Nonsequential Double Recombination in Intense Laser Fields

A second plateau in the harmonic spectra of laser-driven two-electron atoms is observed both in the numerical solution of a low-dimensional model helium atom and using an extended strong field approximation. It is shown that the harmonics well beyond the usual cut-off are due to the simultaneous recombination of the two electrons, which were emitted during different, previous half-cycles. The new cut-off is explained in terms of classical trajectories. Classical predictions and the time-frequency analysis of the ab initio quantum results are in excellent agreement. The mechanism corresponds to the inverse single photon double ionization process in the presence of a (low frequency) laser field.Comment: 4 pages, RevTeX, v2 with an extended strong field approximation treatment of the process; instead, v1 describes an attosecond control scheme to enhance the proces

Discrete element model for brittle materials

We adopt the discrete element method (DEM) to study the fracture behavior of brittle materials. We propose an approach which relates crack initiation to crack growth. The material consists of a set of particles in contact, which allows us to derive an expression for the stress intensity factor as a function of the contact forces and displacements. A classical failure criterion, based on the material’s toughness, is then adopted for the analysis of crack propagation, represented by the loss of cohesion forces between particles. Afterwards, we apply our discrete criterion to uncracked materials under homogenous stress conditions, obtaining a Rankine like behavior. The work results in a simple discrete model which is totally compatible to continuum mechanics, where no calibration tests are required, in contrast to most of discrete approaches

Relativistic and retardation effects in the two--photon ionization of hydrogen--like ions

The non-resonant two-photon ionization of hydrogen-like ions is studied in second-order perturbation theory, based on the Dirac equation. To carry out the summation over the complete Coulomb spectrum, a Green function approach has been applied to the computation of the ionization cross sections. Exact second-order relativistic cross sections are compared with data as obtained from a relativistic long-wavelength approximation as well as from the scaling of non-relativistic results. For high-Z ions, the relativistic wavefunction contraction may lower the two-photon ionization cross sections by a factor of two or more, while retardation effects appear less pronounced but still give rise to non-negligible contributions.Comment: 6 pages, 2 figure

Enhancement of Josephson phase diffusion by microwaves

We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position $V_{\rm top}$ of the peak increases with the power $P$ of microwave radiation as $V_{\rm top}\propto\sqrt P$, while its current amplitude weakly decreases with $P$. As the microwave frequency increases, the peak feature evolves into Shapiro steps with finite slope. Our theoretical analysis taking into account the enhancement of incoherent superconducting current by multi-photon absorption is in good agreement with experimental data.Comment: 5 pages, 4 figure

The 'Fast' and 'Slow' Light Induced Defects in Diluted and Undiluted Hydrogenated Amorphous Silicon Solar Cells and Materials

International audienceStudies have been carried out on a-Si:H p-in solar cells and corresponding i-layer films fabricated with and without hydrogen dilution for kinetics with high intensity and 1 sun illuminations. The results show a striking similarity between the kinetics in the fill factors (FF) of the p-in solar cells and the mobility lifetime (µτ) products of the corresponding i layer films. New results are presented on thermal annealing after 10 sun degradation which further substantiate the presence of fast and slow defects in the light induced changes of a-Si:H materials, as do the degradation kinetics of both cells and films under 1 sun illumination to their degraded steady states (DSS). Initial (fast) and subsequent (slow) regimes approaching DSS are present at temperatures between 25°C and 100°C, with the two regimes having distinctly different dependences on temperature. The DSS in the films and cells improve monotonically with temperature whereas the initial regimes show a clear reversal in their temperature dependence between 40°C and 50°C. The inability to express these results of 1 sun kinetics with rate equations containing only single time constants for creation and annealing provides further evidence that more than one defect is responsible for light induced degradation in a-Si:H materials and solar cells