Photon emission induced by elastic exciton--carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton--electron ($X-e^-$) and exciton--hole ($X-h$) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1$s$ exciton--free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrier-assisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi's golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton--carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.Comment: Thoroughly revised version of previous work. Weak and incorrect assumptions have been removed from the paper, and its scope has evolved: see abstract. This is the final version, i.e. as accepted for publication in the European Physical Journal

Biomechanical comparison of the track start and the modified one-handed track start in competitive swimming: an intervention study

This study compared the conventional track and a new one-handed track start in elite age group swimmers to determine if the new technique had biomechanical implications on dive performance. Five male and seven female GB national qualifiers participated (mean ± SD: age 16.7 ± 1.9 years, stretched stature 1.76 ± 0.8 m, body mass 67.4 ± 7.9 kg) and were assigned to a control group (n = 6) or an intervention group (n = 6) that learned the new onehanded dive technique. All swimmers underwent a 4-week intervention comprising 12 ± 3 thirty-minute training sessions. Video cameras synchronized with an audible signal and timing suite captured temporal and kinematic data. A portable force plate and load cell handrail mounted to a swim starting block collected force data over 3 trials of each technique. A MANCOVA identified Block Time (BT), Flight Time (FT), Peak Horizontal Force of the lower limbs (PHF) and Horizontal Velocity at Take-off (Vx) as covariates. During the 10-m swim trial, significant differences were found in Time to 10 m (TT10m), Total Time (TT), Peak Vertical Force (PVF), Flight Distance (FD), and Horizontal Velocity at Take-off (Vx) (p < .05). Results indicated that the conventional track start method was faster over 10 m, and therefore may be seen as a superior start after a short intervention. During training, swimmers and coaches should focus on the most statistically significant dive performance variables: peak horizontal force and velocity at take-off, block and flight time

An Economic Study of the Effect of Android Platform Fragmentation on Security Updates

Vendors in the Android ecosystem typically customize their devices by modifying Android Open Source Project (AOSP) code, adding in-house developed proprietary software, and pre-installing third-party applications. However, research has documented how various security problems are associated with this customization process. We develop a model of the Android ecosystem utilizing the concepts of game theory and product differentiation to capture the competition involving two vendors customizing the AOSP platform. We show how the vendors are incentivized to differentiate their products from AOSP and from each other, and how prices are shaped through this differentiation process. We also consider two types of consumers: security-conscious consumers who understand and care about security, and na\"ive consumers who lack the ability to correctly evaluate security properties of vendor-supplied Android products or simply ignore security. It is evident that vendors shirk on security investments in the latter case. Regulators such as the U.S. Federal Trade Commission have sanctioned Android vendors for underinvestment in security, but the exact effects of these sanctions are difficult to disentangle with empirical data. Here, we model the impact of a regulator-imposed fine that incentivizes vendors to match a minimum security standard. Interestingly, we show how product prices will decrease for the same cost of customization in the presence of a fine, or a higher level of regulator-imposed minimum security.Comment: 22nd International Conference on Financial Cryptography and Data Security (FC 2018

Routine surveillance data on AIDS and HIV infections in the UK: a description of the data available and their use for short-term planning

In the UK surveillance of AIDS and HIV infection is based on routine reporting systems. Whilst attempts are made to ensure that AIDS data are as complete as possible, numbers of reports fluctuate from month to month for reasons which are described. In 1986 there was an increase in death certificates naming AIDS as a cause of death in patients who were not identifiable in the surveillance data. More active surveillance is now undertaken in order to minimize this and other possible discrepancies. It is probable that most cases of AIDS are reported and therefore these data can be used to describe trends in the epidemic by 'risk group'. Laboratory reports of HIV antibody-positive tests could give an earlier indication of trends because of the long incubation period of AIDS. But these laboratory data are difficult to interpret because they represent an incomplete and biased sample of all positive persons. AIDS cases are still being reported at a rate which is increasing approximately exponentially. Short-term predictions are presented showing a growth in the epidemic which is consistent with previously published predictions. Most cases are in the homosexual risk group. New asymptomatic homosexual patients with HIV antibody are still being identified. The epidemic of AIDS in haemophilia patients should be of finite size although new cases of AIDS are likely to continue to be diagnosed for several years. AIDS due to blood transfusion given in the UK before donor screening appears to be a much smaller epidemic. The epidemic in drug abusers is increasing. Heterosexually acquired AIDS and HIV infections are being reported in small but increasing numbers

A neural network-based exploratory learning and motor planning system for co-robots

Collaborative robots, or co-robots, are semi-autonomous robotic agents designed to work alongside humans in shared workspaces. To be effective, co-robots require the ability to respond and adapt to dynamic scenarios encountered in natural environments. One way to achieve this is through exploratory learning, or "learning by doing," an unsupervised method in which co-robots are able to build an internal model for motor planning and coordination based on real-time sensory inputs. In this paper, we present an adaptive neural network-based system for co-robot control that employs exploratory learning to achieve the coordinated motor planning needed to navigate toward, reach for, and grasp distant objects. To validate this system we used the 11-degrees-of-freedom RoPro Calliope mobile robot. Through motor babbling of its wheels and arm, the Calliope learned how to relate visual and proprioceptive information to achieve hand-eye-body coordination. By continually evaluating sensory inputs and externally provided goal directives, the Calliope was then able to autonomously select the appropriate wheel and joint velocities needed to perform its assigned task, such as following a moving target or retrieving an indicated object

Keyword-Based Delegable Proofs of Storage

Cloud users (clients) with limited storage capacity at their end can outsource bulk data to the cloud storage server. A client can later access her data by downloading the required data files. However, a large fraction of the data files the client outsources to the server is often archival in nature that the client uses for backup purposes and accesses less frequently. An untrusted server can thus delete some of these archival data files in order to save some space (and allocate the same to other clients) without being detected by the client (data owner). Proofs of storage enable the client to audit her data files uploaded to the server in order to ensure the integrity of those files. In this work, we introduce one type of (selective) proofs of storage that we call keyword-based delegable proofs of storage, where the client wants to audit all her data files containing a specific keyword (e.g., "important"). Moreover, it satisfies the notion of public verifiability where the client can delegate the auditing task to a third-party auditor who audits the set of files corresponding to the keyword on behalf of the client. We formally define the security of a keyword-based delegable proof-of-storage protocol. We construct such a protocol based on an existing proof-of-storage scheme and analyze the security of our protocol. We argue that the techniques we use can be applied atop any existing publicly verifiable proof-of-storage scheme for static data. Finally, we discuss the efficiency of our construction.Comment: A preliminary version of this work has been published in International Conference on Information Security Practice and Experience (ISPEC 2018

Nozzle geometry-induced vortices in supersonic wind tunnels

Streamwise-coherent structures were observed in schlieren images of a Mach 2.5 flow in an empty supersonic wind tunnel with rectangular cross-section. These features are studied using RANS computations in combination with wind tunnel experiments. The structures are identified as regions of streamwise vorticity embedded in the sidewall boundary layers. These vortices locally perturb the sidewall boundary layers, and can increase their thickness by as much as 37%. The vortices are caused by a region of separation upstream of the nozzle where there is a sharp geometry change, typical in supersonic wind tunnels with interchangeable nozzle blocks. Despite originating in the corners, the vortices are transported by secondary flows in the sidewall boundary layers so they end up near the tunnel centre-height, well away from any corners. The successful elimination of these sidewall vortices from the flow is achieved by replacing the sharp corner with a more rounded geometry, so that the flow here remains attached.Air Force Office of Scientific Research: Award No. FA9550-16-1-043

More Discriminants with the Brezing-Weng Method

The Brezing-Weng method is a general framework to generate families of pairing-friendly elliptic curves. Here, we introduce an improvement which can be used to generate more curves with larger discriminants. Apart from the number of curves this yields, it provides an easy way to avoid endomorphism rings with small class number

Lymphocyte Culture: Induction of Colonies by Conditioned Medium from Human Lymphoid Cell Lines

It has been shown that macrophage and granulocyte colonies can be induced m semisolid agar (1-3) in the presence of substances termed colony-stimulating factors (CSF), which are released predominantly by monocytes (4). However, attempts to induce formation of lymphoid colonies with CSF have so far proved unsuccessful. In the mouse, B lymphoid colonies are formed in the presence of 2-mercaptoethanol (5), and T lymphoid colonies can be induced with the plant lectins phytohemagglutinin (PHA) and concanavalin A (6). T lymphoid colonies can also be established from human peripheral blood lymphocytes in the presence of PHA (7-9), whereas with pokeweed mitogen mixed T and B lymphoid colonies are formed (9). Established human lymphoid cell lines multiply spontaneously in the absence of plant lectins or mercaptoethanol, and it seemed possible that such cells might release growth-stimulating substances into the culture medium. We have therefore investigated the ability of conditioned medium (CM) obtained from lymphoid cell lines to induce normal human peripheral blood lymphocytes (PBL) to form lymphoid colonies in agar

On the morphological deviation in additive manufacturing of porous Ti6Al4V scaffold: a design consideration

Additively manufactured Ti scaffolds have been used for bone replacement and orthopaedic applications. In these applications, both morphological and mechanical properties are important for their in vivo performance. Additively manufactured Ti6Al4V triply periodic minimal surface (TPMS) scaffolds with diamond and gyroid structures are known to have high stiffness and high osseointegration properties, respectively. However, morphological deviations between the as-designed and as-built types of these scaffolds have not been studied before. In this study, the morphological and mechanical properties of diamond and gyroid scaffolds at macro and microscales were examined. The results demonstrated that the mean printed strut thickness was greater than the designed target value. For diamond scaffolds, the deviation increased from 7.5 μm (2.5% excess) for vertical struts to 105.4 μm (35.1% excess) for horizontal struts. For the gyroid design, the corresponding deviations were larger, ranging from 12.6 μm (4.2% excess) to 198.6 μm (66.2% excess). The mean printed pore size was less than the designed target value. For diamonds, the deviation of the mean pore size from the designed value increased from 33.1 μm (-3.0% excess) for vertical struts to 92.8 μm (-8.4% excess) for horizontal struts. The corresponding deviation for gyroids was larger, ranging from 23.8 μm (-3.0% excess) to 168.7 μm (-21.1% excess). Compressive Young's modulus of the bulk sample, gyroid and diamond scaffolds was calculated to be 35.8 GPa, 6.81 GPa and 7.59 GPa, respectively, via the global compression method. The corresponding yield strength of the samples was measured to be 1012, 108 and 134 MPa. Average microhardness and Young's modulus from α and β phases of Ti6Al4V from scaffold struts were calculated to be 4.1 GPa and 131 GPa, respectively. The extracted morphology and mechanical properties in this study could help understand the deviation between the as-design and as-built matrices, which could help develop a design compensation strategy before the fabrication of the scaffolds