Diffusion of Monochromatic Classical Waves

We study the diffusion of monochromatic classical waves in a disordered acoustic medium by scattering theory. In order to avoid artifacts associated with mathematical point scatterers, we model the randomness by small but finite insertions. We derive expressions for the configuration-averaged energy flux, energy density, and intensity for one, two and three dimensional (1D, 2D and 3D) systems with an embedded monochromatic source using the ladder approximation to the Bethe-Salpeter equation. We study the transition from ballistic to diffusive wave propagation and obtain results for the frequency-dependence of the medium properties such as mean free path and diffusion coefficient as a function of the scattering parameters. We discover characteristic differences of the diffusion in 2D as compared to the conventional 3D case, such as an explicit dependence of the energy flux on the mean free path and quite different expressions for the effective transport velocity.Comment: 11 pages, 2 figure

Sensing Noncollinear Magnetism at the Atomic Scale Combining Magnetic Exchange and Spin-Polarized Imaging

Storing and accessing information in atomic-scale magnets requires magnetic imaging techniques with single-atom resolution. Here, we show simultaneous detection of the spin-polarization and exchange force, with or without the flow of current, with a new method, which combines scanning tunneling microscopy and non-contact atomic force microscopy. To demonstrate the application of this new method, we characterize the prototypical nano-skyrmion lattice formed on a monolayer of Fe/Ir(111). We resolve the square magnetic lattice by employing magnetic exchange force microscopy, demonstrating its applicability to non-collinear magnetic structures, for the first time. Utilizing distance-dependent force and current spectroscopy, we quantify the exchange forces in comparison to the spin-polarization. For strongly spin-polarized tips, we distinguish different signs of the exchange force which we suggest arises from a change in exchange mechanisms between the probe and a skyrmion. This new approach may enable both non-perturbative readout combined with writing by current-driven reversal of atomic-scale magnets

An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T

We present the design of a highly compact High Field Scanning Probe Microscope (HF-SPM) for operation at cryogenic temperatures in an extremely high magnetic field, provided by a water-cooled Bitter magnet able to reach 38 T. The HF-SPM is 14 mm in diameter: an Attocube nano-positioner controls the coarse approach of a piezo resistive AFM cantilever to a scanned sample. The Bitter magnet constitutes an extreme environment for SPM due to the high level of vibrational noise; the Bitter magnet noise at frequencies up to 300 kHz is characterized and noise mitigation methods are described. The performance of the HF-SPM is demonstrated by topographic imaging and noise measurements at up to 30 T. Additionally, the use of the SPM as a three-dimensional dilatometer for magnetostriction measurements is demonstrated via measurements on a magnetically frustrated spinel sample.Comment: 6 pages, 5 figure

Challenges with using estimates when calculating ART need among adults in South Africa

Background. The Foundation for Professional Development (FPD) collects information annually on HIV/AIDS service provision and estimates service needs in the City of Tshwane Metropolitan Municipality (CTMM).Methods. Antiretroviral therapy (ART) data from the Department of Health and Statistics South Africa (SSA) mid-year population estimates were used to approximate the ART need among adults in the CTMM.Results. According to SSA data, ART need decreased dramatically from 2010 to 2011 and was lower than the number of adults receiving ART. Although the noted difference was probably due to changes in the calculations by SSA, no detailed or confirmed explanation could be offered.Conclusions. We provide a constructive contribution to the discussion regarding the use of model-derived estimates of ART need

BANDWIDTH STUDIES OF AN INJECTION-SEEDED BETA-BARIUM BORATE OPTICAL PARAMETRIC OSCILLATOR

Spectral and temporal properties of a scanning injection-seeded beta-barium borate optical parametric oscillator pumped by the third harmonic of a 10-Hz Nd:YAG laser have been studied. The seed source was a cw diode laser with a wavelength of 830 nm tunable over a range of 50 GHz. We measured the bandwidth of the seeded optical parameter oscillator, using a two-photon resonance in barium and a Fabry-Perot etalon, to be approximately 400 MHz for pump power levels more than two times above threshold. This is similar to 2 times the Fourier-transform-limited bandwidth. At lower pump powers the bandwidth was smaller

The Star Formation Threshold in NGC 6822

We investigate the star formation threshold in NGC 6822, a nearby Local Group dwarf galaxy, on sub-kpc scales using high-resolution, wide-field, deep HI, Halpha and optical data. In a study of the HI velocity profiles we identify a cool and warm neutral component in the Interstellar Medium of NGC 6822. We show that the velocity dispersion of the cool component (~4 km/s) when used with a Toomre-Q criterion gives an optimal description of ongoing star formation in NGC 6822, superior to that using the more conventional dispersion value of 6 km/s. However, a simple constant surface density criterion for star formation gives an equally superior description. We also investigate the two-dimensional distribution of Q and the star formation threshold and find that these results also hold locally. The range in gas density in NGC 6822 is much larger than the range in critical density, and we argue that the conditions for star formation in NGC 6822 are fully driven by this density criterion. Star formation is local, and in NGC 6822 global rotational or shear parameters are apparently not important.Comment: Accepted for publication in The Astronomical Journal. Version with high-resolution figures available at http://www.mso.anu.edu.au/~edeblok/papers/6822_paper2.ps.g

On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy

As part of a continuing survey of nearby galaxies, we have mapped the neutral gas content of the low surface brightness, Magellanic-type galaxy NGC 3109 --- and its environment, including the Antlia dwarf galaxy --- at unprecedented velocity resolution and brightness sensitivity. The HI mass of NGC 3109 is measured to be (3.8 +/- 0.5) x 10^8 Msun. A substantial warp in the disk of NGC 3109 is detected in the HI emission image in the form of an extended low surface brightness feature. We report a positive detection in HI of the nearby Antlia dwarf galaxy, and measure its total neutral gas mass to be (6.8 +/- 1.4) x 10^5 Msun. We show the warp in NGC 3109 to lie at exactly the same radial velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. HI data for a further eight galaxies detected in the background are presented.Comment: Accepted for publication in A

Comparison of cooling methods to induce and maintain normo- and hypothermia in intensive care unit patients: a prospective intervention study

Contains fulltext : 52086.pdf ( ) (Open Access)BACKGROUND: Temperature management is used with increased frequency as a tool to mitigate neurological injury. Although frequently used, little is known about the optimal cooling methods for inducing and maintaining controlled normo- and hypothermia in the intensive care unit (ICU). In this study we compared the efficacy of several commercially available cooling devices for temperature management in ICU patients with various types of neurological injury. METHODS: Fifty adult ICU patients with an indication for controlled mild hypothermia or strict normothermia were prospectively enrolled. Ten patients in each group were assigned in consecutive order to conventional cooling (that is, rapid infusion of 30 ml/kg cold fluids, ice and/or coldpacks), cooling with water circulating blankets, air circulating blankets, water circulating gel-coated pads and an intravascular heat exchange system. In all patients the speed of cooling (expressed as degrees C/h) was measured. After the target temperature was reached, we measured the percentage of time the patient's temperature was 0.2 degrees C below or above the target range. Rates of temperature decline over time were analyzed with one-way analysis of variance. Differences between groups were analyzed with one-way analysis of variance, with Bonferroni correction for multiple comparisons. A p < 0.05 was considered statistically significant. RESULTS: Temperature decline was significantly higher with the water-circulating blankets (1.33 +/- 0.63 degrees C/h), gel-pads (1.04 +/- 0.14 degrees C/h) and intravascular cooling (1.46 +/- 0.42 degrees C/h) compared to conventional cooling (0.31 +/- 0.23 degrees C/h) and the air-circulating blankets (0.18 +/- 0.2 degrees C/h) (p < 0.01). After the target temperature was reached, the intravascular cooling device was 11.2 +/- 18.7% of the time out of range, which was significantly less compared to all other methods. CONCLUSION: Cooling with water-circulating blankets, gel-pads and intravascular cooling is more efficient compared to conventional cooling and air-circulating blankets. The intravascular cooling system is most reliable to maintain a stable temperature