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    ABSTRACT: We report on the first completely simultaneous observation of a gamma-ray burst (GRB) using an array of Imaging Atmospheric Cherenkov Telescopes, which is sensitive to photons in the very high energy (VHE) γ -ray range (100 GeV). On 2006 June 2, the Swift Burst Alert Telescope (BAT) registered an unusually soft γ -ray burst (GRB 060602B). The burst position was under observation using the High Energy Stereoscopic System (HESS) at the time the burst occurred. Data were taken before, during, and after the burst. A total of 5 hr of observations were obtained during the night of 2006 June 2–3, and five additional hours were obtained over the next three nights. No VHE γ -ray signal was found during the period covered by the HESS observations. The 99% confidence level flux upper limit (> 1 TeV) for the prompt phase (9 s) of GRB 060602B is 2.9 × 10 −9 erg cm −2 s −1 . Due to the very soft BAT spectrum of the burst compared with other Swift GRBs and its proximity to the Galactic center, the burst is likely associated with a Galactic X-ray burster, although the possibility of it being a cosmological GRB cannot be ruled out. We discuss the implications of our flux limits in the context of these two bursting scenarios.
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    ABSTRACT: The question of how to introduce thermal fluctuations in the equation of motion of a magnetic system is addressed. Using the approach of the fluctuation-dissipation theorem we calculate the properties of the noise for both, the fluctuating field and the additive fluctuating torque (force) representation. In contrast to earlier calculations we consider the general case of a system of interacting magnetic moments. We show that the interactions do not result in any correlations of thermal fluctuations in the field representation and that the same widely used formula can be used in the most general case. We further prove that close to the equilibrium where the fluctuation-dissipation theorem is valid, both, field and additive torque (force) representations coincide, being different far away from it. We also show that the uncorrelated character of the noise is due to the form of the Landau-Lifshitz (or Gilbert) damping and under different damping formalisms, the normal mode analysis is proper.
  • Chapter: Space
    Keywords in Sound, Edited by David Novak, Matt Sakakeeny, 01/2015; Duke University Press.
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    ABSTRACT: In many areas of physics, the Kramers-Kronig (KK) relations are used to extract information about the real part of the optical response of a medium from its imaginary counterpart. In this paper we discuss an alternative but mathematically equivalent approach based on the Hilbert transform. We apply the Hilbert transform to transmission spectra to find the group and refractive indices of a Cs vapor, and thereby demonstrate how the Hilbert transform allows indirect measurement of the refractive index, group index and group delay whilst avoiding the use of complicated experimental set ups.
    Physical Review A 11/2014; DOI:10.1103/PhysRevA.91.032513
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    ABSTRACT: Modern developmental biology relies on optically sectioning fluorescence microscope techniques to produce non-destructive in vivo images of developing specimens at high resolution in three dimensions. As optimal performance of these techniques is reliant on the three-dimensional (3D) intensity profile of the illumination employed, the ability to directly record and analyze these profiles is of great use to the fluorescence microscopist or instrument builder. Though excitation beam profiles can be measured indirectly using a sample of fluorescent beads and recording the emission along the microscope detection path, we demonstrate an alternative approach where a miniature camera sensor is used directly within the illumination beam. Measurements taken using our approach are solely concerned with the illumination optics as the detection optics are not involved. We present a miniature beam profiling device and high dynamic range flux reconstruction algorithm that together are capable of accurately reproducing quantitative 3D flux maps over a large focal volume. Performance of this beam profiling system is verified within an optical test bench and demonstrated for fluorescence microscopy by profiling the low NA illumination beam of a single plane illumination microscope. The generality and success of this approach showcases a widely flexible beam amplitude diagnostic tool for use within the life sciences.
    Review of Scientific Instruments 10/2014; 85(10):103713. DOI:10.1063/1.4899208
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    ABSTRACT: The effect of a commonly used hole injection layer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT–PSS), on polymer light-emitting diode (PLED) performance has been investigated. A series of four different types of commercial PEDOT–PSS, with varying resistivity and work function were examined in devices with the structure Indium Tin Oxide (ITO)/PEDOT–PSS/High Molecular Weight Poly(n-vinylcarbazole) (PVKH): 30% N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD)/Low molecular Weight Poly(n-vinylcarbazole) (PVKL): 40% 2-(4-Biphenyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole (PBD): 8% Ir(ppy)3. It was found that the PEDOT–PSS with the highest work function and resistivity produced the devices with the highest efficiencies; this is due to the improved hole injection effect, the decrease in electron leakage current and the prevention of pixel crosstalk. A maximum device current efficiency of 33.4 cd A−1 has been achieved for the most resistive PEDOT; this corresponded to an external quantum efficiency (E.Q.E.) of 11%. Increasing the work function of the PEDOT used resulted in a 60% increase in E.Q.E. and device efficiency for PEDOTs in the same resistivity range. Drift–diffusion simulations, carried out using SEmiconducting Thin Film Optics Simulation software (SETFOS) 3.2, produced J–V curves in good agreement with the experimentally observed results; this allowed us to extract qualitative values for the effective device mobility along with the PEDOT work function and resistivity.
    Organic Electronics 01/2014; 15(1):245–250. DOI:10.1016/j.orgel.2013.11.029
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    ABSTRACT: The absolute density of SD radicals in a supersonic jet has been measured down to $(1.1\pm0.1)\times10^5$ cm$^{-3}$ in a modestly specified apparatus that uses a cross-correlated combination of cavity ring-down and laser-induced fluorescence detection. Such a density corresponds to $215\pm21$ molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantum-noise-limited and measured to be $(7.9\pm0.6)\times10^{-11}$ cm$^{-1}$, in 200 s of acquisition time, corresponding to a noise-equivalent absorption sensitivity for the apparatus of $(1.6\pm0.1)\times10^{-9}$ cm$^{-1}$ Hz$^{-1/2}$.
    Physical Chemistry Chemical Physics 10/2013; 15(45). DOI:10.1039/c3cp53394h
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    ABSTRACT: Nickel nanoparticles have been created in an organic-based matrix by the reaction of Ni(COD)2 (COD = 1,5-bis-cyclooctadiene) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4). The size of the nickel nanoparticles can be controlled by the use of different solvents and inclusion of tetrahydrofuran (THF) within the reaction to stabilise the Ni(0) atoms from the Ni(COD)2. Materials are characterised with a combination of X-ray diffraction, electron microscopy and magnetometry and it is found that samples made using a halocarbon solvent resulted in clustered bulk Ni particles (size ≤ 10 nm) with anomalously high superparamagnetic blocking temperatures. Using an isocyanide solvent produces smaller (size ∼ 1 nm), well dispersed particles that show little evidence of superparamagnetic blocking in the range of temperatures investigated (>2 K). In all samples there is another component which dominates the magnetic response at low temperatures and shows an interesting temperature dependent scaling behaviour when plotted as M vs. B/T which we believe is related to the organo-metallic matrix that the particles are trapped within. We propose that the enhanced blocking temperature of particles synthesised using halocarbon solvents can be attributed to inter-particle dipolar interactions and nanoparticle-matrix exchange interactions.
    Nanoscale 10/2013; 5(24). DOI:10.1039/c3nr04883g
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    ABSTRACT: We demonstrate a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=-0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.
    Physical Review Letters 09/2013; 111(11):113901. DOI:10.1103/PhysRevLett.111.113901
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    ABSTRACT: Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.
    Advanced Materials 07/2013; 25(27). DOI:10.1002/adma.201300753
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Physical Chemistry Chemical Physics 09/2013; 16(11). DOI:10.1039/c3cp52603h
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The Journal of Physical Chemistry C 05/2015; 119(23). DOI:10.1021/jp5012941
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