Site symmetry analysis of the 738 nm defect in diamond

Based on a detailed analysis of polarized Raman and luminescence measurements of a ‘‘mosaic’’ diamond film, symmetry properties of a ubiquitous point defect observed in diamond films are determined. Specifically, the defect, which gives rise to emission at 738 nm, is determined unequivocally to be a 〈110〉‐oriented defect with the transition dipole moment of the center oriented along the 〈110〉 symmetry axis. These results represent the first analysis of the symmetry properties of this point defect and aid in the development of structural model of the center. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69825/2/JAPIAU-78-6-4069-1.pd

Fluctuations and oscillations in a simple epidemic model

We show that the simplest stochastic epidemiological models with spatial correlations exhibit two types of oscillatory behaviour in the endemic phase. In a large parameter range, the oscillations are due to resonant amplification of stochastic fluctuations, a general mechanism first reported for predator-prey dynamics. In a narrow range of parameters that includes many infectious diseases which confer long lasting immunity the oscillations persist for infinite populations. This effect is apparent in simulations of the stochastic process in systems of variable size, and can be understood from the phase diagram of the deterministic pair approximation equations. The two mechanisms combined play a central role in explaining the ubiquity of oscillatory behaviour in real data and in simulation results of epidemic and other related models.Comment: acknowledgments added; a typo in the discussion that follows Eq. (3) is corrected

If cooperation is likely punish mildly: Insights from economic experiments based on the snowdrift game

Punishment may deter antisocial behavior. Yet to punish is costly, and the costs often do not offset the gains that are due to elevated levels of cooperation. However, the effectiveness of punishment depends not only on how costly it is, but also on the circumstances defining the social dilemma. Using the snowdrift game as the basis, we have conducted a series of economic experiments to determine whether severe punishment is more effective than mild punishment. We have observed that severe punishment is not necessarily more effective, even if the cost of punishment is identical in both cases. The benefits of severe punishment become evident only under extremely adverse conditions, when to cooperate is highly improbable in the absence of sanctions. If cooperation is likely, mild punishment is not less effective and leads to higher average payoffs, and is thus the much preferred alternative. Presented results suggest that the positive effects of punishment stem not only from imposed fines, but may also have a psychological background. Small fines can do wonders in motivating us to chose cooperation over defection, but without the paralyzing effect that may be brought about by large fines. The later should be utilized only when absolutely necessary.Comment: 15 pages, 6 figures; accepted for publication in PLoS ON

Photoluminescence investigation of GaN films grown by metalorganic chemical vapor deposition on (100) GaAs

GaN films were grown on (100) GaAs substrates by metalorganic chemical vapor deposition and were found to be of (200) cubic or (111) cubic/(0002) hexagonal phase. Their photoluminescence characteristics remained invariant with material phase. We report assignment of band‐edge photoluminescence near 3.36 eV and 3.15–3.31 eV in apparently cubic GaN to intrinsic/bound excitons and phonon‐assisted, donor‐acceptor pair recombination respectively, on the basis of observed temperature and intensity dependences. A free exciton energy of 3.375 eV is deduced at 6.5 K. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69757/2/JAPIAU-77-4-1705-1.pd

Bistable emission of a black-body radiator

Bistable black-body emission is reported from resonantly excited Er3+,Yb3+:Y2O3Er3+,Yb3+:Y2O3 nanopowders. A simple model based on thermo-optic nonlinear response in the strongly scattering random medium explains the observed behavior.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69863/2/APPLAB-85-23-5517-1.pd

Direct glass bonded high specific power silicon solar cells for space applications

A lightweight, radiation hard, high performance, ultra-thin silicon solar cell is described that incorporates light trapping and a cover glass as an integral part of the device. The manufacturing feasibility of high specific power, radiation insensitive, thin silicon solar cells was demonstrated experimentally and with a model. Ultra-thin, light trapping structures were fabricated and the light trapping demonstrated experimentally. The design uses a micro-machined, grooved back surface to increase the optical path length by a factor of 20. This silicon solar cell will be highly tolerant to radiation because the base width is less than 25 microns making it insensitive to reduction in minority carrier lifetime. Since the silicon is bonded without silicone adhesives, this solar cell will also be insensitive to UV degradation. These solar cells are designed as a form, fit, and function replacement for existing state of the art silicon solar cells with the effect of simultaneously increasing specific power, power/area, and power supply life. Using a 3-mil thick cover glass and a 0.3 g/sq cm supporting Al honeycomb, a specific power for the solar cell plus cover glass and honeycomb of 80.2 W/Kg is projected. The development of this technology can result in a revolutionary improvement in high survivability silicon solar cell products for space with the potential to displace all existing solar cell technologies for single junction space applications

Observation of near‐band‐gap luminescence from boron nitride films

We report results from cathodoluminescence spectroscopy of boron nitride films grown on Si(100) substrates by ion‐source‐assisted magnetron sputtering of a hexagonal BN target. Three main peaks are observed in the near‐band‐gap region for hexagonal boron nitride films at energies of 4.90, 5.31, and 5.50 eV. We also report deep‐level emission spectra of predominantly cubic boron nitride films which are correlated with sample growth conditions. In particular we show that the emission intensity, position, and linewidth are strongly dependent on the substrate bias voltage used during sample growth.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69948/2/APPLAB-65-10-1251-1.pd

Site symmetry analysis of the 738 nm defect in diamond

Based on a detailed analysis of polarized Raman and luminescence measurements of a "mosaic" diamond film, symmetry properties of a ubiquitous point defect observed in diamond films are determined. Specifically, the defect, which gives rise to emission at 738 nm, is determined unequivocally to be a (1 lO)-oriented defect with the transition dipole moment of the center oriented along the (110) symmetry axis. These results represent the first analysis of the symmetry properties of this point defect and aid in the development of structural model of the center. 0 199.5 American Institute of Physics