Scattering Mechanism in Modulation-Doped Shallow Two-Dimensional Electron Gases

We report on a systematic investigation of the dominant scattering mechanism in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped GaAs/Al_{x}Ga_{1-x}As heterostructures. The power-law exponent of the electron mobility versus density, mu \propto n^{alpha}, is extracted as a function of the 2DEG's depth. When shallower than 130 nm from the surface, the power-law exponent of the 2DEG, as well as the mobility, drops from alpha \simeq 1.65 (130 nm deep) to alpha \simeq 1.3 (60 nm deep). Our results for shallow 2DEGs are consistent with theoretical expectations for scattering by remote dopants, in contrast to the mobility-limiting background charged impurities of deeper heterostructures.Comment: 4 pages, 3 figures, modified version as accepted in AP

Survival of Campylobacter spp. on inoculated pork skin or meat.

Campylobacter is one of the main causes of human foodborne bacterial zoonoses due to food consumption in developed countries. Nine to 32% of pig carcasses are contaminated by Campylobacter. The purpose of the study was to improve our knowledge of the survival of implanted campylobacters from the two kinds of pork matrix meat (skin, muscle) during meat cold domestic storage. One hundred and twenty pork skin and 120 skinless chine samples (25 cm2/sample) were inoculated with two C. jejuni and four C. coli strains and stored in closed box at 4 oc for 1, 4, 8, 15 and 22 days. Campylobacter were isolated from sample suspensions after mechanical pummeling and numbered by direct plating. We calculated the shoulder time (ST), the D value (the time for one log decrease} and the R, value (the time to reach 10% of the initial population R, = ST +D). We compared them in a stratified approach according to pork matrix and strain. According to matrixes, mean D, TS and R, value varied significantly between pork skin (4.3 days, 1.3 days, 5.6 days, respectively} and spare rib (7 .2 days, 3.5 days, 10.8 days, respectively}. On spare rib, R1 was higher (16 days) with one C. coli strain (CCV55). Statistical effects between TS and R, value on spare rib and strain were noticed. This study shows that the survival of campylobacters on pork meat is similar to the survival of Campylobacter on poultry meat. Consequently, good hygiene practices are needed to manage the risk of pork Campylobacter contamination and further studies focusing on survival factors may complete this risk analysis on the pork food chain

Influence of metallic nanoparticles on upconversion processes

It is well known that Raman scattering and fluorescence can be enhanced by the presence of metallic nanoparticles. Here, we derive simple equations to analyse the influence of metallic nanoparticles on upconversion processes such as non-radiative energy transfer or excited state absorption. We compare the resulting expressions with the more familiar Raman and fluorescence cases, and find significant differences. We use numerical simulations to calculate the upconverted signal enhancement achievable by means of metallic spheres of different radii, and find particles of 100-400nm radius at infrared frequencies to be favorable. We also discuss the considerable challenges involved in using metallic particles to enhance upconversion for solar energy.Comment: Changes mostly made on the structure of the text and on the notation to improve clarity. Other changes attempt to better clarify the conditions under which the equations and simulations are valid. Main results and conclusions essentially unchange

Mechanical Flip-Chip for Ultra-High Electron Mobility Devices

Electrostatic gates are of paramount importance for the physics of devices based on high-mobility two-dimensional electron gas (2DEG) since they allow depletion of electrons in selected areas. This field-effect gating enables the fabrication of a wide range of devices such as, for example, quantum point contacts (QPC), electron interferometers and quantum dots. To fabricate these gates, processing is usually performed on the 2DEG material, which is in many cases detrimental to its electron mobility. Here we propose an alternative process which does not require any processing of the 2DEG material other than for the ohmic contacts. This approach relies on processing a separate wafer that is then mechanically mounted on the 2DEG material in a flip-chip fashion. This technique proved successful to fabricate quantum point contacts on both GaAs/AlGaAs materials with both moderate and ultra-high electron mobility.Comment: 5 pages, 3 figure

Heap Formation in Granular Media

Using molecular dynamics (MD) simulations, we find the formation of heaps in a system of granular particles contained in a box with oscillating bottom and fixed sidewalls. The simulation includes the effect of static friction, which is found to be crucial in maintaining a stable heap. We also find another mechanism for heap formation in systems under constant vertical shear. In both systems, heaps are formed due to a net downward shear by the sidewalls. We discuss the origin of net downward shear for the vibration induced heap.Comment: 11 pages, 4 figures available upon request, Plain TeX, HLRZ-101/9