2,821 research outputs found
Development of UHF measurements
Collector gauge and orbitron gauge for ultrahigh vacuum measurement
Semiclassical Dynamics of Electrons in Magnetic Bloch Bands: a Hamiltonian Approach
y formally diagonalizing with accuracy the Hamiltonian of electrons
in a crystal subject to electromagnetic perturbations, we resolve the debate on
the Hamiltonian nature of semiclassical equations of motion with Berry-phase
corrections, and therefore confirm the validity of the Liouville theorem. We
show that both the position and momentum operators acquire a Berry-phase
dependence, leading to a non-canonical Hamiltonian dynamics. The equations of
motion turn out to be identical to the ones previously derived in the context
of electron wave-packets dynamics.Comment: 4 page
From Feynman Proof of Maxwell Equations to Noncommutative Quantum Mechanics
In 1990, Dyson published a proof due to Feynman of the Maxwell equations
assuming only the commutation relations between position and velocity. With
this minimal assumption, Feynman never supposed the existence of Hamiltonian or
Lagrangian formalism. In the present communication, we review the study of a
relativistic particle using ``Feynman brackets.'' We show that Poincar\'e's
magnetic angular momentum and Dirac magnetic monopole are the consequences of
the structure of the Lorentz Lie algebra defined by the Feynman's brackets.
Then, we extend these ideas to the dual momentum space by considering
noncommutative quantum mechanics. In this context, we show that the
noncommutativity of the coordinates is responsible for a new effect called the
spin Hall effect. We also show its relation with the Berry phase notion. As a
practical application, we found an unusual spin-orbit contribution of a
nonrelativistic particle that could be experimentally tested. Another practical
application is the Berry phase effect on the propagation of light in
inhomogeneous media.Comment: Presented at the 3rd Feynman Festival (Collage Park, Maryland,
U.S.A., August 2006
Semiclassical quantization of electrons in magnetic fields: the generalized Peierls substitution
A generalized Peierls substitution which takes into account a Berry phase
term must be considered for the semiclassical treatment of electrons in a
magnetic field. This substitution turns out to be an essential element for the
correct determination of the semiclassical equations of motion as well as for
the semiclassical Bohr-Sommerfeld quantization condition for energy levels. A
general expression for the cross-sectional area is derived and used as an
illustration for the calculation of the energy levels of Bloch and Dirac
electrons
Eco Global Evaluation: Cross Benefits of Economic and Ecological Evaluation
This paper highlights the complementarities of cost and environmental evaluation in a sustainable approach. Starting with the needs and limits for whole product lifecycle evaluation, this paper begins with the modeling, data capture and performance indicator aspects. In a second step, the information issue, regarding the whole lifecycle of the product is addressed. In order to go further than the economical evaluations/assessment, the value concept (for a product or a service) is discussed. Value could combine functional requirements, cost objectives and environmental impact. Finally, knowledge issues which address the complexity of integrating multi-disciplinary expertise to the whole lifecycle of a product are discussing.EcoSD NetworkEcoSD networ
Affinity and dose of TCR engagement yield proportional enhancer and gene activity in CD4+ T cells.
Affinity and dose of T cell receptor (TCR) interaction with antigens govern the magnitude of CD4+ T cell responses, but questions remain regarding the quantitative translation of TCR engagement into downstream signals. We find that while the response of mouse CD4+ T cells to antigenic stimulation is bimodal, activated cells exhibit analog responses proportional to signal strength. Gene expression output reflects TCR signal strength, providing a signature of T cell activation. Expression changes rely on a pre-established enhancer landscape and quantitative acetylation at AP-1 binding sites. Finally, we show that graded expression of activation genes depends on ERK pathway activation, suggesting that an ERK-AP-1 axis plays an important role in translating TCR signal strength into proportional activation of enhancers and genes essential for T cell function
Space Shuttle Crawler Transporter Truck Shoe Qualification Tests and Analyses for Return-to-Flight
A vital element to Launch Complex 39 (LC39) and NASA's Kennedy Space Center (KSC) mobile launch transfer operation is a 3 million kilogram behemoth known as the Crawler Transporter (CT). Built in the 1960's, two CT's have accumulated over 1700+ miles each and have been used for the Apollo and the Space Shuttle programs. Recent observation of fatigue cracks on the CT shoes led to a comprehensive engineering, structural and metallurgical evaluation to assess the root cause that necessitated procurement of over 1000 new shoes. This paper documents the completed dynamic and compression tests on the old and new shoes respectively, so as to certify them for Space Shuttle's return-to-flight (RTF). Measured strain data from the rollout tests was used to develop stress/loading spectra and static equivalent load for qualification testing of the new shoes. Additionally, finite element analysis (FEA) was used to conduct sensitivity analyses of various contact parameters and structural characteristics for acceptance of new shoes
Improving the forward kinematics of cable-driven parallel robots through cable angle sensors
This paper presents a sensor fusion method that aims at improving the accuracy of cable-driven planar parallel mechanisms (CDPMs) and simplifying the kinematic resolution. While the end-effector pose of the CDPM is usually obtained with the cable lengths, the proposed method combines the cable length measurement with the cable angle by using a data fusion algorithm. This allows for a resolution based on the loop closure equations and a weighted least squares method. The paper first presents the resolution of the forward kinematics for planar parallel mechanisms using cable angle only. Then, the proposed sensor fusion scheme is detailed. Finally, an experiment comparing the different procedures for obtaining the pose of the CDPM is carried out, in order to demonstrate the efficiency of the proposed fusion method
Workspace and Singularity analysis of a Delta like family robot
Workspace and joint space analysis are essential steps in describing the task
and designing the control loop of the robot, respectively. This paper presents
the descriptive analysis of a family of delta-like parallel robots by using
algebraic tools to induce an estimation about the complexity in representing
the singularities in the workspace and the joint space. A Gr{\"o}bner based
elimination is used to compute the singularities of the manipulator and a
Cylindrical Algebraic Decomposition algorithm is used to study the workspace
and the joint space. From these algebraic objects, we propose some certified
three dimensional plotting describing the the shape of workspace and of the
joint space which will help the engineers or researchers to decide the most
suited configuration of the manipulator they should use for a given task. Also,
the different parameters associated with the complexity of the serial and
parallel singularities are tabulated, which further enhance the selection of
the different configuration of the manipulator by comparing the complexity of
the singularity equations.Comment: 4th IFTOMM International Symposium on Robotics and Mechatronics, Jun
2015, Poitiers, France. 201
Analysis of highâfrequency and longâterm data in undergraduate ecology classes improves quantitative literacy
Ecologists are increasingly analyzing long-term and high-frequency sensor datasets as part of their research. As ecology becomes a more data-rich scientific discipline, the next generation of ecologists needs to develop the quantitative literacy required to effectively analyze, visualize, and interpret large datasets. We developed and assessed three modules to teach undergraduate freshwater ecology students both scientific concepts and quantitative skills needed to work with large datasets. These modules covered key ecological topics of phenology, physical mixing, and the balance between primary production and respiration, using lakes as model systems with high-frequency or long-term data. Our assessment demonstrated that participating in these modules significantly increased student comfort using spreadsheet software and their self-reported competence in performing a variety of quantitative tasks. Interestingly, students with the lowest pre-module comfort and skills achieved the biggest gains. Furthermore, students reported that participating in the modules helped them better understand the concepts presented and that they appreciated practicing quantitative skills. Our approach demonstrates that working with large datasets in ecology classrooms helps undergraduate students develop the skills and knowledge needed to help solve complex ecological problems and be more prepared for a data-intensive future
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