Anomalous Symmetry Fractionalization and Surface Topological Order

In addition to possessing fractional statistics, anyon excitations of a 2D topologically ordered state can realize symmetry in distinct ways , leading to a variety of symmetry enriched topological (SET) phases. While the symmetry fractionalization must be consistent with the fusion and braiding rules of the anyons, not all ostensibly consistent symmetry fractionalizations can be realized in 2D systems. Instead, certain `anomalous' SETs can only occur on the surface of a 3D symmetry protected topological (SPT) phase. In this paper we describe a procedure for determining whether an SET of a discrete, onsite, unitary symmetry group $G$ is anomalous or not. The basic idea is to gauge the symmetry and expose the anomaly as an obstruction to a consistent topological theory combining both the original anyons and the gauge fluxes. Utilizing a result of Etingof, Nikshych, and Ostrik, we point out that a class of obstructions are captured by the fourth cohomology group $H^4( G, \,U(1))$, which also precisely labels the set of 3D SPT phases, with symmetry group $G$. We thus establish a general bulk-boundary correspondence between the anomalous SET and the 3d bulk SPT whose surface termination realizes it. We illustrate this idea using the chiral spin liquid ($U(1)_2$) topological order with a reduced symmetry $\mathbb{Z}_2 \times \mathbb{Z}_2 \subset SO(3)$, which can act on the semion quasiparticle in an anomalous way. We construct exactly solved 3d SPT models realizing the anomalous surface terminations, and demonstrate that they are non-trivial by computing three loop braiding statistics. Possible extensions to anti-unitary symmetries are also discussed.Comment: 18 pages + 3 pages appendices. v2: added section on identifying the SPT order from nontrivial loop statistics in the 3D bulk with projective semion surface stat

Time reversal invariant gapped boundaries of the double semion state

The boundary of a fractionalized topological phase can be gapped by condensing a proper set of bosonic quasiparticles. Interestingly, in the presence of a global symmetry, such a boundary can have different symmetry transformation properties. Here we present an explicit example of this kind, in the double semion state with time reversal symmetry. We find two distinct cases where the semionic excitations on the boundary can transform either as time reversal singlets or as time reversal (Kramers) doublets, depending on the coherent phase factor of the Bose condensate. The existence of these two possibilities are demonstrated using both field theory argument and exactly solvable lattice models. Furthermore, we study the domain walls between these two types of gapped boundaries and find that the application of time reversal symmetry tunnels a semion between them.Comment: 11 pages, 8 figure

Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401-33 from XMM-Newton observations

We observed two relic radio sources, 0917+75 and 1401-33, with the XMM-Newton X-ray observatory. We did not detect any X-ray emission, thermal or non-thermal, in excess of the local background level from either target. This imposes new upper limits on the X-ray flux due to inverse Compton scattering of photons from the cosmic microwave background by relativistic electrons in the relic sources, and new lower limits on the magnetic field strength from the relative strength of the radio and X-ray emission. The combination of radio and X-ray observations provides a measure of the magnetic field independent of equipartition or minimum energy assumptions. Due to increasing sensitivity of radio observations, the known population of cluster relics has been growing; however, studies of non-thermal X-ray emission from relics remain scarce. Our study adds to the small sample of relics studied in X-rays. In both relics, our field strength lower limits are slightly larger than estimates of the equipartition magnetic field.Comment: 11 pages, 5 figures. Accepted by MNRA

NuSTAR observations of the young, energetic radio pulsar PSR B1509-58

We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of the young rotation-powered radio pulsar PSR B1509$-$58 in the supernova remnant MSH 15$-$52. We confirm the previously reported curvature in the hard X-ray spectrum, showing that a log parabolic model provides a statistically superior fit to the spectrum compared with the standard power law. The log parabolic model describes the NuSTAR data, as well as previously published gamma-ray data obtained with COMPTEL and AGILE, all together spanning 3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak of the broadband high energy spectrum to be at 2.6$\pm$0.8 MeV, an improvement of nearly an order of magnitude in precision over previous measurements. In addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm previously reported variations of photon indices with phase. Finally, we measure the pulsed fraction of PSR B1509$-$58 in the hard X-ray energy band for the first time. Using the energy resolved pulsed fraction results, we estimate that the pulsar's off-pulse emission has a photon index value between 1.26 and 1.96. Our results support a model in which the pulsar's lack of GeV emission is due to viewing geometry, with the X-rays originating from synchrotron emission from secondary pairs in the magnetosphere.Comment: 10 pages, 8 figures, 6 tables, ApJ accepte

Web-Conferencing Based Education: An Empirical Comparison with Face-to-Face Education

The advancement of technology and the widespread availability of the Internet have enabled web-conferencing basededucation. This research examines the relative efficacy of a web-conferencing based learning environment using AdobeConnect versus the traditional face-to-face classroom learning environment. Two instructional strategies – interactive versusdirect – in these learning environments and their effects on perceived learning and satisfaction are studied. Our findingssuggest that there is an interaction effect of learning environment and instructional strategy. In the interactive instructionalsessions, students in a web-conferencing based learning environment experienced a higher level of classroom interactivitythan those in a face-to-face classroom environment. Also, in the interactive instructional sessions, students in the webconferencingbased learning environment experienced higher perceived learning and satisfaction than those in the face-tofacelearning environment. No significant difference is observed between the direct instructional sessions of the twoenvironments

Effects of Bulk and Surface Conductivity on the Performance of CdZnTe Pixel Detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip

Comorbidities only account for a small proportion of excess mortality after fracture: A record linkage study of individual fracture types

Background: Non-hip non-vertebral fractures (NHNV) constitute the majority of osteoporotic fractures but few studies have examined the association between these fractures, co-morbidity and mortality. Objective: To examine the relationship between individual non-hip non-vertebral fractures, co-morbidities and mortality. Methods: Prospective population-based cohort of 267,043 subjects (45 and Up Study, Australia) had baseline questionnaires linked to hospital administrative and all-cause mortality data from 2006 - 2013. Associations between fracture and mortality examined using multivariate, time dependent Cox models, adjusted for age, prior fracture, body mass index, smoking and co-morbidities (cardiovascular disease, diabetes, stroke, thrombosis and cancer) and survival function curves. Population attributable fraction calculated for each level of risk exposure. Results: During 1,490,651 person-years, women and men experienced 7,571 and 4,571 fractures and 7,064 deaths and 11,078 deaths, respectively. In addition to hip and vertebral fractures, pelvis, humerus, clavicle, rib, proximal tibia/fibula, elbow and distal forearm fractures in both sexes, and ankle fractures in men, were associated with increased multivariable adjusted mortality hazard ratios ranging from 1.3 to 3.4. Co-morbidity independently added to mortality such that a woman with a humeral fracture and one co-morbidity had a similarly reduced 5 year survival to that of a woman with a hip fracture and no co-morbidities. Population mortality attributable to any fracture without co-morbidity was 9.2% in women and 5.3% in men. Conclusion: All proximal non-hip, non-vertebral fractures in women and men were associated with increased mortality risk. Co-existent co-morbidities independently further increased mortality. Population attributable risk for mortality for fracture was similar to cardiovascular disease and diabetes, highlighting their importance and potential benefit for early intervention and treatment

3-D Virtual Worlds and Higher Education

Conducting education in three-dimensional (3-D) virtual worlds is an emerging phenomenon in the educational arena. The objective of this research is to investigate the factors influencing students’ intention to adopt the 3-D virtual worlds for delivery of education. Drawing on existing technology acceptance models as well as studies in traditional and distance education, we developed a model to study students’ acceptance of using a 3-D virtual world for education and propose to test the model empirically using survey data collected from college students. We also study the use of two instructional methods in the 3-D virtual world. This study contributes to both academic research and practice. Studies on the use of this new, emerging technology for education can help to increase our understanding on the use of 3-D virtual world environment for delivery of higher education

3-D Virtual Worlds: Education and Learning

3-D virtual worlds are increasing in popularity as a medium for higher education. In this research, we assess the efficacy of two instruction strategies in a virtual world environment, Second Life, and their effects on interactivity, social presence, and perceived learning. The two instruction strategies are direct and interactive instruction strategies. Our findings suggest that the interactive instruction strategy is more effective than the direct instruction strategy in increasing perceived learning, social presence, and classroom interactivity in the virtual world environment. The study also captured data on perceived ease of use and usefulness of the virtual world environment for education. The results show that the virtual world environment is perceived by students to be easy to use for both direct and interactive sessions. Students perceived the virtual world environment to be more useful for the interactive session than for the direct instruction session