12,732 research outputs found
Surface magnetic ordering in topological insulators with bulk magnetic dopants
We show that a three dimensional topological insulator doped with magnetic
impurities in the bulk can have a regime where the surface is magnetically
ordered but the bulk is not. This is in contrast to conventional materials
where bulk ordered phases are typically more robust than surface ordered
phases. The difference originates from the topologically protected gapless
surface states characteristic of topological insulators. We study the problem
using a mean field approach in two concrete models that give the same
qualitative result, with some interesting differences. Our findings could help
explain recent experimental results showing the emergence of a spectral gap in
the surface state of Bi2Se3 doped with Mn or Fe atoms, but with no measurable
bulk magnetism.Comment: 8 pages, 6 figure
Resolution-enhanced Mapping Spectrometer
A familiar mapping spectrometer implementation utilizes two dimensional detector arrays with spectral dispersion along one direction and spatial along the other. Spectral images are formed by spatially scanning across the scene (i.e., push-broom scanning). For imaging grating and prism spectrometers, the slit is perpendicular to the spatial scan direction. For spectrometers utilizing linearly variable focal-plane-mounted filters the spatial scan direction is perpendicular to the direction of spectral variation. These spectrometers share the common limitation that the number of spectral resolution elements is given by the number of pixels along the spectral (or dispersive) direction. Resolution enhancement by first passing the light input to the spectrometer through a scanned etalon or Michelson is discussed. Thus, while a detector element is scanned through a spatial resolution element of the scene, it is also temporally sampled. The analysis for all the pixels in the dispersive direction is addressed. Several specific examples are discussed. The alternate use of a Michelson for the same enhancement purpose is also discussed. Suitable for weight constrained deep space missions, hardware systems were developed including actuators, sensor, and electronics such that low-resolution etalons with performance required for implementation would weigh less than one pound
Noise-free high-efficiency photon-number-resolving detectors
High-efficiency optical detectors that can determine the number of photons in
a pulse of monochromatic light have applications in a variety of physics
studies, including post-selection-based entanglement protocols for linear
optics quantum computing and experiments that simultaneously close the
detection and communication loopholes of Bell's inequalities. Here we report on
our demonstration of fiber-coupled, noise-free, photon-number-resolving
transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these
sensors could be made even higher at any wavelength in the visible and
near-infrared spectrum without resulting in a higher dark-count rate or
degraded photon-number resolution.Comment: 4 pages, 4 figures Published in Physical Review A, Rapid
Communications, 17 June 200
Escherichia Coli-Based Cell-Free Protein Synthesis for Iterative Design of Tandem-Core Virus-Like Particles
Tandem-core hepatitis B core antigen (HBcAg) virus-like particles (VLPs), in which two HBcAg monomers are joined together by a peptide linker, can be used to display two different antigens on the VLP surface. We produced universal influenza vaccine candidates that use this scaffold in an Escherichia coli-based cell-free protein synthesis (CFPS) platform. We then used the CFPS system to rapidly test modifications to the arginine-rich region typically found in wild-type HBcAg, the peptide linkers around the influenza antigen inserts, and the plasmid vector backbone to improve titer and quality. Using a minimal plasmid vector backbone designed for CFPS improved titers by at least 1.4-fold over the original constructs. When the linker lengths for the influenza inserts were more consistent in length and a greater variety of codons for glycine and serine were utilized, titers were further increased to over 70 μg/mL (4.0-fold greater than the original construct) and the presence of lower molecular weight product-related impurities was significantly reduced, although improvements in particle assembly were not seen. Furthermore, any constructs with the C-terminal arginine-rich region removed resulted in asymmetric particles of poor quality. This demonstrates the potential for CFPS as a screening platform for VLPs
APPARATUS FOR THE STUDY OF FISSION-GAS RELEASE FROM NEUTRON-ACTIVATED FUELED GRAPHITE
A simple laboratory apparatus for the study of fissiongas release from neutron-activated fueled graphite was developed. Xenon-133 released from a heated specimen is carried in a helium sweep gas to a charcoal trap, where the accumulated activity is monitored continuously by a scintillation detector, ratemeter, and pen recorder. The maximum specimen temperature (2500 deg F) is achieved in 10 min with an induction heater. All instrumentation is commercially available. Data for several neutron-activated fueled-graphite specimens heated in the range from 800 to 2500 deg F are presented to illustrate the typical results obtained with the apparatus. (auth
The development and initial evaluation of the Pornography-Use Avoidance Self-Efficacy Scale
Background and aims This study employed a newly developed questionnaire to evaluate whether men’s self-efficacy to avoid using pornography in each of 18 emotional, social, or sexually arousing situations was associated with either their typical frequency of pornography use or their hypersexuality. Methods Using an Internet-based data collection procedure, 229 male pornography users (Mage = 33.3 years, SD = 12.2) who had sought or considered seeking professional help for their use of pornography completed questionnaires assessing their situationally specific self-efficacy, history of pornography use, self-efficacy to employ specific pornography-reduction strategies, hypersexuality, and demographic characteristics. Results Frequency of pornography use was significantly negatively associated with level of confidence in 12 of the 18 situations. In addition, lower hypersexuality and higher confidence to employ pornography-use-reduction strategies were associated with higher confidence to avoid using pornography in each of the 18 situations. A principal axis factor analysis yielded three clusters of situations: (a) sexual arousal/boredom/opportunity, (b) intoxication/locations/easy access, and (c) negative emotions. Discussion and conclusions This questionnaire could be employed to identify specific high-risk situations for lapse or relapse and as a measure of treatment outcome among therapy clients, but we recommend further examination of the psychometric properties and clinical utility of the questionnaire in treatment samples. Because only one of the three clusters reflected a consistent theme, we do not recommend averaging self-efficacy within factors to create subscales
Beta lives - some statistical perspectives on the capital asset pricing model
This note summarizes some technical issues relevant to the use of the idea of excess return in empirical modelling. We cover the case where the aim is to construct a measure of expected return on an asset and a model of the CAPM type is used. We review some of the problems and show examples where the basic CAPM may be used to develop other results which relate the expected returns on assets both to the expected return on the market and other factors
Prognostic value of lymph node ratio and extramural vascular invasion on survival for patients undergoing curative colon cancer resection
There was no study funding. We are grateful to Tony Rafferty (Tailored Information for the People of Scotland, TIPs) for providing survival data.Peer reviewedPublisher PD
Practical long-distance quantum key distribution system using decoy levels
Quantum key distribution (QKD) has the potential for widespread real-world
applications. To date no secure long-distance experiment has demonstrated the
truly practical operation needed to move QKD from the laboratory to the real
world due largely to limitations in synchronization and poor detector
performance. Here we report results obtained using a fully automated, robust
QKD system based on the Bennett Brassard 1984 protocol (BB84) with low-noise
superconducting nanowire single-photon detectors (SNSPDs) and decoy levels.
Secret key is produced with unconditional security over a record 144.3 km of
optical fibre, an increase of more than a factor of five compared to the
previous record for unconditionally secure key generation in a practical QKD
system.Comment: 9 page
The Tidal Tails of the Ultra-Faint Globular Cluster Palomar 1
Using the Optimal Filter Technique applied to Sloan Digital Sky Survey
photometry, we have found extended tails stretching about 1 degree (or several
tens of half-light radii) from either side of the ultra-faint globular cluster
Palomar 1. The tails contain roughly as many stars as does the cluster itself.
Using deeper Hubble Space Telescope data, we see that the isophotes twist in a
chacteristic S-shape on moving outwards from the cluster centre to the tails.
We argue that the main mechanism forming the tails may be relaxation driven
evaporation and that Pal 1 may have been accreted from a now disrupted dwarf
galaxy ~500 Myr ago.Comment: 6 figures, accepted for publication in MNRAS Letters Changes in v2:
Merged previous figures 3 and 5 and slightly expanded discussio
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