9,398 research outputs found
Influence of the Coulomb potential on above-threshold ionization: a quantum-orbit analysis beyond the strong-field approximation
We perform a detailed analysis of how the interplay between the residual
binding potential and a strong laser field influences above-threshold
ionization (ATI), employing a semi-analytical, Coulomb-corrected strong-field
approximation (SFA) in which the Coulomb potential is incorporated in the
electron propagation in the continuum. We find that the Coulomb interaction
lifts the degeneracy of some SFA trajectories, and we identify a set of orbits
which, for high enough photoelectron energies, may be associated with
rescattering. Furthermore, by performing a direct comparison with the standard
SFA, we show that several features in the ATI spectra can be traced back to the
influence of the Coulomb potential on different electron trajectories. These
features include a decrease in the contrast, a shift towards lower energies in
the interference substructure, and an overall increase in the photoelectron
yield. All features encountered exhibit a very good agreement with the \emph{ab
initio} solution of the time-dependent Schr\"odinger equation.Comment: 12 pages, 10 figure
Formation and interaction of resonance chains in the open 3-disk system
In ballistic open quantum systems one often observes that the resonances in
the complex-energy plane form a clear chain structure. Taking the open 3-disk
system as a paradigmatic model system, we investigate how this chain structure
is reflected in the resonance states and how it is connected to the underlying
classical dynamics. Using an efficient scattering approach we observe that
resonance states along one chain are clearly correlated while resonance states
of different chains show an anticorrelation. Studying the phase space
representations of the resonance states we find that their localization in
phase space oscillate between different regions of the classical trapped set as
one moves along the chains and that these oscillations are connected to a
modulation of the resonance spacing. A single resonance chain is thus no WKB
quantization of a single periodic orbits, but the structure of several
oscillating chains arises from the interaction of several periodic orbits. We
illuminate the physical mechanism behind these findings by combining the
semiclassical cycle expansion with a quantum graph model.Comment: 25 pages, 15 figure
Selective enhancement of topologically induced interface states in a dielectric resonator chain
The recent realization of topological phases in insulators and
superconductors has advanced the quest for robust quantum technologies. The
prospects to implement the underlying topological features controllably has
given incentive to explore optical platforms for analogous realizations. Here
we realize a topologically induced defect state in a chain of dielectric
microwave resonators and show that the functionality of the system can be
enhanced by supplementing topological protection with non-hermitian symmetries
that do not have an electronic counterpart. We draw on a characteristic
topological feature of the defect state, namely, that it breaks a sublattice
symmetry. This isolates the state from losses that respect parity-time
symmetry, which enhances its visibility relative to all other states both in
the frequency and in the time domain. This mode selection mechanism naturally
carries over to a wide range of topological and parity-time symmetric optical
platforms, including couplers, rectifiers and lasers.Comment: 5 pages, 4 figures, + supplementary information (3 pages, 4 figures
Effect of turbulence on electron cyclotron current drive and heating in ITER
Non-linear local electromagnetic gyrokinetic turbulence simulations of the
ITER standard scenario H-mode are presented for the q=3/2 and q=2 surfaces. The
turbulent transport is examined in regions of velocity space characteristic of
electrons heated by electron cyclotron waves. Electromagnetic fluctuations and
sub-dominant micro-tearing modes are found to contribute significantly to the
transport of the accelerated electrons, even though they have only a small
impact on the transport of the bulk species. The particle diffusivity for
resonant passing electrons is found to be less than 0.15 m^2/s, and their heat
conductivity is found to be less than 2 m^2/s. Implications for the broadening
of the current drive and energy deposition in ITER are discussed.Comment: Letter, 5 pages, 5 figures, for submission to Nuclear Fusio
Study of the spectral properties of ELM precursors by means of wavelets
The high confinement regime (H-mode) in tokamaks is accompanied by the occurrence of bursts of MHD activity at the plasma edge, so-called edge localized modes (ELMs), lasting less than 1 ms. These modes are often preceded by coherent oscillations in the magnetic field, the ELM precursors, whose mode numbers along the toroidal and the poloidal directions can be measured from the phase shift between Mirnov pickup coils. When the ELM precursors have a lifetime shorter than a few milliseconds, their toroidal mode number and their nonlinear evolution before the ELM crash cannot be studied reliably with standard techniques based on Fourier analysis, since averaging in time is implicit in the computation of the Fourier coefficients. This work demonstrates significant advantages in studying spectral features of the short-lived ELM precursors by using Morlet wavelets. It is shown that the wavelet analysis is suitable for the identification of the toroidal mode numbers of ELM precursors with the shortest lifetime, as well as for studying their nonlinear evolution with a time resolution comparable to the acquisition rate of the Mirnov coils
Mobility and Legal Migration in the Context of the European Neighbourbood Policy: What Role for the European Union?
The paper examines the evolution of the external dimension of EU immigration policy mainly in the context of European Neighbourhood policy (ENP). The EU is interested in countering irregular migration from Southern neighbours rather than in facilitating legal migration for third country nationals. The latter field falls within the competence of the EU Member States and they are reluctant to open legal migration channels. Only recently Mobility Partnerships (MPs) have
been concluded with Morocco, Tunisia and Jordan. Most recently, the EU has decided to financially support countries of origin/transit in order to strengthen their capacity to tackle migration pressure; migration compacts will be negotiated
not only with Southern neighbours but also with other African countries. In contrast to Southern neighbours, MPs were agreed with all Eastern neighbours, except Belarus; readmission agreements and visa-free agreements were concluded
with three Eastern neighbour countries: Ukraine, Georgia and Moldova. The EU has fostered mobility and people-to-people movements in its relations with Eastern neighbours but the added value of EU legislation designed to favour legal migration
and integration of third country nationals remain limited. The conclusion is that the EU continues to be a «fortress», especially for low-skilled migrants seeking an employment
Quasimodes of a chaotic elastic cavity with increasing local losses
We report non-invasive measurements of the complex field of elastic
quasimodes of a silicon wafer with chaotic shape. The amplitude and phase
spatial distribution of the flexural modes are directly obtained by Fourier
transform of time measurements. We investigate the crossover from real mode to
complex-valued quasimode, when absorption is progressively increased on one
edge of the wafer. The complexness parameter, which characterizes the degree to
which a resonance state is complex-valued, is measured for non-overlapping
resonances and is found to be proportional to the non-homogeneous contribution
to the line broadening of the resonance. A simple two-level model based on the
effective Hamiltonian formalism supports our experimental results
Statistics of eigenfunctions in open chaotic systems: a perturbative approach
We investigate the statistical properties of the complexness parameter which
characterizes uniquely complexness (biorthogonality) of resonance eigenstates
of open chaotic systems. Specifying to the regime of isolated resonances, we
apply the random matrix theory to the effective Hamiltonian formalism and
derive analytically the probability distribution of the complexness parameter
for two statistical ensembles describing the systems invariant under time
reversal. For those with rigid spectra, we consider a Hamiltonian characterized
by a picket-fence spectrum without spectral fluctuations. Then, in the more
realistic case of a Hamiltonian described by the Gaussian Orthogonal Ensemble,
we reveal and discuss the r\^ole of spectral fluctuations
Augmenting Group Performance in Target-Face Recognition via Collaborative Brain-Computer Interfaces for Surveillance Applications
This paper presents a hybrid collaborative brain- computer interface (cBCI) to improve group-based recognition of target faces in crowded scenes recorded from surveillance cameras. The cBCI uses a combination of neural features extracted from EEG and response times to estimate the decision confidence of the users. Group decisions are then obtained by weighing individual responses according to these confidence estimates. Results obtained with 10 participants indicate that the proposed cBCI improves decision errors by up to 7% over traditional group decisions based on majority. Moreover, the confidence estimates obtained by the cBCI are more accurate and robust than the confidence reported by the participants after each decision. These results show that cBCIs can be an effective means of human augmentation in realistic scenarios
CES-530: Collaborative Brain-Computer Interface for Aiding Decision-making
We look at the possibility of integrating the percepts from multiple non-communicating observers as a means of achieving better joint perception and better group decisions. Our approach involves the combination of a brain-computer interface with human behavioural responses. To test ideas in controlled conditions, we asked observers to perform a simple matching task involving the rapid sequential presentation of pairs of visual patterns and the subsequent decision as whether the two patterns in a pair were the same or different. We recorded the response times of observers as well as a neural feature which predicts incorrect decisions and, thus, indirectly indicates the confidence of the decisions made by the observers. We then built a composite neuro-behavioural feature which optimally combines the two measures. For group decisions, we uses a majority rule and three rules which weigh the decisions of each observer based on response times and our neural and neuro-behavioural features. Results indicate that the integration of behavioural responses and neural features can significantly improve accuracy when compared with the majority rule. An analysis of event-related potentials indicates that substantial differences are present in the proximity of the response for correct and incorrect trials, further corroborating the idea of using hybrids of brain-computer interfaces and traditional strategies for improving decision making
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