14,889 research outputs found
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
Suppression of Ground-State Magnetization in Finite-Sized Systems Due to Off-Diagonal Interaction Fluctuations
We study a generic model of interacting fermions in a finite-sized disordered
system. We show that the off-diagonal interaction matrix elements induce
density of states fluctuations which generically favor a minimum spin ground
state at large interaction amplitude, . This effect competes with the
exchange effect which favors large magnetization at large , and it
suppresses this exchange magnetization in a large parameter range. When
off-diagonal fluctuations dominate, the model predicts a spin gap which is
larger for odd-spin ground states as for even-spin, suggesting a simple
experimental signature of this off-diagonal effect in Coulomb blockade
transport measurements.Comment: Final, substantially modified version of the article. Accepted for
publication in Physical Review Letter
An experimental study of the dual-fuel performance of a small compression ignition diesel engine operating with three gaseous fuels
A dual-fuel engine is a compression ignition (CI) engine where the primary gaseous fuel source is premixed with air as it enters the combustion chamber. This homogenous mixture is ignited by a small quantity of diesel, the ‘pilot’, that is injected towards the end of the compression stroke. In the present study, a direct-injection CI engine, was fuelled with three different gaseous fuels: methane, propane, and butane. The engine performance at various gaseous concentrations was recorded at 1500 r/min and quarter, half, and three-quarters relative to full a load of 18.7 kW. In order to investigate the combustion performance, a novel three-zone heat release rate analysis was applied to the data. The resulting heat release rate data are used to aid understanding of the performance characteristics of the engine in dual-fuel mode.
Data are presented for the heat release rates, effects of engine load and speed, brake specific energy consumption of the engine, and combustion phasing of the three different primary gaseous fuels.
Methane permitted the maximum energy substitution, relative to diesel, and yielded the most significant reductions in CO2. However, propane also had significant reductions in CO2 but had an increased diffusional combustion stage which may lend itself to the modern high-speed direct-injection engine
Random-Matrix Theory of Quantum Size Effects on Nuclear Magnetic Resonance in Metal Particles
The distribution function of the local density of states is computed exactly
for the Wigner-Dyson ensemble of random Hamiltonians. In the absence of
time-reversal symmetry, precise agreement is obtained with the "supersymmetry"
theory by Efetov and Prigodin of the NMR lineshape in disordered metal
particles. Upon breaking time-reversal symmetry, the variance of the Knight
shift in the smallest particles is reduced by a universal factor of 2/3. ***To
be published in Physical Review B.****Comment: 4 pages, REVTeX-3.0, 1 postscript figure, INLO-PUB-940819; [2017:
figure included in text
On the isospin dependence of the mean spin-orbit field in nuclei
By the use of the latest experimental data on the spectra of Sb and
Sn and on the analysis of properties of other odd nuclei adjacent to
doubly magic closed shells the isospin dependence of a mean spin-orbit
potential is defined. Such a dependence received the explanation in the
framework of different theoretical approaches.Comment: 52 pages, Revtex, no figure
Signatures of electron correlations in the transport properties of quantum dots
The transition matrix elements between the correlated and
electron states of a quantum dot are calculated by numerical diagonalization.
They are the central ingredient for the linear and non--linear transport
properties which we compute using a rate equation. The experimentally observed
variations in the heights of the linear conductance peaks can be explained. The
knowledge of the matrix elements as well as the stationary populations of the
states allows to assign the features observed in the non--linear transport
spectroscopy to certain transition and contains valuable information about the
correlated electron states.Comment: 4 pages (revtex,27kB) + 3 figures in one file ziped and uuencoded
(postscript,33kB), to appear in Phys.Rev.B as Rapid Communicatio
Generalized Fokker-Planck Equation For Multichannel Disordered Quantum Conductors
The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the
distribution of transmission eigenvalues of multichannel disordered conductors,
has been enormously successful in describing a variety of detailed transport
properties of mesoscopic wires. However, it is limited to the regime of quasi
one dimension only. We derive a one parameter generalization of the DMPK
equation, which should broaden the scope of the equation beyond the limit of
quasi one dimension.Comment: 8 pages, abstract, introduction and summary rewritten for broader
readership. To be published in Phys. Rev. Let
Nonlinear resonant tunneling in systems coupled to quantum reservoirs
An adiabatic approximation in terms of instantaneous resonances is developed
to study the steady-state and time-dependent transport of interacting electrons
in biased resonant tunneling heterostructures. The resulting model consists of
quantum reservoirs coupled to regions where the system is described by
nonlinear ordinary differential equations and has a general conceptual
interest.Comment: 4 pages, 3 postscript figure
Ambient particle inhalation and the cardiovascular system: potential mechanisms
Well-documented air pollution episodes throughout recent history have led to deaths among individuals with cardiovascular and respiratory disease. Although the components of air pollution that cause the adverse health effects in these individuals are unknown, a small proportion by mass but a large proportion by number of the ambient air particles are ultrafine, i.e., less than 100 nm in diameter. This ultrafine component of particulate matter with a mass median aerodynamic diameter less than 10 microm (PM(10) may mediate some of the adverse health effects reported in epidemiologic studies and for which there is toxicologic evidence to support this contention. The exact mechanism by which ultrafine particles have adverse effects is unknown, but these particles have recently been shown to enhance calcium influx on contact with macrophages. Oxidative stress is also to be anticipated at the huge particle surface; this can be augmented by oxidants generated by recruited inflammatory leukocytes. Atheromatous plaques form in the coronary arteries and are major causes of morbidity and death associated epidemiologically with particulate air pollution. In populations exposed to air pollution episodes, blood viscosity, fibrinogen, and C-reactive protein (CRP) were higher. More recently, increases in heart rate in response to rising air pollution have been described and are most marked in individuals who have high blood viscosity. In our study of elderly individuals, there were significant rises in CRP, an index of inflammation. In this present review, we consider the likely interactions between the ultrafine particles the acute phase response and cardiovascular disease
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