefficient that show an intricate relationship
between the charge-carrier density, the damping rate, and the particle
distance, which may be relevant to surface processes and electrochemistry
involving graphene.Comment: 14 pages, 10 figures, accepted for publication in Phys. Rev.
Models of spin-orbit coupled oligomers
We address the stability and dynamics of eigenmodes in linearly-shaped
strings (dimers, trimers, tetramers, and pentamers) built of droplets of a
binary Bose-Einstein condensate (BEC). The binary BEC is composed of atoms in
two pseudo-spin states with attractive interactions, dressed by properly
arranged laser fields, which induce the (pseudo-) spin-orbit (SO) coupling. We
demonstrate that the SO-coupling terms help to create eigenmodes of particular
types in the strings. Dimer, trimer, and pentamer eigenmodes of the linear
system, which correspond to the zero eigenvalue (EV, alias chemical potential)
extend into the nonlinear ones, keeping an exact analytical form, while
tetramers do not admit such a continuation, because the respective spectrum
does not contain a zero EV. Stability areas of these modes shrink with the
increasing nonlinearity. Besides these modes, other types of nonlinear states,
which are produced by the continuation of their linear counterparts
corresponding to some nonzero EVs, are found in a numerical form (including
ones for the tetramer system). They are stable in nearly entire existence
regions in trimer and pentamer systems, but only in a very small area for the
tetramers. Similar results are also obtained, but not displayed in detail, for
hexa- and septamers.Comment: Chaos, in pres
Interface solitons in locally linked two-dimensional lattices
Existence, stability and dynamics of soliton complexes, centered at the site
of a single transverse link connecting two parallel 2D (two-dimensional)
lattices, are investigated. The system with the on-site cubic self-focusing
nonlinearity is modeled by the pair of discrete nonlinear Schr\"{o}dinger
equations linearly coupled at the single site. Symmetric, antisymmetric and
asymmetric complexes are constructed by means of the variational approximation
(VA) and numerical methods. The VA demonstrates that the antisymmetric soliton
complexes exist in the entire parameter space, while the symmetric and
asymmetric modes can be found below a critical value of the coupling parameter.
Numerical results confirm these predictions. The symmetric complexes are
destabilized via a supercritical symmetry-breaking pitchfork bifurcation, which
gives rise to stable asymmetric modes. The antisymmetric complexes are subject
to oscillatory and exponentially instabilities in narrow parametric regions. In
bistability areas, stable antisymmetric solitons coexist with either symmetric
or asymmetric ones.Comment: 9 figure
On Bright and Dark Breathers in Lattices with Saturable Nonlinearity
The moving bright and dark localized modes in one-dimensional optical lattices with saturable nonlinearity are considered with respect to the grand canonical free energy concept and linear stability analysis of the eigenvalue spectra
Extreme events in discrete nonlinear lattices
We perform statistical analysis on discrete nonlinear waves generated though
modulational instability in the context of the Salerno model that interpolates
between the intergable Ablowitz-Ladik (AL) equation and the nonintegrable
discrete nonlinear Schrodinger (DNLS) equation. We focus on extreme events in
the form of discrete rogue or freak waves that may arise as a result of rapid
coalescence of discrete breathers or other nonlinear interaction processes. We
find power law dependence in the wave amplitude distribution accompanied by an
enhanced probability for freak events close to the integrable limit of the
equation. A characteristic peak in the extreme event probability appears that
is attributed to the onset of interaction of the discrete solitons of the AL
equation and the accompanied transition from the local to the global
stochasticity monitored through the positive Lyapunov exponent of a nonlinear
map.Comment: 5 pages, 4 figures; reference added, figure 2 correcte
Surface solitons in trilete lattices
Fundamental solitons pinned to the interface between three semi-infinite
one-dimensional nonlinear dynamical chains, coupled at a single site, are
investigated. The light propagation in the respective system with the
self-attractive on-site cubic nonlinearity, which can be implemented as an
array of nonlinear optical waveguides, is modeled by the system of three
discrete nonlinear Schr\"{o}dinger equations. The formation, stability and
dynamics of symmetric and asymmetric fundamental solitons centered at the
interface are investigated analytically by means of the variational
approximation (VA) and in a numerical form. The VA predicts that two asymmetric
and two antisymmetric branches exist in the entire parameter space, while four
asymmetric modes and the symmetric one can be found below some critical value
of the inter-lattice coupling parameter -- actually, past the symmetry-breaking
bifurcation. At this bifurcation point, the symmetric branch is destabilized
and two new asymmetric soliton branches appear, one stable and the other
unstable. In this area, the antisymmetric branch changes its character, getting
stabilized against oscillatory perturbations. In direct simulations, unstable
symmetric modes radiate a part of their power, staying trapped around the
interface. Highly unstable asymmetric modes transform into localized breathers
traveling from the interface region across the lattice without significant
power loss.Comment: Physica D in pres
Real-time chest-wall-motion tracking by a single optical fibre grating:a prospective method for ventilator triggering
Objective: The ventilators involved in non-invasive mechanical ventilation commonly provide ventilator support via a facemask. The interface of the mask with a patient promotes air leaks that cause errors in the feedback information provided by a pneumatic sensor and hence patient-ventilator asynchrony with multiple negative consequences. Our objective is to test the possibility of using chest-wall motion measured by an optical fibre-grating sensor as a more accurate non-invasive ventilator triggering mechanism. Approach: The basic premise of our approach is that the measurement accuracy can be improved by using a triggering signal that precedes pneumatic triggering in the neuro-ventilatory coupling sequence. We propose a technique that uses the measurement of chest-wall curvature by a long-period fibre-grating sensor. The sensor was applied externally to the rib-cage and interrogated in the lateral (edge) filtering scheme. The study was performed on 34 healthy volunteers. Statistical data analysis of the time lag between the fibre-grating sensor and the reference pneumotachograph was preceded by the removal of the unwanted heartbeat signal by wavelet transform processing. Main results: The results show a consistent fibre-grating signal advance with respect to the standard pneumatic signal by (230 ± 100) ms in both the inspiratory and expiratory phases. We further show that heart activity removal yields a tremendous improvement in sensor accuracy by reducing it from 60 ml to 0.3 ml. Significance: The results indicate that the proposed measurement technique may lead to a more reliable triggering decision. Its imperviousness to air leaks, non-invasiveness, low-cost and ease of implementation offer good prospects for applications in both clinical and homecare ventilation
Extreme events in two dimensional disordered nonlinear lattices
Spatiotemporal complexity is induced in a two dimensional nonlinear
disordered lattice through the modulational instability of an initially weakly
perturbed excitation. In the course of evolution we observe the formation of
transient as well as persistent localized structures, some of which have
extreme magnitude. We analyze the statistics of occurrence of these extreme
collective events and find that the appearance of transient extreme events is
more likely in the weakly nonlinear regime. We observe a transition in the
extreme events recurrence time probability from exponential, in the
nonlinearity dominated regime, to power law for the disordered one.Comment: 5 figures, 5 page
ECG derived feature combination versus single feature in predicting defibrillation success in out-of-hospital cardiac arrested patients
Objective: Algorithms to predict shock outcome based on ventricular fibrillation (VF) waveform features are potentially useful tool to optimize defibrillation strategy (immediate defibrillation versus cardiopulmonary resuscitation). Researchers have investigated numerous predictive features and classification methods using single VF feature and/or their combinations, however reported predictabilities are not consistent. The purpose of this study was to validate whether combining VF features can enhance the prediction accuracy in comparison to single feature. Approach: The analysis was performed in 3 stages: feature extraction, preprocessing and feature selection and classification. Twenty eight predictive features were calculated on 4s episode of the pre-shock VF signal. The preprocessing included instances normalization and oversampling. Seven machine learning algorithms were employed for selecting the best performin single feature and combination of features using wrapper method: Logistic Regression (LR), Naïve-Bayes (NB), Decision tree (C4.5), AdaBoost.M1 (AB), Support Vector Machine (SVM), Nearest Neighbour (NN) and Random Forest (RF). Evaluation of the algorithms was performed by nested 10 fold cross-validation procedure. Main results: A total of 251 unbalanced first shocks (195 unsuccessful and 56 successful) were oversampled to 195 instances in each class. Performance metric based on average accuracy of feature combination has shown that LR and NB exhibit no improvement, C4.5 and AB an improvement not greater than 1% and SVM, NN and RF an improvement greater than 5% in predicting defibrillation outcome in comparison to the best single feature. Significance: By performing wrapper method to select best performing feature combination the non-linear machine learning strategies (SVM, NN, RF) can improve defibrillation prediction performance