623 research outputs found
Local Density of States in Mesoscopic Samples from Scanning Gate Microscopy
We study the relationship between the local density of states (LDOS) and the
conductance variation in scanning-gate-microscopy experiments on
mesoscopic structures as a charged tip scans above the sample surface. We
present an analytical model showing that in the linear-response regime the
conductance shift is proportional to the Hilbert transform of the
LDOS and hence a generalized Kramers-Kronig relation holds between LDOS and
. We analyze the physical conditions for the validity of this
relationship both for one-dimensional and two-dimensional systems when several
channels contribute to the transport. We focus on realistic Aharonov-Bohm rings
including a random distribution of impurities and analyze the LDOS-
correspondence by means of exact numerical simulations, when localized states
or semi-classical orbits characterize the wavefunction of the system.Comment: 8 pages, 8 figure
Theoretical investigation of the performance of an Alpha Stirling engine for low temperature applications
The purpose of this paper is to explore the applicability and peculiarities of Alpha engines at low heat source temperature levels of 100 to 200°C. A parameter study of an Alpha engine has been carried out using the commercial Stirling software Sage. The obtained results revealed some interesting insights into the peculiarities of low-temperature Alpha-SEs. A method for optimising the system design-parameters for a SE is described in this paper.
The compact design of a double-acting Alpha engine helps to reduce not only engine size and complexity but also costs. At low temperature differences single-cylinder displacer-type (gamma) engines are well documented, while little is known about the performance of Alpha multi-cylinder engines. In order to achieve the highest possible power output not only the thermodynamic side but also the mechanical side of the engine has to be optimised. This is especially important at low temperature differentials, where the conversion efficiencies are inherently small and the driving force for heat absorption and rejection by the working gas is low. It is mandatory not only to convert as much heat as possible to indicated work but also to transfer as much of this hard gained work to usable power output by minimising internal friction, when trying to keep the engine size as small as possible.
The indicated power output of an engine of a specific swept volume is dependent on the temperature and pressure levels, the frequency, the phase angle between the two pistons, the working fluid, and the design of the heat exchangers and the regenerator. A simple model of an Alpha engine was created using the commercial SE simulation tool Sage. In order to explore the relations between those parameters a model was developed, where the design parameters (temperature, mean pressure, frequency, and phase angle) were varied, and the heat exchangers and the regenerator were then optimised for each of the parameter combinations to reach the maximum power output. Temperature and mean pressure are shown to have the expected positive influence on the power output. For the frequency and the phase angle, optimum values can be found that differ significantly from those found for high temperature engines. Helium is used as the benchmark working gas. It can be shown that the use of Nitrogen instead cuts the power output in half, whereas Hydrogen doubles the achievable power output.
The mechanical efficiency of a kinematic SE is largely dependent on the load that is transferred from the pistons to the crankshaft during expansion and vice versa during compression, as it increases frictional losses. In double-acting engines an identical pressure oscillation acts on the opposing faces of a piston, although it is out of phase. The resulting forces can balance each other to some extent depending on the phase, and thus the net force applied to the crankshaft can be reduced. For a four cylinder engine it is shown that the Siemens arrangement balances internal forces on the pistons to a larger extent than the Franchot arrangement, so that the mechanical losses are inherently smaller and thus the brake power is larger even though the indicated work is identical for both engine types.
The contribution of this work is the provision of a modelling methodology, and the identification of a number of insights for system-design considerations for low-temperature applications
Transport inefficiency in branched-out mesoscopic networks: An analog of the Braess paradox
We present evidence for a counter-intuitive behavior of semiconductor
mesoscopic networks that is the analog of the Braess paradox encountered in
classical networks. A numerical simulation of quantum transport in a two-branch
mesoscopic network reveals that adding a third branch can paradoxically induce
transport inefficiency that manifests itself in a sizable conductance drop of
the network. A scanning-probe experiment using a biased tip to modulate the
transmission of one branch in the network reveals the occurrence of this
paradox by mapping the conductance variation as a function of the tip voltage
and position.Comment: 2nd version with minor stylistic corrections. To appear in Phys. Rev.
Lett.: Editorially approved for publication 6 January 201
Structure and transport in multi-orbital Kondo systems
We consider Kondo impurity systems with multiple local orbitals, such as rare
earth ions in a metallic host or multi--level quantum dots coupled to metallic
leads. It is shown that the multiplet structure of the local orbitals leads to
multiple Kondo peaks above the Fermi energy , and to ``shadow'' peaks
below . We use a slave boson mean field theory, which recovers the strong
coupling Fermi liquid fixed point, to calculate the Kondo peak positions,
widths, and heights analytically at T=0, and NCA calculations to fit the
temperature dependence of high--resolution photoemission spectra of Ce
compounds. In addition, an approximate conductance quantization for transport
through multi--level quantum dots or single--atom transistors in the Kondo
regime due to a generalized Friedel sum rule is demonstrated.Comment: 4 pages, 3 figures. Invited article, 23rd International Conference on
Low Temperature Physics LT23, Hiroshima, Japan 200
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy
Quantum point contacts exhibit mysterious conductance anomalies in addition
to well known conductance plateaus at multiples of 2e^2/h. These 0.7 and
zero-bias anomalies have been intensively studied, but their microscopic origin
in terms of many-body effects is still highly debated. Here we use the charged
tip of a scanning gate microscope to tune in situ the electrostatic potential
of the point contact. While sweeping the tip distance, we observe repetitive
splittings of the zero-bias anomaly, correlated with simultaneous appearances
of the 0.7 anomaly. We interpret this behaviour in terms of alternating
equilibrium and non-equilibrium Kondo screenings of different spin states
localized in the channel. These alternating Kondo effects point towards the
presence of a Wigner crystal containing several charges with different
parities. Indeed, simulations show that the electron density in the channel is
low enough to reach one-dimensional Wigner crystallization over a size
controlled by the tip position
Métacognition : intervention thérapeutique autour de la conscience des troubles chez des patients souffrant de traumatisme crùnio-cérébral grave
Unawareness related to brain injury has implications for participation in rehabilitation, functional outcomes, and the emotional well being of patients after an acquired brain injury. However, the development of interventions for improving self-awareness is at an early stage, and research on the effectiveness of interventions is limited. The present paper is an investigation into the efficacy of a metacognitive program on self-awareness in people who have had an acquired brain injury. An experimental group of three patients were included in a âcross-over designâ protocol including data from neuropsychological evaluations, cognitive complaints, behavioral and emotional changes. In comparison, a social intervention (communicative abilities, social interaction) was proposed to four other patients in order to estimate the specific effects of the metacognitive program. While no specific effect of the metacognitive program was highlighted on cognitive impairments, a post-intervention assessment indicated that participants had significantly improved behavioral limitations, suggesting a benefit of the metacognitive and social rehabilitations on behavior skills. Because benefits are similar whatever the program, it seems that social functioning implication in rehabilitation process needs to be taken into account in forthcoming interventions
The RN locus for meat quality maps to pig chromosome 15
International audienc
Hadamard Regularization
Motivated by the problem of the dynamics of point-particles in high
post-Newtonian (e.g. 3PN) approximations of general relativity, we consider a
certain class of functions which are smooth except at some isolated points
around which they admit a power-like singular expansion. We review the concepts
of (i) Hadamard ``partie finie'' of such functions at the location of singular
points, (ii) the partie finie of their divergent integral. We present and
investigate different expressions, useful in applications, for the latter
partie finie. To each singular function, we associate a partie-finie (Pf)
pseudo-function. The multiplication of pseudo-functions is defined by the
ordinary (pointwise) product. We construct a delta-pseudo-function on the class
of singular functions, which reduces to the usual notion of Dirac distribution
when applied on smooth functions with compact support. We introduce and analyse
a new derivative operator acting on pseudo-functions, and generalizing, in this
context, the Schwartz distributional derivative. This operator is uniquely
defined up to an arbitrary numerical constant. Time derivatives and partial
derivatives with respect to the singular points are also investigated. In the
course of the paper, all the formulas needed in the application to the physical
problem are derived.Comment: 50 pages, to appear in Journal of Mathematical Physic
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