120 research outputs found
Conductance of a Quantum Point Contact in the presence of a Scanning Probe Microscope Tip
Using the recursive Green's function technique, we study the coherent
electron conductance of a quantum point contact in the presence of a scanning
probe microscope tip. Images of the coherent fringe inside a quantum point
contact for different widths are obtained. It is found that the conductance of
a specific channel is reduced while other channels are not affected as long as
the tip is located at the positions correspending to that channel. Moreover,
the coherent fringe is smoothed out by increasing the temperature or the
voltage across the device. Our results are consistent with the experiments
reported by Topinka et al.[Science 289, 2323 (2000)].Comment: 5 page
A C*-Algebraic Model for Locally Noncommutative Spacetimes
Locally noncommutative spacetimes provide a refined notion of noncommutative
spacetimes where the noncommutativity is present only for small distances. Here
we discuss a non-perturbative approach based on Rieffel's strict deformation
quantization. To this end, we extend the usual C*-algebraic results to a
pro-C*-algebraic framework.Comment: 13 pages, LaTeX 2e, no figure
Transport properties of strongly correlated metals:a dynamical mean-field approach
The temperature dependence of the transport properties of the metallic phase
of a frustrated Hubbard model on the hypercubic lattice at half-filling are
calculated. Dynamical mean-field theory, which maps the Hubbard model onto a
single impurity Anderson model that is solved self-consistently, and becomes
exact in the limit of large dimensionality, is used. As the temperature
increases there is a smooth crossover from coherent Fermi liquid excitations at
low temperatures to incoherent excitations at high temperatures. This crossover
leads to a non-monotonic temperature dependence for the resistance,
thermopower, and Hall coefficient, unlike in conventional metals. The
resistance smoothly increases from a quadratic temperature dependence at low
temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar
a/e^2 (where "a" is a lattice constant) associated with mean-free paths less
than a lattice constant. Further signatures of the thermal destruction of
quasiparticle excitations are a peak in the thermopower and the absence of a
Drude peak in the optical conductivity. The results presented here are relevant
to a wide range of strongly correlated metals, including transition metal
oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure
A Biased Review of Sociophysics
Various aspects of recent sociophysics research are shortly reviewed:
Schelling model as an example for lack of interdisciplinary cooperation,
opinion dynamics, combat, and citation statistics as an example for strong
interdisciplinarity.Comment: 16 pages for J. Stat. Phys. including 2 figures and numerous
reference
Academic freedom: in justification of a universal ideal
This paper examines the justification for, and benefits of, academic freedom to academics, students, universities and the world at large. The paper surveys the development of the concept of academic freedom within Europe, more especially the impact of the reforms at the University of Berlin instigated by Wilhelm von Humboldt. Following from this, the paper examines the reasons why the various facets of academic freedom are important and why the principle should continue to be supported
Adaptive Approach for Modelling Variability in Pharmacokinetics
We present an improved adaptive approach for studying systems of ODEs affected by parameter variability and state space uncertainty. Our approach is based on a reformulation of the ODE problem as a transport problem of a probability density describing the evolution of the ensemble of systems in time. The resulting multidimensional problem is solved by representing the probability density w.r.t. an adaptively
chosen Galerkin ansatz space of Gaussian distributions. Due to our improvements in adaptivity control, we substantially improved the overall performance of the original algorithm and moreover inherited the theoretical property that the number of Gaussian distribution stays constant for linear ODEs to the numerical scheme. We illustrate the approach
in application to dynamical systems describing the pharmacokinetics of drugs and xenobiotics, where variability in physiological parameters is important to be considered
On twisted Fourier analysis and convergence of Fourier series on discrete groups
We study norm convergence and summability of Fourier series in the setting of
reduced twisted group -algebras of discrete groups. For amenable groups,
F{\o}lner nets give the key to Fej\'er summation. We show that Abel-Poisson
summation holds for a large class of groups, including e.g. all Coxeter groups
and all Gromov hyperbolic groups. As a tool in our presentation, we introduce
notions of polynomial and subexponential H-growth for countable groups w.r.t.
proper scale functions, usually chosen as length functions. These coincide with
the classical notions of growth in the case of amenable groups.Comment: 35 pages; abridged, revised and update
n-Hexadecane hydrocracking Single-Event MicroKinetics on Pt/H-beta
[EN] The Single-Event MicroKinetic (SEMK) model constructed for gas-phase hydroconversion of light n-alkanes on large-pore USY zeolites was applied, for the first time, to the hydrocracking of n-hexadecane on a Pt/H-Beta catalyst. Despite the 12-ringed pore channels, shape selectivity was observed in the formation of ethyl side chains. Additionally, heavy feed molecules such as n-hexadecane lead to physisorption saturation of the catalyst pores by strong Van der Waals interactions of the long alkyl chains with the zeolite framework. Intermolecular interactions and packing efficiencies in the pores induce deviations from typical Henry-regime physisorption characteristics as the physisorption selectivity, which is expected to increase with increasing carbon number, appeared to be independent of the latter. Micropore saturation effects were described by the 'size entropy' which quantifies the difference in standard entropy loss between physisorption in the Henry regime and hindered physisorption on a saturated surface. The size entropy is proportional to the catalyst loading with physisorbed species and the adsorbate carbon number. The addition of a size entropy term in the SEMK model, amounting to 102J mol(-1) K-1 for a hexadecane molecule at full saturation, allowed accurately reproducing the contribution of secondary isomerization and cracking reactions, as quantified by means of a contribution analysis. (C) 2012 Elsevier B.V. All rights reserved.This work was funded by the European Research Institute of
Catalysis and the European Community’s Sixth Framework Programme. This work was also supported by the Research Board
of Ghent University (BOF), the Interuniversity Attraction Poles
Programme–Belgian State–Belgian Science Policy and the Long
Term Structural Methusalem Funding by the Flemish Government.
Financial support by the Comisión Interministerial de Ciencia y Tecnología (CICYT) of Spain through the Project CTQ2010-17988/PPQ
is also gratefully acknowledged.Vandegehuchte, BD.; Thybaut, JW.; Martinez Feliu, A.; Arribas Viana, MDLD.; Marin, GB. (2012). n-Hexadecane hydrocracking Single-Event MicroKinetics on Pt/H-beta. Applied Catalysis A General. 441:10-20. doi:10.1016/j.apcata.2012.06.054S102044
Two super-Earths at the edge of the habitable zone of the nearby M dwarf TOI-2095
The main scientific goal of TESS is to find planets smaller than Neptune
around stars bright enough to allow further characterization studies. Given our
current instrumentation and detection biases, M dwarfs are prime targets to
search for small planets that are in (or nearby) the habitable zone of their
host star. Here we use photometric observations and CARMENES radial velocity
measurements to validate a pair of transiting planet candidates found by TESS.
The data was fitted simultaneously using a Bayesian MCMC procedure taking into
account the stellar variability present in the photometric and spectroscopic
time series. We confirm the planetary origin of the two transiting candidates
orbiting around TOI-2095 (TIC 235678745). The star is a nearby M dwarf ( pc, K, mag) with a
stellar mass and radius of and , respectively. The planetary system is composed of
two transiting planets: TOI-2095b with an orbital period of days and TOI-2095c with days. Both planets have similar sizes with and for planet b and c,
respectively. We put upper limits on the masses of these objects with for the inner and for the outer planet
(95\% confidence level). These two planets present equilibrium temperatures in
the range of 300 - 350 K and are close to the inner edge of the habitable zone
of their star.Comment: Submitted to Astronomy & Astrophysic
The Theory of Brown Dwarfs and Extrasolar Giant Planets
Straddling the traditional realms of the planets and the stars, objects below
the edge of the main sequence have such unique properties, and are being
discovered in such quantities, that one can rightly claim that a new field at
the interface of planetary science and and astronomy is being born. In this
review, we explore the essential elements of the theory of brown dwarfs and
giant planets, as well as of the new spectroscopic classes L and T. To this
end, we describe their evolution, spectra, atmospheric compositions, chemistry,
physics, and nuclear phases and explain the basic systematics of
substellar-mass objects across three orders of magnitude in both mass and age
and a factor of 30 in effective temperature. Moreover, we discuss the
distinctive features of those extrasolar giant planets that are irradiated by a
central primary, in particular their reflection spectra, albedos, and transits.
Aspects of the latest theory of Jupiter and Saturn are also presented.
Throughout, we highlight the effects of condensates, clouds, molecular
abundances, and molecular/atomic opacities in brown dwarf and giant planet
atmospheres and summarize the resulting spectral diagnostics. Where possible,
the theory is put in its current observational context.Comment: 67 pages (including 36 figures), RMP RevTeX LaTeX, accepted for
publication in the Reviews of Modern Physics. 30 figures are color. Most of
the figures are in GIF format to reduce the overall size. The full version
with figures can also be found at:
http://jupiter.as.arizona.edu/~burrows/papers/rm
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