Spin-dependent direct gap emission in tensile-strained Ge films on Si substrates

The circular polarization of direct gap emission of Ge is studied in optically-excited tensile-strained Ge-on-Si heterostructures as a function of doping and temperature. Owing to the spin-dependent optical selection rules, the radiative recombinations involving strain-split light (cG-LH) and heavy hole (cG-HH) bands are unambiguously resolved. The fundamental cG-LH transition is found to have a low temperature circular polarization degree of about 85% despite an off-resonance excitation of more than 300 meV. By photoluminescence (PL) measurements and tight binding calculations we show that this exceptionally high value is due to the peculiar energy dependence of the optically-induced electron spin population. Finally, our observation of the direct gap doublet clarifies that the light hole contribution, previously considered to be negligible, can dominate the room temperature PL even at low tensile strain values of about 0.2%

Interferometric length metrology for the dimensional control of ultra-stable Ring Laser Gyroscopes

We present the experimental test of a method for controlling the absolute length of the diagonals of square ring laser gyroscopes. The purpose is to actively stabilize the ring cavity geometry and to enhance the rotation sensor stability in order to reach the requirements for the detection of the relativistic Lense-Thirring effect with a ground-based array of optical gyroscopes. The test apparatus consists of two optical cavities 1.32 m in length, reproducing the features of the ring cavity diagonal resonators of large frame He-Ne ring laser gyroscopes. The proposed measurement technique is based on the use of a single diode laser, injection locked to a frequency stabilized He-Ne/Iodine frequency standard, and a single electro-optic modulator. The laser is modulated with a combination of three frequencies allowing to lock the two cavities to the same resonance frequency and, at the same time, to determine the cavity Free Spectral Range (FSR). We obtain a stable lock of the two cavities to the same optical frequency reference, providing a length stabilization at the level of 1 part in $10^{11}$, and the determination of the two FSRs with a relative precision of 0.2 ppm. This is equivalent to an error of 500 nm on the absolute length difference between the two cavities

OLIVE FRUIT FLY: A threat to the South African olive industry?

Olive fruit fly (Bactrocera oleae) is the most serious pest of cultivated olives in the Mediterranean basin. to date we have not seen the same level of damage in south africa, but the question remained whether it poses a similar threat as the local olive industry expands. From this study it is clear that the climatic con- ditions during the period preceding harvest are un- favourable for rapid population growth of OFF in the Western Cape, in contrast to conditions in the coastal area of Trapani province in Sicily. Climate, and not parasitism, appears to be the main factor limiting OFF population levels in the Western Cape. While sporadic outbreaks of economically damaging OFF infestations can be expected in areas where the climatic conditions during a particular season or part of a season are favourable for OFF, the generally un- favourable climatic conditions mean that OFF is not expected to pose a similar threat to olive production in the Western Cape as it does in the Mediterranean basin

Rotational sensitivity of the "G-Pisa" gyrolaser

G-Pisa is an experiment investigating the possibility to operate a high sensitivity laser gyroscope with area less than $1 \rm m^2$ for improving the performances of the mirrors suspensions of the gravitational wave antenna Virgo. The experimental set-up consists in a He-Ne ring laser with a 4 mirrors square cavity. The laser is pumped by an RF discharge where the RF oscillator includes the laser plasma in order to reach a better stability. The contrast of the Sagnac fringes is typically above 50% and a stable regime has been reached with the laser operating both single mode or multimode. The effect of hydrogen contamination on the laser was also checked. A low-frequency sensitivity, below $1 \rm Hz$, in the range of $10^{-8} \rm {(rad / s)/ \sqrt{Hz}}$ has been measured.Comment: 6 pages, 6 figures, presented at the EFTF-IFCS joint conference 200

A 1.82 m^2 ring laser gyroscope for nano-rotational motion sensing

We present a fully active-controlled He-Ne ring laser gyroscope, operating in square cavity 1.35 m in side. The apparatus is designed to provide a very low mechanical and thermal drift of the ring cavity geometry and is conceived to be operative in two different orientations of the laser plane, in order to detect rotations around the vertical or the horizontal direction. Since June 2010 the system is active inside the Virgo interferometer central area with the aim of performing high sensitivity measurements of environmental rotational noise. So far, continuous not attempted operation of the gyroscope has been longer than 30 days. The main characteristics of the laser, the active remote-controlled stabilization systems and the data acquisition techniques are presented. An off-line data processing, supported by a simple model of the sensor, is shown to improve the effective long term stability. A rotational sensitivity at the level of ten nanoradiants per squareroot of Hz below 1 Hz, very close to the required specification for the improvement of the Virgo suspension control system, is demonstrated for the configuration where the laser plane is horizontal

Electrostatic internal energy using the method of images

For several configurations of charges in the presence of conductors, the method of images permits us to obtain some observables associated with such a configuration by replacing the conductors with some image charges. However, simple inspection shows that the potential energy associated with both systems does not coincide. Nevertheless, it can be shown that for a system of a grounded or neutral conductor and a distribution of charges outside, the external potential energy associated with the real charge distribution embedded in the field generated by the set of image charges is twice the value of the internal potential energy associated with the original system. This assertion is valid for any size and shape of the conductor, and regardless of the configuration of images required. In addition, even in the case in which the conductor is not grounded nor neutral, it is still possible to calculate the internal potential energy of the original configuration through the method of images. These results show that the method of images could also be useful for calculations of the internal potential energy of the original system.Comment: 5 pages, 3 figures. New discussions added. Minor change

DNA Barcoding to Improve the Taxonomy of the Afrotropical Hoverflies (Insecta: Diptera: Syrphidae)

The identification of Afrotropical hoverflies is very difficult because of limited recent taxonomic revisions and the lack of comprehensive identification keys. In order to assist in their identification, and to improve the taxonomy of this group, we constructed a reference dataset of 513 COI barcodes of 90 of the more common nominal species from Ghana, Togo, Benin and Nigeria (W Africa) and added ten publically available COI barcodes from nine nominal Afrotropical species to this (total: 523 COI barcodes; 98 nominal species; 26 genera). The identification accuracy of this dataset was evaluated with three methods (K2P distance-based, Neighbor-Joining (NJ) / Maximum Likelihood (ML) analysis, and using SpeciesIdentifier). Results of the three methods were highly congruent and showed a high identification success. Nine species pairs showed a low ( 0.03) maximum intraspecific K2P distance was observed in eight species and barcodes of these species not always formed single clusters in the NJ / ML analayses which may indicate the occurrence of cryptic species. Optimal K2P thresholds to differentiate intra- from interspecific K2P divergence were highly different among the three subfamilies (Eristalinae: 0.037, Syrphinae: 0.06, Microdontinae: 0.007–0.02), and among the different general suggesting that optimal thresholds are better defined at the genus level. In addition to providing an alternative identification tool, our study indicates that DNA barcoding improves the taxonomy of Afrotropical hoverflies by selecting (groups of) taxa that deserve further taxonomic study, and by attributing the unknown sex to species for which only one of the sexes is known

Anisotropic pinning enhancement in Nb films with arrays of submicrometric Ni lines

Arrays of submicrometric Ni lines have been fabricated in superconducting Nb films by electron beam lithography. In the mixed state, these arrays induce strong anisotropy in the dissipation behavior. The dissipation is reduced several orders of magnitude, in the whole applied magnetic field range, when the vortex motion is perpendicular to the Ni lines (applied current parallel to them) in comparison with dissipation of vortices moving parallel to the lines. In addition, for the samples studied in this work, a change in the slope of the ρ(B) curves is observed when the vortices move perpendicular to the lines and the vortex lattice parameter matches the width of the Ni lines. © 2002 American Institute of Physics.The authors want to thank Spanish CICYT MAT2002- 04543 and ESF Vortex program. D.J. and E.M.G. thank Ministerio de Ciencia y Tecnologı´a ~Spain! for Ramo´n y Cajal Contracts.Peer reviewe