314 research outputs found
Field-Dependent Critical Current in Type-II Superconducting Strips: Combined Effect of Bulk Pinning and Geometrical Edge Barrier
Recent theoretical and experimental research on low-bulk-pinning
superconducting strips has revealed striking dome-like magnetic-field
distributions due to geometrical edge barriers. The observed magnetic-flux
profiles differ strongly from those in strips in which bulk pinning is
dominant. In this paper we theoretically describe the current and field
distributions of a superconducting strip under the combined influence of both a
geometrical edge barrier and bulk pinning at the strip's critical current Ic,
where a longitudinal voltage first appears. We calculate Ic and find its
dependence upon a perpendicular applied magnetic field Ha. The behavior is
governed by a parameter p, defined as the ratio of the bulk-pinning critical
current Ip to the geometrical-barrier critical current Is0. We find that when p
> 2/pi and Ip is field-independent, Ic vs Ha exhibits a plateau for small Ha,
followed by the dependence Ic-Ip ~ 1/Ha in higher magnetic fields.Comment: 4 pages, 2 figures, Fig. 1 revised, submitted to Phys. Rev.
Generation of Ultrastable Microwaves via Optical Frequency Division
There has been increased interest in the use and manipulation of optical
fields to address challenging problems that have traditionally been approached
with microwave electronics. Some examples that benefit from the low
transmission loss, agile modulation and large bandwidths accessible with
coherent optical systems include signal distribution, arbitrary waveform
generation, and novel imaging. We extend these advantages to demonstrate a
microwave generator based on a high-Q optical resonator and a frequency comb
functioning as an optical-to-microwave divider. This provides a 10 GHz
electrical signal with fractional frequency instability <8e-16 at 1 s, a value
comparable to that produced by the best microwave oscillators, but without the
need for cryogenic temperatures. Such a low-noise source can benefit radar
systems, improve the bandwidth and resolution of communications and digital
sampling systems, and be valuable for large baseline interferometry, precision
spectroscopy and the realization of atomic time
Role of substrate quality on the performance of semipolar (11 2 - 2) InGaN light-emitting diodes
We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar (11 2 - 2) GaN substrate (Bulk-GaN) and a low-cost large-size (11 2 - 2) GaN template created on patterned (10 1 - 2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼ and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ∼2 × 108cm-2 and BSF density of ∼1 × 103cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density
AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy
We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 109 cm-2. However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %)
Inclusive Production Cross Sections from 920 GeV Fixed Target Proton-Nucleus Collisions
Inclusive differential cross sections and
for the production of \kzeros, \lambdazero, and
\antilambda particles are measured at HERA in proton-induced reactions on C,
Al, Ti, and W targets. The incident beam energy is 920 GeV, corresponding to
GeV in the proton-nucleon system. The ratios of differential
cross sections \rklpa and \rllpa are measured to be and , respectively, for \xf . No significant dependence upon the
target material is observed. Within errors, the slopes of the transverse
momentum distributions also show no significant
dependence upon the target material. The dependence of the extrapolated total
cross sections on the atomic mass of the target material is
discussed, and the deduced cross sections per nucleon are
compared with results obtained at other energies.Comment: 17 pages, 7 figures, 5 table
Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope
The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and hyperspectral cathodoluminescence imaging (CL) provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a materials' light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires
Estimating the potential survival gains by eliminating socioeconomic and sex inequalities in stage at diagnosis of melanoma.
BACKGROUND: Although inequalities in cancer survival are thought to reflect inequalities in stage at diagnosis, little evidence exists about the size of potential survival gains from eliminating inequalities in stage at diagnosis. METHODS: We used data on patients diagnosed with malignant melanoma in the East of England (2006-2010) to estimate the number of deaths that could be postponed by completely eliminating socioeconomic and sex differences in stage at diagnosis after fitting a flexible parametric excess mortality model. RESULTS: Stage was a strong predictor of survival. There were pronounced socioeconomic and sex inequalities in the proportion of patients diagnosed at stages III-IV (12 and 8% for least deprived men and women and 25 and 18% for most deprived men and women, respectively). For an annual cohort of 1025 incident cases in the East of England, eliminating sex and deprivation differences in stage at diagnosis would postpone approximately 24 deaths to beyond 5 years from diagnosis. Using appropriate weighting, the equivalent estimate for England would be around 215 deaths, representing 11% of all deaths observed within 5 years from diagnosis in this population. CONCLUSIONS: Reducing socioeconomic and sex inequalities in stage at diagnosis would result in substantial reductions in deaths within 5 years of a melanoma diagnosis.This article is an independent research supported by different
funding bodies, beyond the authors’ own employing organisations.
MJR was partially funded by a Cancer Research UK Postdoctoral
Fellowship (CRUK_A13275). GL is supported by a Postdoctoral
Fellowship award by the National Institute for Health Research
(NIHR PDF-2011-04-047) to end of 2014 and a Cancer Research
UK Clinician Scientist Fellowship award (A18180) from January
2015. The views expressed in this publication are those of the
authors and not necessarily those of the National Health Service
(NHS), the National Institute for Health Research, the Department
of Health, Cancer Research UK, or any other organisation. We
thank all staff at the National Cancer Registration Service, Public
Health England, Eastern Office, who helped collect and code data
used in this study. We particularly acknowledge the help of Dr
Clement H Brown and Dr Brian A Rous who were responsible for
staging.This is the final published version. It first appeared at http://www.nature.com/bjc/journal/v112/n1s/full/bjc201550a.html
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