3,004 research outputs found
Dirac Particles in a Gravitational Field
The semiclassical approximation for the Hamiltonian of Dirac particles
interacting with an arbitrary gravitational field is investigated. The time
dependence of the metrics leads to new contributions to the in-band energy
operator in comparison to previous works on the static case. In particular we
find a new coupling term between the linear momentum and the spin, as well as
couplings which contribute to the breaking of the particle - antiparticle
symmetry
Mechanics of a Plant in Fluid Flow
Plants live in constantly moving fluid, whether air or water. In response to
the loads associated with fluid motion, plants bend and twist, often with great
amplitude. These large deformations are not found in traditional engineering
application and thus necessitate new specialised scientific developments.
Studying Fluid-Structure Interactions (FSI) in botany, forestry and
agricultural science is crucial to the optimisation of biomass production for
food, energy, and construction materials. FSI are also central in the study of
the ecological adaptation of plants to their environment. This review paper
surveys the mechanics of FSI on individual plants. We present a short refresher
on fluids mechanics then dive in the statics and dynamics of plant-fluid
interactions. For every phenomenon considered, we present the appropriate
dimensionless numbers to characterise the problem, discuss the implications of
these phenomena on biological processes, and propose future research avenues.
We cover the concept of reconfiguration while considering poroelasticity,
torsion, chirality, buoyancy, and skin friction. We also cover the dynamical
phenomena of wave action, flutter, and vortex-induced vibrations.Comment: 26 pages, 8 figure
Spirited Imperialism: The Formation and Command of the First Canadian Expeditionary Force in South Africa
This article explores the role played by Chamberlain, Minto and Hutton in shaping the organizational and command structure of the Canadian expeditionary force that deployed to South Africa. In 1899, the war raised not only the vital question about the kind of imperial war Canada ought to participate in, but also the form of that participation. As this article demonstrates, the British politician, the colonial administrator and the general saw the war as an opportunity to advance their specific agenda, strongly fuelled by imperialist sentiments. In doing so, the ideas that shaped their actions between July and October 1899 provided the base for a succession of seemingly unrelated decisions that had a substantial influence on Canada’s contribution and on the command arrangements in South Africa. The Canadian soldiers who sailed to South Africa were, for the first time, grouped in a national military formation commanded by a Canadian officer, setting a precedent for the country’s participation in future conflicts
Special treatment reduces helium permeation of glass in vacuum systems
Internal surfaces of the glass component of a vacuum system are exposed to cesium in gaseous form to reduce helium permeation. The cesium gas is derived from decomposition of cesium nitrate through heating. Several minutes of exposure of the internal surfaces of the glass vessel are sufficient to complete the treatment
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
Bosons Doubling
It is shown that next-nearest-neighbor interactions may lead to unusual
paramagnetic or ferromagnetic phases which physical content is radically
different from the standard phases. Actually there are several particles
described by the same quantum field in a manner similar to the species doubling
of the lattice fermions. We prove the renormalizability of the theory at the
one loop level.Comment: 12 page
Buckling of a beam extruded into highly viscous fluid
Inspired by microscopic paramecies which use trichocyst extrusion to propel
themselves away from thermal aggressions, we propose a macroscopic experiment
to study the stability of a slender beam extruded in a highly viscous fluid.
Piano wires were extruded axially at constant speed in a tank filled with corn
syrup. The force necessary to extrude the wire was measured to increase
linearly at first until the compressive viscous force causes the wire to
buckle. A numerical model, coupling a lengthening elastica formulation with
resistive force theory, predicts a similar behaviour. The model is used to
study the dynamics at large time when the beam is highly deformed. It is found
that at large time, a large deformation regime exists in which the force
necessary to extrude the beam at constant speed becomes constant and
length-independent. With a proper dimensional analysis, the beam can be shown
to buckle at a critical length based on the extrusion speed, the bending
rigidity and the dynamic viscosity of the fluid. Hypothesising that the
trichocysts of paramercies must be sized to maximise their thrust per unit
volume as well as avoid buckling instabilities, we predict that their bending
rigidity must be about . The
verification of this prediction is left for future work.Comment: Accepted for publication in PRE on November 18 2014, 7 pages, 6
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Extreme vacuum technology including cryosorption, diffusion pump and pressure calibration studies Summary technical report, 1 Feb. 1965 - 1 Mar. 1966
Cryosorption, diffusion pump, and pressure calibration studies in extreme vacuum science and technology application progra
Berry Curvature in Graphene: A New Approach
In the present paper we have directly computed the Berry curvature terms
relevant for Graphene in the presence of an \textit{inhomogeneous} lattice
distortion. We have employed the generalized Foldy Wouthuysen framework,
developed by some of us \cite{ber0,ber1,ber2}. We show that a non-constant
lattice distortion leads to a valley-orbit coupling which is responsible to a
valley-Hall effect. This is similar to the valley-Hall effect induced by an
electric field proposed in \cite{niu2} and is the analogue of the spin-Hall
effect in semiconductors \cite{MURAKAMI, SINOVA}. Our general expressions for
Berry curvature, for the special case of homogeneous distortion, reduce to the
previously obtained results \cite{niu2}. We also discuss the Berry phase in the
quantization of cyclotron motion.Comment: Slightly modified version, to appear in EPJ
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