384 research outputs found
Multiplication and excess noise characteristics of thin 4H-SiC UV avalanche photodiodes
The avalanche multiplication and excess noise characteristics of thin 4H-SiC avalanche photodiodes with an i-region width of 0.1 µm have been investigated. The diodes are found to exhibit multiplication characteristics which change significantly when the wavelength of the illuminating light changes from 230 to 365 nm. These multiplication characteristics show unambiguously that β > α in 4H-SiC and that the β/α ratio remains large even in thin 4H-SiC diodes. Low excess noise, corresponding to k=0.1 in the local model where k=α/β for hole injection, was measured using 325-nm light. The results indicate that 4H-SiC is a suitable material for realizing low-noise UV avalanche photodiodes requiring good visible-blind performance
Nonlocal effects in thin 4H-SiC UV avalanche photodiodes
The avalanche multiplication and excess noise characteristics of 4H-SiC avalanche photodiodes with i-region widths of 0.105 and 0.285 mum have been investigated using 230-365-nm light, while the responsivities of the photodiodes at unity gain were examined for wavelengths up to 375 nm. Peak unity gain responsivities of more than 130 mA/W at 265 nm, equivalent to quantum efficiencies of more than 60%, were obtained for both structures. The measured avalanche characteristics show, that beta > alpha and that the beta/alpha ratio remains large even in thin 4H-SiC avalanche regions. Very low excess noise, corresponding to k(eff) < 0.15 in the local noise model, where k(eff) = alpha/beta(beta/alpha) for hole (electron) injection, was measured with 365-nm light in both structures. Modeling the experimental results using a simple quantum efficiency model and a nonlocal description yields effective ionization threshold energies of 12 and 8 eV for electrons and holes, respectively, and suggests that the dead space in 4H-SiC is soft. Although dead space is important, pure hole injection is still required to ensure low excess noise in thin 4H-SiC APDs owing to beta/alpha ratios that remain large, even at very high fields
Використання словникових технологій у процесі вивчення іноземних мов
Методичні рекомендації щодо використання словникових технологій складаються з теоретичної і прикладної частин. Теоретична частина подає характеристики основних типів словників і рекомендації щодо користування ними. Прикладна частина містить добірку завдань з основних видів робіт з урахуванням тем навчальних програм для студентів різних спеціальностей денної та заочної форми навчання, які володіють основами граматики, необхідним лексичним запасом для розуміння запропонованого матеріалу й навичками усного мовлення. Рекомендуються для використання на практичних заняттях, а також для самостійної роботи студентів із залученням словників різних типів. Ці методичні рекомендації розроблені з метою надання методичної допомоги викладачам і студентам у процесі роботи зі словником під час вивчення іноземних мов. Книга включає безліч пропозицій для формування вмінь і навичок роботи зі словником на заняттях і під час самостійної роботи. Теоретична частина написана українською мовою та подає роз’яснення особливостей словників різних типів, способів їх використання й умови оптимального вибору словника. Прикладна частина на матеріалі англійської мови пропонує основні види роботи зі словником, спрямовані на формування вмінь і навичок правильної вимови, читання, письма, граматики, перекдау, а також збагаченя власного словникового запасу. Для кожного виду роботи зі словником підібрано п’ять завдань по темах робочих програм з навчальної дисципліни “Англійська мова” для студентів вищих навчальних закладів. Крім того, запропоновані матеріали розкривають такі теми, як сполученя слів, фразові дієслова, ідіоми та словотворення. Види роботи зі словником спрямовані на використання не тільки традиційних паперових, а й електронних та інших нестандартних спеціалізованих словників. Матеріал цих рекомендацій може бути використаний як джерело навчальної та практичної інформації для викладачів і студентів різних спеціальностей. Автор сподівається, що методичні рекомендації будуть цінними для будь-якого користувача, який хоче поліпшити своє особисте знання словника
Primordial Black Holes: Observational Characteristics of The Final Evaporation
Many early universe theories predict the creation of Primordial Black Holes
(PBHs). PBHs could have masses ranging from the Planck mass to 10^5 solar
masses or higher depending on the size of the universe at formation. A Black
Hole (BH) has a Hawking temperature which is inversely proportional to its
mass. Hence a sufficiently small BH will quasi-thermally radiate particles at
an ever-increasing rate as emission lowers its mass and raises its temperature.
The final moments of this evaporation phase should be explosive and its
description is dependent on the particle physics model. In this work we
investigate the final few seconds of BH evaporation, using the Standard Model
and incorporating the most recent Large Hadron Collider (LHC) results, and
provide a new parameterization for the instantaneous emission spectrum. We
calculate for the first time energy-dependent PBH burst light curves in the
GeV/TeV energy range. Moreover, we explore PBH burst search methods and
potential observational PBH burst signatures. We have found a unique signature
in the PBH burst light curves that may be detectable by GeV/TeV gamma-ray
observatories such as the High Altitude Water Cerenkov (HAWC) observatory. The
implications of beyond the Standard Model theories on the PBH burst
observational characteristics are also discussed, including potential
sensitivity of the instantaneous photon detection rate to a squark threshold in
the 5 -10 TeV range.Comment: Accepted to Astroparticle Physics Journal (71 Pages, 22 Figures
Multiscale magnetic underdense regions on the solar surface: Granular and Mesogranular scales
The Sun is a non-equilibrium dissipative system subjected to an energy flow
which originates in its core. Convective overshooting motions create
temperature and velocity structures which show a temporal and spatial
evolution. As a result, photospheric structures are generally considered to be
the direct manifestation of convective plasma motions. The plasma flows on the
photosphere govern the motion of single magnetic elements. These elements are
arranged in typical patterns which are observed as a variety of multiscale
magnetic patterns. High resolution magnetograms of quiet solar surface revealed
the presence of magnetic underdense regions in the solar photosphere, commonly
called voids, which may be considered a signature of the underlying convective
structure. The analysis of such patterns paves the way for the investigation of
all turbulent convective scales from granular to global. In order to address
the question of magnetic structures driven by turbulent convection at granular
and mesogranular scales we used a "voids" detection method. The computed voids
distribution shows an exponential behavior at scales between 2 and 10 Mm and
the absence of features at 5-10 Mm mesogranular scales. The absence of
preferred scales of organization in the 2-10 Mm range supports the multiscale
nature of flows on the solar surface and the absence of a mesogranular
convective scale
Lyapunov exponents, bifurcation currents and laminations in bifurcation loci
Bifurcation loci in the moduli space of degree rational maps are shaped
by the hypersurfaces defined by the existence of a cycle of period and
multiplier 0 or . Using potential-theoretic arguments, we
establish two equidistribution properties for these hypersurfaces with respect
to the bifurcation current. To this purpose we first establish approximation
formulas for the Lyapunov function. In degree , this allows us to build
holomorphic motions and show that the bifurcation locus has a lamination
structure in the regions where an attracting basin of fixed period exists
The Fueling and Evolution of AGN: Internal and External Triggers
In this chapter, I review the fueling and evolution of active galactic nuclei
(AGN) under the influence of internal and external triggers, namely intrinsic
properties of host galaxies (morphological or Hubble type, color, presence of
bars and other non-axisymmetric features, etc) and external factors such as
environment and interactions. The most daunting challenge in fueling AGN is
arguably the angular momentum problem as even matter located at a radius of a
few hundred pc must lose more than 99.99 % of its specific angular momentum
before it is fit for consumption by a BH. I review mass accretion rates,
angular momentum requirements, the effectiveness of different fueling
mechanisms, and the growth and mass density of black BHs at different epochs. I
discuss connections between the nuclear and larger-scale properties of AGN,
both locally and at intermediate redshifts, outlining some recent results from
the GEMS and GOODS HST surveys.Comment: Invited Review Chapter to appear in LNP Volume on "AGN Physics on All
Scales", Chapter 6, in press. 40 pages, 12 figures. Typo in Eq 5 correcte
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
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