Detailed analytical investigation of magnetic field line random walk in turbulent plasmas: II. Isotropic turbulence

The random walk of magnetic field lines in the presence of magnetic turbulence in plasmas is investigated from first principles. An isotropic model is employed for the magnetic turbulence spectrum. An analytical investigation of the asymptotic behavior of the field-line mean-square displacement is carried out. in terms of the position variable z. It is shown that varies as similar to z ln z for large distance z. This result corresponds to a superdiffusive behavior of field line wandering. This investigation complements previous work, which relied on a two-component model for the turbulence spectrum. Contrary to that model, quasilinear theory appears to provide an adequate description of the field line random walk for isotropic turbulence.

Evaluation of Random Delay Insertion against DPA on FPGAs

Side-channel attacks (SCA) threaten electronic cryptographic devices and can be carried out by monitoring the physical characteristics of security circuits. Differential Power Analysis (DPA) is one the most widely studied side-channel attacks. Numerous countermeasure techniques, such as Random Delay Insertion (RDI), have been proposed to reduce the risk of DPA attacks against cryptographic devices. The RDI technique was first proposed for microprocessors but it was shown to be unsuccessful when implemented on smartcards as it was vulnerable to a variant of the DPA attack known as the Sliding-Window DPA attack.Previous research by the authors investigated the use of the RDI countermeasure for Field Programmable Gate Array (FPGA) based cryptographic devices. A split-RDI technique wasproposed to improve the security of the RDI countermeasure. A set of critical parameters wasalso proposed that could be utilized in the design stage to optimize a security algorithm designwith RDI in terms of area, speed and power. The authors also showed that RDI is an efficientcountermeasure technique on FPGA in comparison to other countermeasures.In this article, a new RDI logic design is proposed that can be used to cost-efficiently implementRDI on FPGA devices. Sliding-Window DPA and realignment attacks, which were shown to beeffective against RDI implemented on smartcard devices, are performed on the improved RDIFPGA implementation. We demonstrate that these attacks are unsuccessful and we also proposea realignment technique that can be used to demonstrate the weakness of RDI implementations.

Increasing entanglement through engineered disorder in the random Ising chain

The ground-state entanglement entropy between block of sites in the random Ising chain is studied by means of the Von Neumann entropy. We show that in presence of strong correlations between the disordered couplings and local magnetic fields the entanglement increases and becomes larger than in the ordered case. The different behavior with respect to the uncorrelated disordered model is due to the drastic change of the ground state properties. The same result holds also for the random three-state quantum Potts model.

Observational tests of interstellar methanol formation

Context. It has been established that the classical gas-phase production of interstellar methanol (CH3OH) cannot explain observed abundances. Instead it is now generally thought that the main formation path has to be by successive hydrogenation of solid CO on interstellar grain surfaces. Aims. While theoretical models and laboratory experiments show that methanol is efficiently formed from CO on cold grains, our aim is to test this scenario by astronomical observations of gas associated with young stellar objects (YSOs). Methods. We have observed the rotational transition quartets J = 2K – 1K of 12CH3OH and 13CH3OH at 96.7 and 94.4 GHz, respectively, towards a sample of massive YSOs in different stages of evolution. In addition, the J = 1-0 transitions of 12C18O and 13C18O were observed towards some of these sources. We use the 12C/13C ratio to discriminate between gas-phase and grain surface origin: If methanol is formed from CO on grains, the ratios should be similar in CH3OH and CO. If not, the ratio should be higher in CH3OH due to 13C fractionation in cold CO gas. We also estimate the abundance ratios between the nuclear spin types of methanol (E and A). If methanol is formed on grains, this ratio is likely to have been thermalized at the low physical temperature of the grain, and therefore show a relative over-abundance of A-methanol. Results. We show that the 12C/13C isotopic ratio is very similar in gas-phase CH3OH and C18O, on the spatial scale of about 40 arcsec, towards four YSOs. For two of our sources we find an overabundance of A-methanol as compared to E-methanol, corresponding to nuclear spin temperatures of 10 and 16 K. For the remaining five sources, the methanol E/A ratio is less than unity. Conclusions. While the 12C/13C ratio test is consistent with methanol formation from hydrogenation of CO on grain surfaces, the result of the E/A ratio test is inconclusive.

Combined antenna and localized plasmon resonance in Raman scattering from Random arrays of silver-coated, vertically aligned multiwalled carbon nanotubes

The electric field enhancement associated with detailed structure within novel optical antenna nanostructures is modeled using the surface integral equation technique in the context of surface-enhanced Raman scattering (SERS). The antennae comprise random arrays of vertically aligned, multi-walled carbon nanotubes dressed with highly granular Ag. Different types of "hot-spot" underpinning the SERS are identified, but contrasting characteristics are revealed. Those at the outer edges of the Ag grains are antenna driven with field enhancement amplified in antenna antinodes while intergrain hotspots are largely independent of antenna activity. Hot-spots between the tops of antennae leaning towards each other also appear to benefit from antenna amplification.

The relationship between agricultural intensification and biological control: experimental tests across Europe

Agricultural intensification can affect biodiversity and related ecosystem services such as biological control, but large-scale experimental evidence is missing. We examined aphid pest populations in cereal fields under experimentally reduced densities of (1) ground-dwelling predators (-G), (2) vegetation-dwelling predators and parasitoids (-V), (3) a combination of (1) and (2) (-G-V),compared with open-fields (control), in contrasting landscapes with low vs. high levels of agricultural intensification (AI), and in five European regions. Aphid populations were 28%, 97%, and 199% higher in -G, -V, and -G -V treatments, respectively, compared to the open fields, indicating synergistic effects of both natural-enemy groups. Enhanced parasitoid : host and predator : prey ratios were related to reduced aphid population density and population growth. The relative importance of parasitoids and vegetation-dwelling predators greatly differed among European regions, and agricultural intensification affected biological control and aphid density only in some regions. This shows a changing role of species group identity in diverse enemy communities and a need to consider region-specific landscape management.

Random neighbor sandpile model with constrained total energy

We propose as energy-constrained sandpile model with random neighbors. The critical behavior of the model is in the same universality class as the mean-field self-organized criticality sandpile. The critical energy E-c depends on the number of neighbors n of each site, but the various exponents do not. For n = 6, we got that E-c = 0.4545; and a self-similar structure of the energy distribution function with five major peaks is also observed. This is a natural result of system dynamics and the way the system is disturbed.

Universality and self-similarity of an energy-constrained sandpile model with random neighbors

We study an energy-constrained sandpile model with random neighbors. The critical behavior of the model is in the same universality class as the mean-field self-organized criticality sandpile. The critical energy E-c depends on the number of neighbors n for each site, but the various exponents are independent of n. A self-similar structure with n-1 major peaks is developed for the energy distribution p(E) when the system approaches its stationary state. The avalanche dynamics contributes to the major peaks appearing at E-Pk = 2k/(2n - 1) with k = 1,2,...,n-1, while the fine self-similar structure is a natural result of the way the system is disturbed. [S1063-651X(99)10307-6].