2,726 research outputs found
Evaluation of performance impairment by spacecraft contaminants
The environmental contaminants (isolated as off-gases in Skylab and Apollo missions) were evaluated. Specifically, six contaminants were evaluated for their effects on the behavior of juvenile baboons. The concentrations of contaminants were determined through preliminary range-finding studies with laboratory rats. The contaminants evaluated were acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), trichloroethylene (TCE), heptane and Freon 21. When the studies of the individual gases were completed, the baboons were also exposed to a mixture of MEK and TCE. The data obtained revealed alterations in the behavior of baboons exposed to relatively low levels of the contaminants. These findings were presented at the First International Symposium on Voluntary Inhalation of Industrial Solvents in Mexico City, June 21-24, 1976. A preprint of the proceedings is included
Current-Carrying Ground States in Mesoscopic and Macroscopic Systems
We extend a theorem of Bloch, which concerns the net orbital current carried
by an interacting electron system in equilibrium, to include mesoscopic
effects. We obtain a rigorous upper bound to the allowed ground-state current
in a ring or disc, for an interacting electron system in the presence of static
but otherwise arbitrary electric and magnetic fields. We also investigate the
effects of spin-orbit and current-current interactions on the upper bound.
Current-current interactions, caused by the magnetic field produced at a point
r by a moving electron at r, are found to reduce the upper bound by an amount
that is determined by the self-inductance of the system. A solvable model of an
electron system that includes current-current interactions is shown to realize
our upper bound, and the upper bound is compared with measurements of the
persistent current in a single ring.Comment: 7 pager, Revtex, 1 figure available from [email protected]
Phonons in a Nanoparticle Mechanically Coupled to a Substrate
The discrete nature of the vibrational modes of an isolated nanometer-scale
solid dramatically modifies its low-energy electron and phonon dynamics from
that of a bulk crystal. However, nanocrystals are usually coupled--even if only
weakly--to an environment consisting of other nanocrystals, a support matrix,
or a solid substrate, and this environmental interaction will modify the
vibrational properties at low frequencies. In this paper we investigate the
modification of the vibrational modes of an insulating spherical nanoparticle
caused by a weak {\it mechanical} coupling to a semi-infinite substrate. The
phonons of the bulk substrate act as a bath of harmonic oscillators, and the
coupling to this reservoir shifts and broadens the nanoparticle's modes. The
vibrational density of states in the nanoparticle is obtained by solving the
Dyson equation for the phonon propagator, and we show that environmental
interaction is especially important at low frequencies. As a probe of the
modified phonon spectrum, we consider nonradiative energy relaxation of a
localized electronic impurity state in the nanoparticle, for which good
agreement with experiment is found.Comment: 10 pages, Revte
Local Phonon Density of States in an Elastic Substrate
The local, eigenfunction-weighted acoustic phonon density of states (DOS)
tensor is calculated for a model substrate consisting of a semi-infinite
isotropic elastic continuum with a stress-free surface. On the surface, the
local DOS is proportional to the square of the frequency, as for the
three-dimensional Debye model, but with a constant of proportionality that is
considerably enhanced compared to the Debye value, a consequence of the
Rayleigh surface modes. The local DOS tensor at the surface is also
anisotropic, as expected. Inside the substrate the local DOS is both spatially
anisotropic and non-quadratic in frequency. However, at large depths, the local
DOS approaches the isotropic Debye value. The results are applied to a Si
substrate.Comment: 7 pages, 2 figures, RevTe
On the shot-noise limit of a thermal current
The noise power spectral density of a thermal current between two macroscopic
dielectric bodies held at different temperatures and connected only at a
quantum point contact is calculated. Assuming the thermal energy is carried
only by phonons, we model the quantum point contact as a mechanical link,
having a harmonic spring potential. In the weak coupling, or weak-link limit,
we find the thermal current analog of the well-known electronic shot-noise
expression.Comment: 4 pages, 1 figur
Electron cyclotron resonance near the axis of the gas-dynamic trap
Propagation of an extraordinary electromagnetic wave in the vicinity of
electron cyclotron resonance surface in an open linear trap is studied
analytically, taking into account inhomogeneity of the magnetic field in
paraxial approximation. Ray trajectories are derived from a reduced dispersion
equation that makes it possible to avoid the difficulty associated with a
transition from large propagation angles to the case of strictly longitudinal
propagation. Our approach is based on the theory, originally developed by the
Zvonkov and Timofeev [1], who used the paraxial approximation for the magnetic
field strength, but did not consider the slope of the magnetic field lines,
which led to considerable error, as has been recently noted by Gospodchikov and
Smolyakova [2]. We have found ray trajectories in analytic form and
demonstrated that the inhomogeneity of both the magnetic field strength and the
field direction can qualitatively change the picture of wave propagation and
significantly affect the efficiency of electron cyclotron heating of a plasma
in a linear magnetic trap. Analysis of the ray trajectories has revealed a
criterion for the resonance point on the axis of the trap to be an attractor
for the ray trajectories. It is also shown that a family of ray trajectories
can still reach the resonance point on the axis if the latter generally repels
the ray trajectories.
As an example, results of general theory are applied to the electron
cyclotron resonance heating experiment which is under preparation on the Gas
Dynamic Trap in the Budker Institute of Nuclear Physics [3]
Quantum interference and electron-electron interactions at strong spin-orbit coupling in disordered systems
Transport and thermodynamic properties of disordered conductors are
considerably modified when the angle through which the electron spin precesses
due to spin-orbit interaction (SOI) during the mean free time becomes
significant. Cooperon and Diffusion equations are solved for the entire range
of strength of SOI. The implications of SOI for the electron-electron
interaction and interference effects in various experimental settings are
discussed.Comment: 4 pages, REVTEX, 1 eps.figure Submitted to Phys. Rev. Let
Mesopotamian Eye Disease Texts
This volume is the first English edition of the Nineveh Series on eye disease from the royal library of Ashurbanipal, 7th century BCE. It is the longest surviving ancient work on opthalmology, anticipating by centuries the Hippocratic treatise on the eye. The Nineveh series represents a systematic array of eye symptoms and therapies, also showing commonalities with Egyptian and Greco-Roman medicine
Infrared catastrophe and tunneling into strongly correlated electron systems: Exact solution of the x-ray edge limit for the 1D electron gas and 2D Hall fluid
In previous work we have proposed that the non-Fermi-liquid spectral
properties in a variety of low-dimensional and strongly correlated electron
systems are caused by the infrared catastrophe, and we used an exact functional
integral representation for the interacting Green's function to map the
tunneling problem onto the x-ray edge problem, plus corrections. The
corrections are caused by the recoil of the tunneling particle, and, in systems
where the method is applicable, are not expected to change the qualitative form
of the tunneling density of states (DOS). Qualitatively correct results were
obtained for the DOS of the 1D electron gas and 2D Hall fluid when the
corrections to the x-ray edge limit were neglected and when the corresponding
Nozieres-De Dominicis integral equations were solved by resummation of a
divergent perturbation series. Here we reexamine the x-ray edge limit for these
two models by solving these integral equations exactly, finding the expected
modifications of the DOS exponent in the 1D case but finding no changes in the
DOS of the 2D Hall fluid with short-range interaction. We also provide, for the
first time, an exact solution of the Nozieres-De Dominicis equation for the 2D
electron gas in the lowest Landau level.Comment: 6 pages, Revte
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