1,639 research outputs found
Failure of conductance quantization in two-dimensional topological insulators due to non-magnetic impurities
Despite topological protection and the absence of magnetic impurities,
two-dimensional topological insulators display quantized conductance only in
surprisingly short channels, which can be as short as 100 nm for atomically
thin materials. We show that the combined action of short-range nonmagnetic
impurities located near the edges and on site electron-electron interactions
effectively creates noncollinear magnetic scatterers, and, hence, results in
strong backscattering. The mechanism causes deviations from quantization even
at zero temperature and for a modest strength of electron-electron
interactions. Our theory provides a straightforward conceptual framework to
explain experimental results, especially those in atomically thin crystals,
plagued with short-range edge disorder.Comment: 8 pages, 9 figures, 5 appendice
Superconductors as ideal spin sources for spintronics
Spin-polarized transport is investigated in normal metal-superconductor (NS)
junctions as a function of interface transmissivity as well as temperature when
the density of states of a superconductor is Zeeman-split in response to an
exchange field (h_exc). Similarly to the "absolute spin-valve effect" predicted
by D. Huertas-Hernando et al. [Phys. Rev. Lett. 88, 047003 (2002)] in
superconducting proximity structures, we show that NS junctions can be used to
generate highly spin-polarized currents, in alternative to half-metallic
ferromagnets. In particular, the spin-polarized current obtained is largely
tunable in magnitude and sign by acting on bias voltage and h_exc. While for
tunnel contacts the current polarization can be as high as 100%, for
transparent junctions it is dominated by the minority spin species. The effect
can be enhanced by electron "cooling" provided by the superconducting gap.Comment: 4 pages, 4 color figures, published versio
Electronic Hong-Ou-Mandel interferometer for multi-mode entanglement detection
We show that multi-mode entanglement of electrons in a mesoscopic conductor
can be detected by a measurement of the zero-frequency current correlations in
an electronic Hong-Ou-Mandel interferometer. By this mean, one can further
establish a lower bound to the entanglement of formation of two-electron input
states. Our results extend the work of Burkard and Loss [Phys. Rev. Lett. 91,
087903 (2003)] to many channels and provide a way to test the existence of
entangled states involving both orbital and spin degrees of freedom.Comment: 6 pages. Revised version. Ref. adde
Manipulating nonequilibrium magnetism through superconductors
Electrostatic control of the magnetization of a normal mesoscopic conductor
is analyzed in a hybrid superconductor-normal-superconductor system. This
effect stems from the interplay between the non-equilibrium condition in the
normal region and the Zeeman splitting of the quasiparticle density of states
of the superconductor subjected to a static in-plane magnetic field. Unexpected
spin-dependent effects such as magnetization suppression, diamagnetic-like
response of the susceptibility as well as spin-polarized current generation are
the most remarkable features presented. The impact of scattering events is
evaluated and let us show that this effect is compatible with realistic
material properties and fabrication techniques.Comment: 5 pages, 4 figure
Crowdsourcing malaria parasite quantification: an online game for analyzing images of infected thick blood smears
Background: There are 600,000 new malaria cases daily worldwide. The gold standard for estimating the parasite burden and the corresponding severity of the disease consists in manually counting the number of parasites in blood smears through a microscope, a process that can take more than 20 minutes of an expert microscopist’s time.
Objective: This research tests the feasibility of a crowdsourced approach to malaria image analysis. In particular, we investigated whether anonymous volunteers with no prior experience would be able to count malaria parasites in digitized images of thick blood smears by playing a Web-based game.
Methods: The experimental system consisted of a Web-based game where online volunteers were tasked with detecting parasites in digitized blood sample images coupled with a decision algorithm that combined the analyses from several players to produce an improved collective detection outcome. Data were collected through the MalariaSpot website. Random images of thick blood films containing Plasmodium falciparum at medium to low parasitemias, acquired by conventional optical microscopy, were presented to players. In the game, players had to find and tag as many parasites as possible in 1 minute. In the event that players found all the parasites present in the image, they were presented with a new image. In order to combine the choices of different players into a single crowd decision, we implemented an image processing pipeline and a quorum algorithm that judged a parasite tagged when a group of players agreed on its position.
Results: Over 1 month, anonymous players from 95 countries played more than 12,000 games and generated a database of more than 270,000 clicks on the test images. Results revealed that combining 22 games from nonexpert players achieved a parasite counting accuracy higher than 99%. This performance could be obtained also by combining 13 games from players trained for 1 minute. Exhaustive computations measured the parasite counting accuracy for all players as a function of the number of games considered and the experience of the players. In addition, we propose a mathematical equation that accurately models the collective parasite counting performance.
Conclusions: This research validates the online gaming approach for crowdsourced counting of malaria parasites in images of thick blood films. The findings support the conclusion that nonexperts are able to rapidly learn how to identify the typical features of malaria parasites in digitized thick blood samples and that combining the analyses of several users provides similar parasite counting accuracy rates as those of expert microscopists. This experiment illustrates the potential of the crowdsourced gaming approach for performing routine malaria parasite quantification, and more generally for solving biomedical image analysis problems, with future potential for telediagnosis related to global health challenges
Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces
Because the physical interpretation of the spin-polarization of a ferromagnet
determined by point-contact Andreev reflection (PCAR) is non-trivial, we have
carried out parameter-free calculations of PCAR spectra based upon a
scattering-theory formulation of Andreev reflection generalized to
spin-polarized systems and a tight-binding linear muffin tin orbital method for
calculating the corresponding scattering matrices. PCAR is found to measure the
spin-dependent interface transparency rather than the bulk polarization of the
ferromagnet which is strongly overestimated by free electron model fitting.Comment: 4 pages, 1figure. submitte
Intense physical activity is associated with cognitive performance in the elderly
Numerous studies have reported positive impacts of physical activity on cognitive function. However, the majority of these studies have utilised physical activity questionnaires or surveys, thus results may have been influenced by reporting biases. Through the objective measurement of routine levels of physical activity via actigraphy, we report a significant association between intensity, but not volume, of physical activity and cognitive functioning. A cohort of 217 participants (aged 60–89 years) wore an actigraphy unit for 7 consecutive days and underwent comprehensive neuropsychological assessment. The cohort was stratified into tertiles based on physical activity intensity. Compared with individuals in the lowest tertile of physical activity intensity, those in the highest tertile scored 9%, 9%, 6% and 21% higher on the digit span, digit symbol, Rey Complex Figure Test (RCFT) copy and Rey Figure Test 30-min recall test, respectively. Statistically, participants in the highest tertile of physical activity intensity performed significantly better on the following cognitive tasks: digit symbol, RCFT copy and verbal fluency test (all P<0.05). The results indicate that intensity rather than quantity of physical activity may be more important in the association between physical activity and cognitive function
- …