3,012 research outputs found
Recursion relations for generalized Fresnel coefficients: Casimir force in a planar cavity
We emphasize and demonstrate that, besides using the usual recursion
relations involving successive layers, generalized Fresnel coefficients of a
multilayer can equivalently be calculated using the recursion relations
involving stacks of layers, as introduced some time ago [M. S. Tomas, Phys.
Rev. A 51, 2545 (1995)]. Moreover, since the definition of the generalized
Fresnel coefficients employed does not imply properties of the stacks, these
nonstandard recursion relations can be used to calculate Fresnel coefficients
not only for local systems but also for a general multilayer consisting of
various types (local, nonlocal, inhomogeneous etc.) of layers. Their utility is
illustrated by deriving a few simple algorithms for calculating the
reflectivity of a Bragg mirror and extending the formula for the Casimir force
in a planar cavity to arbitrary media.Comment: 5 pages, 2 figures, slightly expande
Merit - An evaluation tool for 100% renewable energy provision
Islands represent an interesting challenge in terms of energy supply. A great deal of work has been carried out to look at specific aspects of this issue on different islands. Unfortunately, results from one study cannot be easily applied to other islands due to island-specific resources and energy-use profiles. A quantitative evaluation tool (MERIT) is presented here, which is able to match half-hourly energy demands (heat, electricity, hot water and transport) with local supplies. The program examines the energy balance on any scale, from an individual building through to an entire country, thereby providing a powerful and generic aid to decision making. This paper demonstrates the generality and usefulness of MERIT by using it to analyse the options for creating an energy-autonomous community on a typical, small island off the west coast of Scotland. Results are presented showing the feasibility of accomplishing 100% renewable provision on this island using available local resources
On the physical origins of the negative index of refraction
The physical origins of negative refractive index are derived from a dilute
microscopic model, producing a result that is generalized to the dense
condensed phase limit. In particular, scattering from a thin sheet of electric
and magnetic dipoles driven above resonance is used to form a fundamental
description for negative refraction. Of practical significance, loss and
dispersion are implicit in the microscopic model. While naturally occurring
negative index materials are unavailable, ferromagnetic and ferroelectric
materials provide device design opportunities.Comment: 4 pages, 1 figur
Rigorous derivation of coherent resonant tunneling time and velocity in finite periodic systems
The velocity of resonant tunneling electrons in finite periodic
structures is analytically calculated in two ways. The first method is based on
the fact that a transmission of unity leads to a coincidence of all still
competing tunneling time definitions. Thus, having an indisputable resonant
tunneling time we apply the natural definition
to calculate the velocity. For the second method we
combine Bloch's theorem with the transfer matrix approach to decompose the wave
function into two Bloch waves. Then the expectation value of the velocity is
calculated. Both different approaches lead to the same result, showing their
physical equivalence. The obtained resonant tunneling velocity is
smaller or equal to the group velocity times the magnitude of the complex
transmission amplitude of the unit cell. Only at energies where the unit cell
of the periodic structure has a transmission of unity equals the
group velocity. Numerical calculations for a GaAs/AlGaAs superlattice are
performed. For typical parameters the resonant velocity is below one third of
the group velocity.Comment: 12 pages, 3 figures, LaTe
A technique for optimal temperature estimation for modeling sunrise/sunset thermal snap disturbance torque
A predictive temperature estimation technique which can be used to drive a model of the Sunrise/Sunset thermal 'snap' disturbance torque experienced by low Earth orbiting spacecraft is described. The twice per orbit impulsive disturbance torque is attributed to vehicle passage in and out of the Earth's shadow cone (umbra), during which large flexible appendages undergo rapidly changing thermal conditions. Flexible members, in particular solar arrays, experience rapid cooling during umbra entrance (Sunset) and rapid heating during exit (Sunrise). The thermal 'snap' phenomena has been observed during normal on-orbit operations of both the LANDSAT-4 satellite and the Communications Technology Satellite (CTS). Thermal 'snap' has also been predicted to be a dominant source of error for the TOPEX satellite. The fundamental equations used to model the Sunrise/Sunset thermal 'snap' disturbance torque for a typical solar array like structure will be described. For this derivation the array is assumed to be a thin, cantilevered beam. The time varying thermal gradient is shown to be the driving force behind predicting the thermal 'snap' disturbance torque and therefore motivates the need for accurate estimates of temperature. The development of a technique to optimally estimate appendage surface temperature is highlighted. The objective analysis method used is structured on the Gauss-Markov Theorem and provides an optimal temperature estimate at a prescribed location given data from a distributed thermal sensor network. The optimally estimated surface temperatures could then be used to compute the thermal gradient across the body. The estimation technique is demonstrated using a typical satellite solar array
Wave and Particle Limit for Multiple Barrier Tunneling
The particle approach to one-dimensional potential scattering is applied to
non relativistic tunnelling between two, three and four identical barriers. We
demonstrate as expected that the infinite sum of particle contributions yield
the plane wave results. In particular, the existence of resonance/transparency
for twin tunnelling in the wave limit is immediately obvious. The known
resonances for three and four barriers are also derived. The transition from
the wave limit to the particle limit is exhibit numerically.Comment: 15 pages, 3 figure
Adiabatic and Non-Adiabatic Contributions to the Free Energy from the Electron-Phonon Interaction for Na, K, Al, and Pb
We calculate the adiabatic contributions to the free energy due to the
electron--phonon interaction at intermediate temperatures, for the elemental metals Na, K, Al, and Pb. Using our
previously published results for the nonadiabatic contributions we show that
the adiabatic contribution, which is proportional to at low
temperatures and goes as at high temperatures, dominates the
nonadiabatic contribution for temperatures above a cross--over temperature,
, which is between 0.5 and 0.8 , where is the melting
temperature of the metal. The nonadiabatic contribution falls as for
temperatures roughly above the average phonon frequency.Comment: Updated versio
Dust as a Standard of Space and Time in Canonical Quantum Gravity
The coupling of the metric to an incoherent dust introduces into spacetime a
privileged dynamical reference frame and time foliation. The comoving
coordinates of the dust particles and the proper time along the dust worldlines
become canonical coordinates in the phase space of the system. The Hamiltonian
constraint can be resolved with respect to the momentum that is canonically
conjugate to the dust time. Imposition of the resolved constraint as an
operator restriction on the quantum states yields a functional Schr\"{o}dinger
equation. The ensuing Hamiltonian density has an extraordinary feature: it
depends only on the geometric variables, not on the dust coordinates or time.
This has three important consequences. First, the functional Schr\"{o}dinger
equation can be solved by separating the dust time from the geometric
variables. Second, the Hamiltonian densities strongly commute and therefore can
be simultaneously defined by spectral analysis. Third, the standard constraint
system of vacuum gravity is cast into a form in which it generates a true Lie
algebra. The particles of dust introduce into space a privileged system of
coordinates that allows the supermomentum constraint to be solved explicitly.
The Schr\"{o}dinger equation yields a conserved inner product that can be
written in terms of either the instantaneous state functionals or the solutions
of constraints. Examples of gravitational observables are given, though neither
the intrinsic metric nor the extrinsic curvature are observables. Disregarding
factor--ordering difficulties, the introduction of dust provides a satisfactory
phenomenological approach to the problem of time in canonical quantum gravity.Comment: 56 pages (REVTEX file + 3 postscipt figure files
Fock space exploration by angle resolved transmission through quantum diffraction grating of cold atoms in an optical lattice
Light transmission or diffraction from different quantum phases of cold atoms
in an optical lattice has recently come up as a useful tool to probe such ultra
cold atomic systems. The periodic nature of the optical lattice potential
closely resembles the structure of a diffraction grating in real space, but
loaded with a strongly correlated quantum many body state which interacts with
the incident electromagnetic wave, a feature that controls the nature of the
light transmission or dispersion through such quantum medium. In this paper we
show that as one varies the relative angle between the cavity mode and the
optical lattice, the peak of the transmission spectrum through such cavity also
changes reflecting the statistical distribution of the atoms in the illuminated
sites. Consequently the angle resolved transmission spectrum of such quantum
diffraction grating can provide a plethora of information about the Fock space
structure of the many body quantum state of ultra cold atoms in such an optical
cavity that can be explored in current state of the art experiments.Comment: 40 double spaced, single column pages, 40 .eps figures, accepted for
publication in Physical Review
Disentangling multipole resonances through a full x-ray polarization analysis
Complete polarization analysis applied to resonant x-ray scattering at the Cr
K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted
into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994
eV). The physical mechanism behind this phenomenon is a subtle interference
effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions,
leading to a phase shift between the respective scattering amplitudes. This
effect may be exploited to disentangle two close-lying resonances that appear
as a single peak in a conventional energy scan, in this way allowing to single
out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure
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