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Facing the market in North Africa
No full textThis article examines the results of economic reform programs since the mid-1980s in Morocco, Algeria, Tunisia, and Egypt. Although these states have liberalized their economies in the face of international and domestic market forces, ruling elites have been adept at maintaining control over the distribution of resources. Selective reforms have prevented the emergence of competitive markets and powerful, autonomous private sectors and have yet to induce a transition to political liberalism and accountable government in North Africa
On the generation of ELF/VLF waves for long-distance propagation via steerable HF heating of the lower ionosphere
No full textELF/VLF radio waves (300 Hz to 30 kHz) have been successfully generated via modulated HF (3-10 MHz) heating of the lower ionosphere in the presence of natural currents, most recently with the HAARP facility in Alaska. Generation is possible via amplitude modulation or via two techniques involving motion of the HF beam during the ELF/VLF cycle, known as beam painting and geometric modulation, described and measured by Cohen et al. (2010b). In this paper, we describe a theoretical model describing the HF heating and ionospheric responses, followed by a full-wave calculation of ELF/VLF propagation, and utilize this end-to-end model to derive the predicted radiated ELF/VLF pattern up to 1000 km from the HF heater in the Earth-ionosphere waveguide. We quantitatively compare the generated ELF/VLF signals on the ground from various generation techniques and find it to be generally in agreement with earlier measurements. We apply a simplified ELF/VLF propagation model to quantify the contribution of the ELF/VLF phased array in the radiation pattern resulting from geometric modulation and find this contribution to be significant. We also use a limited HF heating model to quantify the degree to which the current power level of HAARP is sufficient for the beam painting technique, since this technique requires high HF power densities at high altitudes
Dye lasing in optically manipulated liquid aerosols
No full textWe present dye lasing from optically manipulated glycerol-water aerosols with diameters ranging between 7.7 and 11.0 mu m confined in optical tweezers. While being optically trapped near the focal point of an infrared laser, the droplets stained with Rhodamine B were pumped with a Q-switched green laser and their fluorescence emission spectra featuring whispering gallery modes (WGMs) were recorded with a spectrograph. Nonlinear dependence of the intensity of the droplet WGMs on the pump laser fluence indicates dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by adjusting the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium
Dynamical properties of a coupled nonlinear dielectric waveguide-surface-plasmon system as another type of Josephson junction
No full textWe demonstrate that a weakly-coupled nonlinear dielectric waveguide surface-plasmon (DWSP-JJ) system can be formulated in analogy to bosonic Josephson junction of atomic condensates at very low temperatures, yet it exhibits different dynamical features. Such a system can be realized along a metal - dielectric interface where the dielectric medium hosts a nonlinear waveguide (e. g. fiber) for soliton propagation. The inherently dynamic coupling parameter generates novel features in the phase space
Dimer-atom scattering between two identical fermions and a third particle
No full textWe use the diagrammatic T-matrix approach to wanalyze the three-body scattering problem between two identical fermions and a third particle (which could be a different species of fermion or a boson). We calculate the s-wave dimer-atom scattering length for all mass ratios, and our results exactly match the results of Petrov. In particular, we list the exact dimer-atom scattering lengths for all available two-species Fermi-Fermi and Bose-Fermi mixtures. In addition, unlike that of the equal-mass-particles case where the three-body scattering T matrix decays monotonically as a function of the outgoing momentum, we show that, after an initial rapid drop, this function changes sign and becomes negative at large momenta and then decays slowly to zero when the mass ratio of the fermions to the third particle is higher than a critical value (around 6.5). As the mass ratio gets higher, modulations of the T matrix become more apparent with multiple sign changes, related to the "fall of a particle to the center" phenomenon and to the emergence of three-body Efimov bound states
Optimal control of molecular motion expressed through quantum fluid dynamics
No full textA quantum fluid-dynamic (QFD) control formulation is presented for optimally manipulating atomic and molecular systems. In QFD the control quantum system is expressed in terms of the probability density rho and the quantum current j. This choice of variables is motivated by the generally expected slowly varying spatial-temporal dependence of the fluid-dynamical variables. The QFD approach is illustrated for manipulation of the ground electronic state dynamics of HCl induced by an external electric field
Optical spectral singularities as threshold resonances
No full textSpectral singularities are among generic mathematical features of complex scattering potentials. Physically they correspond to scattering states that behave like zero-width resonances. For a simple optical system, we show that a spectral singularity appears whenever the gain coefficient coincides with its threshold value and other parameters of the system are selected properly. We explore a concrete realization of spectral singularities for a typical semiconductor gain medium and propose a method of constructing a tunable laser that operates at threshold gain
Trapped Yb-173 Fermi gas across an orbital Feshbach resonance
No full textStarting with the two-band description of an orbital Feshbach resonance, we study superfluid properties of a trapped Yb-173 Fermi gas under the assumptions of a local-density approximation for the trapping potential and a mean-field approximation for the intraband Cooper pairings. In particular, we investigate the competition and interplay between the pair-breaking effect that is caused by the interband detuning energy, and the pair-breaking and thermal-broadening effects that are simultaneously caused by the temperature. We predict several experimental signatures that are directly caused by this interplay including a spatial separation of superfluid and normal phases within the trap, and could play decisive roles in probing two-band superfluidity in these systems
Composition-induced structural transitions in mixed rare-gas clusters
No full textThe low-energy structures of mixed Ar-Xe and Kr-Xe Lennard-Jones clusters are investigated using a newly developed parallel Monte Carlo minimization algorithm with specific exchange moves between particles or trajectories. Tests on the 13- and 19-atom clusters show a significant improvement over the conventional basin-hopping method, the average search length being reduced by more than one order of magnitude. The method is applied to the more difficult case of the 38-atom cluster, for which the homogeneous clusters have a truncated octahedral shape. It is found that alloys of dissimilar elements (Ar-Xe) favor polytetrahedral geometries over octahedra due to the reduced strain penalty. Conversely, octahedra are even more stable in Kr-Xe alloys than in Kr-38 or Xe-38, and they show a core-surface phase separation behavior. These trends are indeed also observed and further analysed on the 55-atom cluster. Finally, we correlate the relative stability of cubic structures in these clusters to the glass forming character of the bulk mixtures
Imaginary-scaling versus indefinite-metric quantization of the Pais-Uhlenbeck oscillator
No full textUsing the Pais-Uhlenbeck oscillator as a toy model, we outline a consistent alternative to the indefinite-metric quantization scheme that does not violate unitarity. We describe the basic mathematical structure of this method by giving an explicit construction of the Hilbert space of state vectors and the corresponding creation and annihilation operators. The latter satisfy the usual bosonic commutation relation and differ from those of the indefinite-metric theories by a sign in the definition of the creation operator. This change of sign achieves a definitization of the indefinite metric that gives life to the ghost states without changing their contribution to the energy spectrum