11,432 research outputs found
Cryogenic masers
Various factors affecting the frequency stability of hydrogen masers are described and related to maser design parameters. The long-term frequency stability of a hydrogen maser is limited by the mechanical stability of the cavity, and the magnitudes of the wall relaxation, spin exchange, and recombination rates which affect the Q of the line. Magnetic resonance studies of hydrogen atoms at temperatures below 1 K and in containers coated with liquid helium films demonstrated that cryogenic masers may allow substantial improvements in all of these parameters. In particular the thermal expansion coefficients of most materials are negligible at 1 K. Spin exchange broadening is three orders of magnitude smaller at 1 K than at room temperature, and the recombination and wall relaxation rates are negligible at 0.52 K where the frequency shift due to the 4 He-coated walls of the container has a broad minimum as a function of temperature. Other advantages of the helium-cooled maser result from the high purity, homogeneity, and resilence of helium-film-coated walls and the natural compatibility of the apparatus with helium-cooled amplifiers
Diffusion of Nonequilibrium Quasiparticles in a Cuprate Superconductor
We report a transport study of nonequilibrium quasiparticles in a high-Tc
cuprate superconductor using the transient grating technique. Low-intensity
laser excitation (at photon energy 1.5 eV) was used to introduce a spatially
periodic density of quasiparticles into a high-quality untwinned single crystal
of YBa2Cu3O6.5. Probing the evolution of the initial density through space and
time yielded the quasiparticle diffusion coefficient, and both inelastic and
elastic scattering rates. The technique reported here is potentially applicable
to precision measurement of quasiparticle dynamics, not only in cuprate
superconductors, but in other electronic systems as well.Comment: 5 pages, 4 figure
Towards Quantum Gravity: A Framework for Probabilistic Theories with Non-Fixed Causal Structure
General relativity is a deterministic theory with non-fixed causal structure.
Quantum theory is a probabilistic theory with fixed causal structure. In this
paper we build a framework for probabilistic theories with non-fixed causal
structure. This combines the radical elements of general relativity and quantum
theory. The key idea in the construction is physical compression. A physical
theory relates quantities. Thus, if we specify a sufficiently large set of
quantities (this is the compressed set), we can calculate all the others. We
apply three levels of physical compression. First, we apply it locally to
quantities (actually probabilities) that might be measured in a particular
region of spacetime. Then we consider composite regions. We find that there is
a second level of physical compression for the composite region over and above
the first level physical compression for the component regions. Each
application of first and second level physical compression is quantified by a
matrix. We find that these matrices themselves are related by the physical
theory and can therefore be subject to compression. This is the third level of
physical compression. This third level of physical compression gives rise to a
new mathematical object which we call the causaloid. From the causaloid for a
particular physical theory we can calculate verything the physical theory can
calculate. This approach allows us to set up a framework for calculating
probabilistic correlations in data without imposing a fixed causal structure
(such as a background time). We show how to put quantum theory in this
framework (thus providing a new formulation of this theory). We indicate how
general relativity might be put into this framework and how the framework might
be used to construct a theory of quantum gravity.Comment: 23 pages. For special issue of Journal of Physics A entitled "The
quantum universe" in honour of Giancarlo Ghirard
Average and worst-case specifications of precipitating auroral electron environment
The precipitation electrons in the auroral environment are highly variable in their energy and intensity in both space and time. As such they are a source of potential hazard to the operation of the Space Shuttle and other large spacecraft operating in polar orbit. In order to assess these hazards both the average and extreme states of the precipitating electrons must be determined. Work aimed at such a specification is presented. First results of a global study of the average characteristics are presented. In this study the high latitude region was divided into spatial elements in magnetic local time and corrected geomagnetic latitude. The average electron spectrum was then determined in each spatial element for seven different levels of activity as measured by K sub p using an extremely large data set of auroral observations. Second a case study of an extreme auroral electron environment is presented, in which the electrons are accelerated through field aligned potential as high as 30,000 volts and in which the spacecraft is seen to charge negatively to a potential approaching .5 kilovolts
Reflective Ghost Imaging through Turbulence
Recent work has indicated that ghost imaging may have applications in
standoff sensing. However, most theoretical work has addressed
transmission-based ghost imaging. To be a viable remote-sensing system, the
ghost imager needs to image rough-surfaced targets in reflection through long,
turbulent optical paths. We develop, within a Gaussian-state framework,
expressions for the spatial resolution, image contrast, and signal-to-noise
ratio of such a system. We consider rough-surfaced targets that create fully
developed speckle in their returns, and Kolmogorov-spectrum turbulence that is
uniformly distributed along all propagation paths. We address both classical
and nonclassical optical sources, as well as a computational ghost imager.Comment: 13 pages, 3 figure
Possible Origins of High-\u3ci\u3eT\u3csub\u3ec\u3c/sub\u3e\u3c/i\u3e, Superconductivity
A new mechanism is proposed to explain high-Tc superconductivity in copper-oxide-based, open perovskitelike systems. It is shown that, should the oxygen ions be moving in a double-well potential, an order-of-magnitude enhancement of the electron-lattice coupling follows automatically from a consistent treatment of this motion. Both theoretical and experimental evidence for the presence of such double wells is cited
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
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