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

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

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

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

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

Optically-induced charge separation and terahertz emission in unbiased dielectrics

Charge separation mediated by linearly-polarized light in transparent insulators is analyzed numerically by integrating the equations of motion of a bound charge. It is shown that large static and transient dipole moments can be induced by the magnetic component of light at nonrelativistic intensities regardless of whether the pumplight is coherent or incoherent. Quantitative estimates show that efficient conversion of optical beams to electrical power is possible in lossless dielectric media and that THz radiation can be generated in unbiased materials through the use of transverse optical magnetism

Controller design with regional pole constraints - Hyperbolic and horizontal strip regions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76652/1/AIAA-1992-4401-149.pd

Parallax of PSR J1744-1134 and the Local Interstellar Medium

We present the annual trigonometric parallax of PSR J1744-1134 derived from an analysis of pulse times of arrival. The measured parallax, pi = 2.8+/-0.3 mas ranks among the most precisely determined distances to any pulsar. The parallax distance of 357+/-39 pc is over twice that derived from the dispersion measure using the Taylor & Cordes model for the Galactic electron distribution. The mean electron density in the path to the pulsar, n_e = (0.0088 +/- 0.0009) cm^{-3}, is the lowest for any disk pulsar. We have compared the n_e for PSR J1744-1134 with those for another 11 nearby pulsars with independent distance estimates. We conclude that there is a striking asymmetry in the distribution of electrons in the local interstellar medium. The electron column densities for pulsars in the third Galactic quadrant are found to be systematically higher than for those in the first. The former correlate with the position of the well known local HI cavity in quadrant three. The excess electrons within the cavity may be in the form of HII clouds marking a region of interaction between the local hot bubble and a nearby superbubble.Comment: revised version accepted for publication in ApJ Letters; reanalysis of uncertainty in parallax measure and changes to fig

Breakdown of Conformal Invariance at Strongly Random Critical Points

We consider the breakdown of conformal and scale invariance in random systems with strongly random critical points. Extending previous results on one-dimensional systems, we provide an example of a three-dimensional system which has a strongly random critical point. The average correlation functions of this system demonstrate a breakdown of conformal invariance, while the typical correlation functions demonstrate a breakdown of scale invariance. The breakdown of conformal invariance is due to the vanishing of the correlation functions at the infinite disorder fixed point, causing the critical correlation functions to be controlled by a dangerously irrelevant operator describing the approach to the fixed point. We relate the computation of average correlation functions to a problem of persistence in the RG flow.Comment: 9 page

Luminous infrared galaxies as possible sources of the UHE cosmic rays

Ultra High Energy (UHE) particles coming from discrete extragalactic sources are potential candidates for EAS events above a few tens of EeV. In particular, galaxies with huge infrared luminosity triggered by collision and merging processes are possible sites of UHECR acceleration. Using the PSCz catalogue of IR galaxies we calculate a large scale anisotropy of UHE protons originating in the population of the luminous infrared galaxies (LIRGs). Small angle particle scattering in weak irregular extragalactic magnetic fields as well as deflection by regular Galactic field are taken into account. We give analytical formulae for deflection angles with included energy losses on cosmic microwave background (CMB). The hypotheses of the anisotropic and isotropic distributions of the experimental data above 40 EeV from AGASA are checked, using various statistical tests. We show that on the basis of the small scale clustering analysis there is a much better correlation of the UHECRs data below GZK cut-off with the predictions of the LIRG origin than with those of isotropy. We derive analytical formulae for a probability of a given number of doublets, triplets and quadruplets for any density distribution of independent events on the sky. The famous AGASA UHE triple event is found to be very well correlated on the sky with the brightest extragalactic infrared source within 70 Mpc - merger galaxies Arp 299 (NGC 3690 + IC 694).Comment: 17 pages, 10 figures, accepted for publ: Journal of Physics