3,156 research outputs found
Characterization of All-Chromium Tunnel Junctions and Single Electron Tunneling Devices Fabricated by Direct-Writing Multilayer Technique
We report about the fabrication and analysis of the properties of Cr/CrO_x/Cr
tunnel junctions and SET transistors, prepared by different variants of
direct-writing multilayer technique. In all cases, the CrO_x tunnel barriers
were formed in air under ambient conditions. From the experiments on single
junctions, values for the effective barrier height and thickness were derived.
For the Cr/CrO_x/Cr SET transistors we achieved minimal junction areas of 17 x
60 nm^2 using a scanning transmission electron microscope for the e-beam
exposure on Si_3N_4 membrane substrate. We discuss the electrical performance
of the transistor samples as well as their noise behavior.Comment: 19 pages, 9 figure
Cooper pair cotunneling in single charge transistors with dissipative electromagnetic environment
We observed current-voltage characteristics of superconducting single charge
transistors with on-chip resistors of R about R_Q = h/4e^2 = 6.45 kOhm, which
are explained in terms of Cooper-pair cotunneling. Both the effective strength
of Josephson coupling and the cotunneling current are modulated by the
gate-induced charge on the transistor island. For increasing values of the
resistance R we found the Cooper pair current at small transport voltages to be
dramatically suppressed.Comment: 4 pages and 2 figure
Primordial Black Hole Formation during First-Order Phase Transitions
Primordial black holes (PBHs) may form in the early universe when
pre-existing adiabatic density fluctuations enter into the cosmological horizon
and recollapse. It has been suggested that PBH formation may be facilitated
when fluctuations enter into the horizon during a strongly first-order phase
transition which proceeds in approximate equilibrium. We employ
general-relativistic hydrodynamics numerical simulations in order to follow the
collapse of density fluctuations during first-order phase transitions. We find
that during late stages of the collapse fluctuations separate into two regimes,
an inner part existing exclusively in the high-energy density phase with energy
density , surrounded by an outer part which exists
exclusively in the low-energy density phase with energy density , where is the latent heat of the transition. We confirm that the
fluctuation density threshold required for the
formation of PBHs during first-order transitions decreases with increasing
and falls below that for PBH formation during ordinary radiation dominated
epochs. Our results imply that, in case PBHs form at all in the early universe,
their mass spectrum is likely dominated by the approximate horizon masses
during epochs when the universe undergoes phase transitions.Comment: 8 pages, 4 figures, revtex style, submitted to PR
Near-Critical Gravitational Collapse and the Initial Mass Function of Primordial Black Holes
The recent discovery of critical phenomena arising in gravitational collapse
near the threshold of black hole formation is used to estimate the initial mass
function of primordial black holes (PBHs). It is argued that the universal
scaling relation between black hole mass and initial perturbation found for a
variety of collapsing space-times also applies to PBH formation, indicating the
possibility of the formation of PBHs with masses much smaller than one horizon
mass. Owing to the natural fine-tuning of initial conditions by the exponential
decline of the probability distribution for primordial density fluctuations,
sub-horizon mass PBHs are expected to form at all epochs. This result suggests
that the constraints on the primordial fluctuation spectrum based on the
abundance of PBHs at different mass scales may have to be revisited.Comment: 4 pages, uses revtex, also available at
http://bigwhirl.uchicago.edu/jcn/pub_pbh.html . To appear in Phys. Rev. Let
On the small-scale stability of thermonuclear flames in Type Ia supernovae
We present a numerical model which allows us to investigate thermonuclear
flames in Type Ia supernova explosions. The model is based on a finite-volume
explicit hydrodynamics solver employing PPM. Using the level-set technique
combined with in-cell reconstruction and flux-splitting schemes we are able to
describe the flame in the discontinuity approximation. We apply our
implementation to flame propagation in Chandrasekhar-mass Type Ia supernova
models. In particular we concentrate on intermediate scales between the flame
width and the Gibson-scale, where the burning front is subject to the
Landau-Darrieus instability. We are able to reproduce the theoretical
prediction on the growth rates of perturbations in the linear regime and
observe the stabilization of the flame in a cellular shape. The increase of the
mean burning velocity due to the enlarged flame surface is measured. Results of
our simulation are in agreement with semianalytical studies.Comment: 9 pages, 7 figures, Uses AASTEX, emulateapj5.sty, onecolfloat.sty.
Replaced with accepted version (ApJ), Figures 1 and 3 are ne
Fauna edáfica como indicadora de contaminação do solo.
Recentemente, o solo tem se tornado foco de preocupação devido à gradativa contaminação de ambientes terrestres e aquáticos, decorrente do progresso e avanços sócio-econômicos no país. Diante dessa realidade, a demanda por atividades antrópicas menos agressivas ao ambiente é cada vez maior. Os possíveis impactos ambientais, portanto, devem ser monitorados, controlados e remediados, a fim de evitar problemas irreversíveis ao meio ambiente e à sociedade. A ecotoxicologia estuda os efeitos dos poluentes sobre os organismos e a interação destes com o habitat. Para se avaliar o impacto de uma substância no solo, ensaios ecotoxicológicos com metodologia padronizada internacionalmente podem ser realizados com invertebrados edáficos, tais como as minhocas, enquitreídeos e colêmbolos, por serem importantes na decomposição da matéria orgânica do solo. Esses ensaios de laboratório, no entanto, precisam ser adaptados, pois a metodologia padrão se baseia em espécies e condições de clima temperado que não condizem com a realidade do Brasil. Adaptações aos ensaios de efeito agudo (mortalidade), efeito crônico (reprodução) e de fuga (comportamento) têm sido estudados por vários grupos e os resultados têm sido positivos com relação ao uso do substrato com o pó da fibra da casca do côco e utilização de temperaturas maiores que 20ºC. Algumas espécies encontradas no Brasil também têm sido testadas, algumas delas apresentando resultados promissores, outras, com limitações. De modo geral, os métodos para avaliação da contaminação do solo em laboratório têm apresentado avanços, entretanto, mais estudos se fazem necessários para o estabelecimento de espécies nativas recomendadas para cada tipo de ensaio.Resumo expandido
Dynamical Symmetry Breaking in Fractal Space
We formulate field theories in fractal space and show the phase diagrams of
the coupling versus the fractal dimension for the dynamical symmetry breaking.
We first consider the 4-dimensional Gross-Neveu (GN) model in the
(4-d)-dimensional randomized Cantor space where the fermions are restricted to
a fractal space by the high potential barrier of Cantor fractal shape. By the
statistical treatment of this potential, we obtain an effective action
depending on the fractal dimension. Solving the 1/N leading Schwinger-Dyson
(SD) equation, we get the phase diagram of dynamical symmetry breaking with a
critical line similar to that of the d-dimensional (2<d<4) GN model except for
the system-size dependence. We also consider QED4 with only the fermions
formally compactified to d dimensions. Solving the ladder SD equation, we
obtain the phase diagram of dynamical chiral symmetry breaking with a linear
critical line, which is consistent with the known results for d=4 (the
Maskawa-Nakajima case) and d=2 (the case with the external magnetic field).Comment: 28 pages, 5 figures, LaTeX with epsf macr
Critical collapse and the primordial black hole initial mass function
It has normally been assumed that primordial black holes (PBHs) always form
with mass approximately equal to the mass contained within the horizon at that
time. Recent work studying the application of critical phenomena in
gravitational collapse to PBH formation has shown that in fact, at a fixed
time, PBHs with a range of masses are formed. When calculating the PBH initial
mass function it is usually assumed that all PBHs form at the same horizon
mass. It is not clear, however, that it is consistent to consider the spread in
the mass of PBHs formed at a single horizon mass, whilst neglecting the range
of horizon masses at which PBHs can form. We use the excursion set formalism to
compute the PBH initial mass function, allowing for PBH formation at a range of
horizon masses, for two forms of the density perturbation spectrum. First we
examine power-law spectra with , where PBHs form on small scales. We find
that, in the limit where the number of PBHs formed is small enough to satisfy
the observational constraints on their initial abundance, the mass function
approaches that found by Niemeyer and Jedamzik under the assumption that all
PBHs form at a single horizon mass. Second, we consider a flat perturbation
spectrum with a spike at a scale corresponding to horizon mass , and compare the resulting PBH mass function with that of the MACHOs
(MAssive Compact Halo Objects) detected by microlensing observations. The
predicted mass spectrum appears significantly wider than the steeply-falling
spectrum found observationally.Comment: 8 pages RevTeX file with ten figures incorporated (uses RevTeX and
epsf). Minor changes to dicussion onl
Achieving equity through 'gender autonomy': the challenges for VET policy and practice
This paper is based on research carried out in an EU Fifth Framework project on 'Gender and Qualification'. The research partners from five European countries investigated the impact of gender segregation in European labour markets on vocational education and training, with particular regard to competences and qualifications. The research explored the part played by gender in the vocational education and training experiences of (i) young adults entering specific occupations in child care, electrical engineering and food preparation/service (ii) adults changing occupations
Discretized Diffusion Processes
We study the properties of the ``Rigid Laplacian'' operator, that is we
consider solutions of the Laplacian equation in the presence of fixed
truncation errors. The dynamics of convergence to the correct analytical
solution displays the presence of a metastable set of numerical solutions,
whose presence can be related to granularity. We provide some scaling analysis
in order to determine the value of the exponents characterizing the process. We
believe that this prototype model is also suitable to provide an explanation of
the widespread presence of power-law in social and economic system where
information and decision diffuse, with errors and delay from agent to agent.Comment: 4 pages 5 figure, to be published in PR
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