Yellow and green luminescence in a freestanding GaN template

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving galliumvacancies and the YL to the same defect, but bound to dislocations, or possibly to structuralsurface defects

Two charge states of dominant acceptor in unintentionally doped GaN: Evidence from photoluminescence study

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence (YL) band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence (GL) band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex

Transient photoluminescence of defect transitions in freestanding GaN

Deep level defects responsible for the 2.4 eV photoluminescence (PL) band in a freestanding GaN template were studied by transient photoluminescence. A nonexponential decay of PL intensity observed at low temperature is attributed to a donor–acceptor pair recombination involving a shallow donor and a deep acceptor. At room temperature, a single-exponential PL decay with a lifetime of 30 μs was observed at the high-energy side of the band, whereas the second component with a lifetime of about 750 μs was detected at the low-energy side of the band. The PL decay and transformation of the PL spectrum at room temperature can be explained by transitions from the conduction band to two deep acceptors. Electron-capture cross section has been estimated as 4×10−21 and 10−19 cm2 for the yellow and green bands, respectively, contributing to the broad 2.4 eV band

Utility and lower limits of frequency detection in surface electrode stimulation for somatosensory brain-computer interface in humans

Objective: Stimulation of the primary somatosensory cortex (S1) has been successful in evoking artificial somatosensation in both humans and animals, but much is unknown about the optimal stimulation parameters needed to generate robust percepts of somatosensation. In this study, the authors investigated frequency as an adjustable stimulation parameter for artificial somatosensation in a closed-loop brain-computer interface (BCI) system. Methods: Three epilepsy patients with subdural mini-electrocorticography grids over the hand area of S1 were asked to compare the percepts elicited with different stimulation frequencies. Amplitude, pulse width, and duration were held constant across all trials. In each trial, subjects experienced 2 stimuli and reported which they thought was given at a higher stimulation frequency. Two paradigms were used: first, 50 versus 100 Hz to establish the utility of comparing frequencies, and then 2, 5, 10, 20, 50, or 100 Hz were pseudorandomly compared. Results: As the magnitude of the stimulation frequency was increased, subjects described percepts that were “more intense” or “faster.” Cumulatively, the participants achieved 98.0% accuracy when comparing stimulation at 50 and 100 Hz. In the second paradigm, the corresponding overall accuracy was 73.3%. If both tested frequencies were less than or equal to 10 Hz, accuracy was 41.7% and increased to 79.4% when one frequency was greater than 10 Hz (p = 0.01). When both stimulation frequencies were 20 Hz or less, accuracy was 40.7% compared with 91.7% when one frequency was greater than 20 Hz (p < 0.001). Accuracy was 85% in trials in which 50 Hz was the higher stimulation frequency. Therefore, the lower limit of detection occurred at 20 Hz, and accuracy decreased significantly when lower frequencies were tested. In trials testing 10 Hz versus 20 Hz, accuracy was 16.7% compared with 85.7% in trials testing 20 Hz versus 50 Hz (p < 0.05). Accuracy was greater than chance at frequency differences greater than or equal to 30 Hz. Conclusions: Frequencies greater than 20 Hz may be used as an adjustable parameter to elicit distinguishable percepts. These findings may be useful in informing the settings and the degrees of freedom achievable in future BCI systems

Weighted Dirac combs with pure point diffraction

A class of translation bounded complex measures, which have the form of weighted Dirac combs, on locally compact Abelian groups is investigated. Given such a Dirac comb, we are interested in its diffraction spectrum which emerges as the Fourier transform of the autocorrelation measure. We present a sufficient set of conditions to ensure that the diffraction measure is a pure point measure. Simultaneously, we establish a natural link to the theory of the cut and project formalism and to the theory of almost periodic measures. Our conditions are general enough to cover the known theory of model sets, but also to include examples such as the visible lattice points.Comment: 44 pages; several corrections and improvement

Spectral density analysis of time correlation functions in lattice QCD using the maximum entropy method

We study various aspects of extracting spectral information from time correlation functions of lattice QCD by means of Bayesian inference with an entropic prior, the maximum entropy method (MEM). Correlator functions of a heavy-light meson-meson system serve as a repository for lattice data with diverse statistical quality. Attention is given to spectral mass density functions, inferred from the data, and their dependence on the parameters of the MEM. We propose to employ simulated annealing, or cooling, to solve the Bayesian inference problem, and discuss practical issues of the approach.Comment: 11 pages, 30 figure files (eps), some references added and updated, requires REVTeX 4.0 and prerequisites (AMS-LaTeX 2.0, graphicx, dcolumn, bm) see http://publish.aps.org/revtex4

ALGAE TO ALKANES

Once considered infeasible and unviable, recently there has been renewed interest in the development of algae-derived transportation fuels. Currently, there are no commercialized algae to fuel ventures, and much debate is centered on the economic viability of such a process. Research conducted by NASA, among others, has expressed skepticism that terrestrially cultivated algae can ever compete with conventional fuels. The purpose of this project is to evaluate the economic feasibility of an algae-to-fuel venture that incorporates the state-of-the-art technologies available in the open literature. Our challenge is to produce 20 thousand barrels per day of n-alkane product that meets the current diesel fuel specifications. To arrive at a recommendation, separate models were built for algae cultivation, lipid extraction, and lipid processing at a scale necessary to reach this target. This analysis departs from prior studies on two major fronts. First, this analysis considers OriginOil’s new method of lipid extraction instead of conventional hexane extraction. Second, the objective of the lipid processing module is to produce n-alkanes from triglycerides, as opposed to producing FAME biodiesel. The n-alkane product from this process is comparable to petroleum-based diesel fuels. Thus it can be readily incorporated into existing energy infrastructure as a diesel blending stock or as a feedstock for other processing units in the refinery. Our economic analysis shows that an algae-to-fuel venture is profitable if the fuel is sold at $3/gallon, the current price of diesel. However, the commercialization of such a process is difficult due to the large total capital investment. At$2.2 billion, the capital investment of algae cultivation is nearly 40 times that of processing, which results in annual depreciation and fixed costs of nearly half of the revenue. Investors would be hesitant to invest such a large amount of money in an algae cultivation process where there is high uncertainty in the cost requirements. Algae-to-fuel economics can be improved by realizing higher value uses of the algae biomass. Biomass composes of over half of algae product, and their potential uses in pharmaceuticals, chemicals, and biomass power generation far surpass their value as animal feed. Proposed carbon-cap-and-trade programs may bring additional revenue. Thus, any algae-to-fuel venture should seek to optimize the value of its byproducts. Governments can support algae-to-fuel ventures by offering tax credits or mandating a market for renewable fuels, but the benefits of these measures are unclear. Additional analysis should address the uncertainties of various costs and look to reduce capital investment

Long-lasting photoluminescence in freestanding GaN templates

We studied time-resolvedphotoluminescence(PL) over a temporal range 10−6–103 s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor–acceptor emission bands can be resolved in the PLspectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor–acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states in this effect is discussed

Statin use and adverse effects among adults \u3e 75 years of age: Insights from the Patient and Provider Assessment of Lipid Management (PALM) registry

Background: Current statin use and symptoms among older adults in routine community practice have not been well characterized since the release of the 2013 American College of Cardiology/American Heart Association guideline. Methods and results: We compared statin use and dosing between adults \u3e75 and ≤75 years old who were eligible for primary or secondary prevention statin use without considering guideline-recommended age criteria. The patients were treated at 138 US practices in the Patient and Provider Assessment of Lipid Management (PALM) registry in 2015. Patient surveys also evaluated reported symptoms while taking statins. Multivariable logistic regression models examined the association between older age and statin use and dosing. Among 6717 people enrolled, 1704 (25%) were \u3e75 years old. For primary prevention, use of any statin or high-dose statin did not vary by age group: any statin, 62.6% in those \u3e75 years old versus 63.1% in those ≤75 years old (P=0.83); high-dose statin, 10.2% versus 12.3% in the same groups (P=0.14). For secondary prevention, older patients were slightly less likely to receive any statin (80.1% versus 84.2% [P=0.003]; adjusted odds ratio, 0.81; 95% confidence interval, 0.66-1.01 [P=0.06]), but were much less likely to receive a high-intensity statin (23.5% versus 36.2% [PP=0.0001]). Among current statin users, older patients were slightly less likely to report any symptoms (41.3% versus 46.6%; P=0.003) or myalgias (27.3% versus 33.3%; Conclusions: Overall use of statins was similar for primary prevention in those aged \u3e75 years versus younger patients, yet older patients were less likely to receive high-intensity statins for secondary prevention. Statins appear to be similarly tolerated in older and younger adult

Names, varieties and ideologies in revived Cornish

The attribution of names is a significant process that often highlights concerns over identity, ideology and ownership. Within the fields of minority languages and Celtic Studies, such concerns are especially pertinent given that the identities in question are frequently perceived as under threat from dominant cultures. The effect of concerns caused by this can be examined with reference to revived Cornish, which became divided into three major varieties in the later twentieth century; by examining the names of these varieties, we can draw conclusions about how they are perceived, or we are invited to perceive them. The motivations of those involved in the Cornish language revival are equally reflected in the names of the organisations and bodies they have formed, which equally contribute to the legitimation of revived Cornish. This paper examines both these categories of name, as well as the phenomenon of Kernowisation, a term coined by Harasta (2013) to refer to the adoption of Cornish personal names, and here extended to the use of Cornish names in otherwise English-language contexts. Examining the names that have been implemented during the Cornish language revival, and the ways in which they are used or indeed refused by those involved, gives us an insight into the various ideologies that steer the revival process. Within the context of the precarious nature of Cornish and Celtic identity, we can identify the concerns of those involved in the Cornish revival movement and highlight the role of naming as an activity of legitimation, showing how the diversity of names that occur reflects an equally diverse range of motivations and influences