Two-dimensional frustrated spin systems in high magnetic fields

We discuss our numerical results on the properties of the S = 1/2 frustrated J1-J2 Heisenberg model on a square lattice as a function of temperature and frustration angle phi = atan(J2/J1) in an applied magnetic field. We cover the full phase diagram of the model in the range -pi <= phi <= pi. The discussion includes the parameter dependence of the saturation field itself, and addresses the instabilities associated with it. We also discuss the magnetocaloric effect of the model and show how it can be used to uniquely determine the effective interaction constants of the compounds which were investigated experimentally.Comment: 4 pages, 5 figures, proceedings of RHMF 200

Toward a numerical deshaker for PFS

The Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEx) is the instrument with the highest spectral resolution observing Mars from orbit since January 2004. It permits studying the atmospheric structure, major and minor compounds. The present time version of the calibration is limited by the effects of mechanical vibration, currently not corrected. We proposed here a new approach to correct for the vibrations based on semi-blind deconvolution of the measurements. This new approach shows that a correction can be done efficiently with 85% reduction of the artefacts, in a equivalent manner to the stacking of 10 spectra. Our strategy is not fully automatic due to the dependence on some regularisation parameters. It may be applied on the complete PFS dataset, correcting the large-scale perturbation due to microvibrations for each spectrum independently. This approach is validated on actual PFS data of Short Wavelength Channel (SWC), perturbed by microvibrations. A coherence check can be performed and also validate our approach. Unfortunately, the coherence check can be done only on the first 310 orbits of MEx only, until the laser line has been switch off. More generally, this work may apply to numerically "deshake" Fourier Transform Spectrometer (FTS), widely used in space experiments or in the laboratory.Comment: 18 pages, 8 figures, submitted to Planetary and Space Scienc

State Selection in Non-Resonantly Excited Wave Packets by Tuning from Non-Adiabatic to Adiabatic Interaction

We show for rotational alignment of diatomic molecules that the crossover from non-adiabatic to adiabatic limits is well described by a convolution of excitation pulse envelope and sinusoidal molecular response and that it takes place in a uniform way in the region between 0.1 and 1 for the ratio of pulse duration to rotational period. In a non-resonant Raman-type excitation, this crossover is used to manipulate the J composition of a rotational wave packet with respect to the initial thermal distribution. By optimizing the duration of a single pulse, arbitrarily narrow distributions at low J levels can be formed. A double pulse excitation, where a longer second pulse acts as a selective dump pulse, allows to prepare non-thermal distributions centered at high J values. With the alignment signal on top of an isotropic background, experimental techniques sensitive to the induced anisotropy are optimally suited for implementation. To demonstrate the efficiency of the method, numerical simulations are carried out for rotational alignment in 14N2 at various experimentally relevant laser intensities. The scheme is transferable to quantum systems with a significant variation of transition frequencies between subsequent levels

Laser-induced alignment and anti-alignment of rotationally excited molecules

We numerically investigate the post-pulse alignment of rotationally excited diatomic molecules upon nonresonant interaction with a linearly polarized laser pulse. In addition to the simulations, we develop a simple model which qualitatively describes the shape and amplitude of post-pulse alignment induced by a laser pulse of moderate power density. In our treatment we take into account that molecules in rotationally excited states can interact with a laser pulse not only by absorbing energy but also by stimulated emission. The extent to which these processes are present in the interaction depends, on the one hand, on the directionality of the molecular angular momentum (given by the M quantum number), and on the other hand on the ratio of transition frequencies and pulse duration (determined by the J number). A rotational wave packet created by a strong pulse from an initially pure state contains a broad range of rotational levels, over which the character of the interaction can change from non-adiabatic to adiabatic. Depending on the laser pulse duration and amplitude, the transition from the non-adiabatic to the adiabatic limit proceeds through a region with dominant rotational heating, or alignment, for short pulses and a large region with rotational cooling, and correspondingly preferred anti-alignment, for longer pulses

Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission

Peripheral collisions with radioactive heavy-ion beams at relativistic energies are discussed as an innovative approach for probing the transient regime experienced by fissile systems evolving towards quasi-equilibrium. A dedicated experiment using the advanced technical installations of GSI, Darmstadt, permitted to realize ideal conditions for the investigation of relaxation effects in the meta-stable well. Combined with a highly sensitive experimental signature, it provides a measure of the transient effects with respect to the flux over the fission barrier. Within a two-step reaction process, 45 proton-rich unstable spherical isotopes produced by projectile-fragmentation of a stable 238U beam have been used as secondary projectiles. The fragmentation of the radioactive projectiles on lead results in nearly spherical compound nuclei which span a wide range in excitation energy and fissility. The decay of these excited systems by fission is studied with a dedicated set-up which permits the detection of both fission products in coincidence and the determination of their atomic numbers with high resolution. The width of the fission-fragment nuclear charge distribution is shown to be specifically sensitive to pre-saddle transient effects and is used to establish a clock for the passage of the saddle point. The comparison of the experimental results with model calculations points to a fission delay of (3.3+/-0.7).10-21s for initially spherical compound nuclei, independent of excitation energy and fissility. This value suggests a nuclear dissipation strength at small deformation of (4.5+/-0.5).1021s-1. The very specific combination of the physics and technical equipment exploited in this work sheds light on previous controversial conclusions.Comment: 38 pages, 15 figure

Computer mapping of LANDSAT data for environmental applications

The author has identified the following significant results. Land cover overlays and maps produced from LANDSAT are providing information on existing land use and resources throughout the 208 study area. The overlays are being used to delineate drainage areas of a predominant land cover type. Information on cover type is also being combined with other pertinent data to develop estimates of sediment and nutrients flows from the drainage area. The LANDSAT inventory of present land cover together with population projects is providing a basis for developing maps of anticipated land use patterns required to evaluate impact on water quality which may result from these patterns. Overlays of forest types were useful for defining wildlife habitat and vegetational resources in the region. LANDSAT data and computer assisted interpretation was found to be a rapid cost effective procedure for inventorying land cover on a regional basis. The entire 208 inventory which include acquisition of ground truth, LANDSAT tapes, computer processing, and production of overlays and coded tapes was completed within a period of 2 months at a cost of about 0.6 cents per acre, a significant improvement in time and cost over conventional photointerpretation and mapping techniques

Quantum effects with an X-ray free electron laser

A quantum kinetic equation coupled with Maxwell's equation is used to estimate the laser power required at an XFEL facility to expose intrinsically quantum effects in the process of QED vacuum decay via spontaneous pair production. A 9 TW-peak XFEL laser with photon energy 8.3 keV could be sufficient to initiate particle accumulation and the consequent formation of a plasma of spontaneously produced pairs. The evolution of the particle number in the plasma will exhibit non-Markovian aspects of the strong-field pair production process and the plasma's internal currents will generate an electric field whose interference with that of the laser leads to plasma oscillations.Comment: 4 pages, LaTeX2