230,280 research outputs found
Induction methods used in low temperature physics
A study has been made of induction bridges used in low temperature physics.\ud
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In Part 1 the design of a mutual inductance bridge of the Hartshorn type is discussed. This design is based on a critical analysis of impurity effects of the different parts of the Hartshorn bridge. With this equipment frequencies up to 0.5 MHz can be used. Two methods have been developed to examine the secondary signal. In one of these use has been made of AD conversion techniques. In the other one, the secondary signal, produced by a superconducting sample, which is generally distorted, is analysed by using a Fourier expansion.\ud
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In Part 2 equipment is described which enables us to measure the phase and amplitude of the harmonics of the output signal of the bridge. For synchronous detection a reference signal of the same frequency of the harmonic of interest is required. This reference signal is generated from the input signal of the bridge by means of a digital frequency multiplier with programmable multiplication factor N.\ud
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In Part 3 some experimental results, showing the possibilities of the equipment, on some superconductors are presented
Brane-World Cosmology, Bulk Scalars and Perturbations
We investigate aspects of cosmology in brane world theories with a bulk
scalar field. We concentrate on a recent model motivated from supergravity in
singular spaces. After discussing the background evolution of such a
brane-world, we present the evolution of the density contrast. We compare our
results to those obtained in the (second) Randall-Sundrum scenario and usual 4D
scalar-tensor theories.Comment: 29 pages, one figure, JHEP3-styl
Photo-ionization modelling of planetary nebulae -- II. Galactic bulge nebulae, a comparison with literature results
We have constructed photo-ionization models of five galactic bulge planetary
nebulae using our automatic method which enables a fully self-consistent
determination of the physical parameters of a planetary nebula. The models are
constrained using the spectrum, the IRAS and radio fluxes and the angular
diameter of the nebula. We also conducted a literature search for physical
parameters determined with classical methods for these nebulae. Comparison of
the distance independent physical parameters with published data shows that the
stellar temperatures generally are in good agreement and can be considered
reliable. The literature data for the electron temperature, electron density
and also for the abundances show a large spread, indicating that the use of
line diagnostics is not reliable and that the accuracy of these methods needs
to be improved. Comparison of the various abundance determinations indicates
that the uncertainty in the electron temperature is the main source of
uncertainty in the abundance determination. The stellar magnitudes predicted by
the photo-ionization models are in good agreement with observed values.Comment: Accepted for publication in MNRA
Gravitational instability and star formation in disk galaxies
We present a general star formation law where star formation rate depends
upon efficiency , timescale of star formation, gas component
of surface mass density and a real exponent . A given exponent
determines which however yields the corresponding star formation
rate. Current nominal Schmidt exponent for our model is .
Based on a gravitational instability parameter and another
dimensionless parameter , where =
pressure, = column density of molecular clouds, we suggest a
general equation for star formation rate which depends upon relative competence
of the two parameters for various physical circumstances. We find that
emerges to be a better parameter for star formation scenario than Toomre
Q-parameter. Star formation rate in the solar neighbourhood is found to be in
good agreement with values inferred from previous studies. Under closed box
approximation model, we obtain a relation between metallicity of gas and the
efficiency of star formation. Our model calculations of metallicity in the
solar neighbourhood agree with earlier estimates. We conclude that metallicity
dispersion for stars of same age may result due to a change in efficiency
through which different sample stars were processed. For no significant change
of metallicity with age, we suggest that all sample stars were born with almost
similar efficiency.Comment: 10 pages, 3 figures, submitted to MNRA
Role of defects and impurities in doping of GaN
We have calculated formation energies and position of the defect levels for
all native defects and for a variety of donor and acceptor impurities employing
first-principles total-energy calculations. An analysis of the numerical
results gives direct insight into defect concentrations and impurity solubility
with respect to growth parameters (temperature, chemical potentials) and into
the mechanisms limiting the doping levels in GaN. We show how compensation and
passivation by native defects or impurities, solubility issues, and
incorporation of dopants on other sites influence the acceptor doping levels.Comment: 8 pages, 3 figures, to appear in "The Physics of Semiconductors
CMB Anisotropies in the Presence of Extra Dimensions
We discuss the effect of the time evolution of extra dimensions on CMB
anisotropies and large-scale structure formation. We study the impact of scalar
fields in a low-energy effective description of a general class of brane world
models on the temperature anisotropy power spectrum. We show that when the
coupling between these scalar fields and matter evolves over cosmological
timescales, current observations of the CMB anisotropies can constrain
primordial values of the fields in a manner complementary to local, late-time
tests of gravity. We also present the effect of these fields on the
polarization anisotropy spectra and the growth of large-scale structure,
showing that future CMB observations will constrain theories of the Universe
involving extra dimensions even further.Comment: 17 pages, 15 figure
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