880 research outputs found
Origin of the scatter in the X-ray luminosity of early-type galaxies observed with ROSAT
Statistical properties of X-ray luminosity and temperature are studied for 52
early-type galaxies based on the ROSAT PSPC data. All of the X-ray luminous
galaxies show largely extended emission with a radius of a few times of
10, while X-ray faint galaxies do not show such a component. This leads to
a division of early-type galaxies into two categories: X-ray extended and X-ray
compact galaxies. Except for a few galaxies in dense cluster environments, the
luminosity and temperature of X-ray compact galaxies are well explained by a
kinematical heating of the gas supplied by stellar mass loss. In contrast,
X-ray extended galaxies indicate large scatter in the X-ray luminosity. We
discuss that X-ray extended galaxies are the central objects of large potential
structures, and the presence and absence of this potential is the main origin
of the large scatter in the X-ray luminosity.Comment: 35 pages, including 8 figures, Accepted for publication in Ap
Phonon distributions of a single bath mode coupled to a quantum dot
The properties of an unconventional, single mode phonon bath coupled to a
quantum dot, are investigated within the rotating wave approximation. The
electron current through the dot induces an out of equilibrium bath, with a
phonon distribution qualitatively different from the thermal one. In selected
transport regimes, such a distribution is characterized by a peculiar selective
population of few phonon modes and can exhibit a sub-Poissonian behavior. It is
shown that such a sub-Poissonian behavior is favored by a double occupancy of
the dot. The crossover from a unequilibrated to a conventional thermal bath is
explored, and the limitations of the rotating wave approximation are discussed.Comment: 21 Pages, 7 figures, to appear in New Journal of Physics - Focus on
Quantum Dissipation in Unconventional Environment
Comment on: "Estimating the Hartree-Fock limit from finite basis set calculations" [Jensen F (2005) Theor Chem Acc 113:267]
We demonstrate that a minor modification of the extrapolation proposed by
Jensen [(2005): Theor Chem Acc 113:267] yields very reliable estimates of the
Hartree-Fock limit in conjunction with correlation consistent basis sets.
Specifically, a two-point extrapolation of the form
yields HF limits
with an RMS error of 0.1 millihartree using aug-cc-pVQZ and
aug-cc-pV5Z basis sets, and of 0.01 millihartree using aug-cc-pV5Z and
aug-cc-pV6Z basis sets.Comment: Theoretical Chemistry Accounts, in pres
Current-oscillator correlation and Fano factor spectrum of quantum shuttle with finite bias voltage and temperature
A general master equation is derived to describe an electromechanical
single-dot transistor in the Coulomb blockade regime. In the equation, Fermi
distribution functions in the two leads are taken into account, which allows
one to study the system as a function of bias voltage and temperature of the
leads. Furthermore, we treat the coherent interaction mechanism between
electron tunneling events and the dynamics of excited vibrational modes.
Stationary solutions of the equation are numerically calculated. We show
current through the oscillating island at low temperature appears step like
characteristics as a function of the bias voltage and the steps depend on mean
phonon number of the oscillator. At higher temperatures the current steps would
disappear and this event is accompanied by the emergence of thermal noise of
the charge transfer. When the system is mainly in the ground state, zero
frequency Fano factor of current manifests sub-Poissonian noise and when the
system is partially driven into its excited states it exhibits super-Poissonian
noise. The difference in the current noise would almost be removed for the
situation in which the dissipation rate of the oscillator is much larger than
the bare tunneling rates of electrons.Comment: 14 pages, 8 figure
2,9,16,23-Tetrakis(1-methylethyl)-5,6,11,12,13,14,19,20,25,26,27,28-dodecadehydrotetrabenzo[a,e,k,o]cycloeicosene1
The title compound, C48H40, is a tetraisopropyl-substituted polyannulenoenyne. The unsubstituted polyannulenoenyne, C36H16 (CSD: RICVEE; CAS: 186494-87-1), has quasi-D
2 (222) symmetry, as determined by least-squares fit (excluding H atoms) to a model optimized in D
2 symmetry by molecular mechanics (r.m.s. deviation = 0.239 Å). The least-squares fits of 36 common C atoms of the title compound (at 90 K) to the parent (at 295 K) and to the optimized model show r.m.s. deviations of 0.419 and 0.426 Å, respectively
How democracies prevail: democratic resilience as a two-stage process
This article introduces a novel conceptualization of democratic resilience - a two-stage process where democracies avoid democratic declines altogether or avert democratic breakdown given that such autocratization is ongoing. Drawing on the Episodes of Regime Transformation (ERT) dataset, we find that democracies have had a high level of resilience to onset of autocratization since 1900. Nevertheless, democratic resilience has become substantially weaker since the end of the Cold War. Fifty-nine episodes of sustained and substantial declines in democratic practices have occurred since 1993, leading to the unprecedented breakdown of 36 democratic regimes. Ominously, we find that once autocratization begins, only one in five democracies manage to avert breakdown. We also analyse which factors are associated with each stage of democratic resilience. The results suggest that democracies are more resilient when strong judicial constraints on the executive are present and democratic institutions were strong in the past. Conversely and adding nuance to the literature, economic development is only associated with resilience to onset of autocratization, not to resilience against breakdown once autocratization has begun
Mission X: Train Like an Astronaut Pilot Study
Mission X: Train Like an Astronaut is an international educational challenge focusing on fitness and nutrition as we encourage students to "train like an astronaut." Teams of students (aged 8-12) learn principles of healthy eating and exercise, compete for points by finishing training modules, and get excited about their future as "fit explorers." The 18 core exercises (targeting strength, endurance, coordination, balance, spatial awareness, and more) involve the same types of skills that astronauts learn in their training and use in spaceflight. This first-of-its-kind cooperative outreach program has allowed 14 space agencies and various partner institutions to work together to address quality health/fitness education, challenge students to be more physically active, increase awareness of the importance of lifelong health and fitness, teach students how fitness plays a vital role in human performance for exploration, and inspire and motivate students to pursue careers in STEM fields. The project was initiated in 2009 in response to a request by the International Space Life Sciences Working Group. USA, Netherlands, Italy, France, Germany, Austria, Colombia, Spain, and United Kingdom hosted teams for the pilot this past spring, and Japan held a modified version of the challenge. Several more agencies provided input into the preparations. Competing on 131 teams, more than 3700 students from 40 cities worldwide participated in the first round of Mission X. OUTCOMES AND BEST PRACTICES Members of the Mission X core team will highlight the outcomes of this international educational outreach pilot project, show video highlights of the challenge, provide the working group s initial assessment of the project and discuss the future potential of the effort. The team will also discuss ideas and best practices for international partnership in education outreach efforts from various agency perspectives and experience
Dephasing in sequential tunneling through a double-dot interferometer
We analyze dephasing in a model system where electrons tunnel sequentially
through a symmetric interference setup consisting of two single-level quantum
dots. Depending on the phase difference between the two tunneling paths, this
may result in perfect destructive interference. However, if the dots are
coupled to a bath, it may act as a which-way detector, leading to partial
suppression of the phase-coherence and the reappearance of a finite tunneling
current. In our approach, the tunneling is treated in leading order whereas
coupling to the bath is kept to all orders (using P(E) theory). We discuss the
influence of different bath spectra on the visibility of the interference
pattern, including the distinction between "mere renormalization effects" and
"true dephasing".Comment: 18 pages, 8 figures; For a tutorial introduction to dephasing see
http://iff.physik.unibas.ch/~florian/dephasing/dephasing.htm
Resonances in a two-dimensional electron waveguide with a single delta-function scatterer
We study the conductance properties of a straight two-dimensional electron
waveguide with an s-like scatterer modeled by a single delta-function potential
with a finite number of modes. Even such a simple system exhibits interesting
resonance phenomena. These resonances are explained in terms of quasi-bound
states both by using a direct solution of the Schroedinger equation and by
studying the Green's function of the system. Using the Green's function we
calculate the survival probability as well as the power absorption and show the
influence of the quasi-bound states on these two quantities.Comment: 5 pages, 6 figures, to be published in Physical Review
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
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