472 research outputs found
Asymptotics of the trap-dominated Gunn effect in p-type Ge
We present an asymptotic analysis of the Gunn effect in a drift-diffusion
model---including electric-field-dependent generation-recombination
processes---for long samples of strongly compensated p-type Ge at low
temperature and under dc voltage bias. During each Gunn oscillation, there are
different stages corresponding to the generation, motion and annihilation of
solitary waves. Each stage may be described by one evolution equation for only
one degree of freedom (the current density), except for the generation of each
new wave. The wave generation is a faster process that may be described by
solving a semiinfinite canonical problem. As a result of our study we have
found that (depending on the boundary condition) one or several solitary waves
may be shed during each period of the oscillation. Examples of numerical
simulations validating our analysis are included.Comment: Revtex, 25 pag., 5 fig., to appear Physica
Nuclear Resonance Vibrational Spectroscopy of Iron Sulfur Proteins
Nuclear inelastic scattering in conjunction with density functional theory
(DFT) calculations has been applied for the identification of vibrational modes
of the high-spin ferric and the high-spin ferrous iron-sulfur center of a
rubredoxin-type protein from the thermophylic bacterium Pyrococcus abysii
Transient elastohydrodynamic lubrication analysis of a novel metal-on-metal hip prosthesis with a non-spherical femoral bearing surface
Effective lubrication performance of metal-on-metal hip implants only requires optimum conformity within the main loaded area, while it is advantageous to increase the clearance in the equatorial region. Such a varying clearance can be achieved by using non-spherical bearing surfaces for either acetabular or femoral components. An elastohydrodynamic lubrication model of a novel metal-on-metal hip prosthesis using a non-spherical femoral bearing surface against a spherical cup was solved under loading and motion conditions specified by ISO standard. A full numerical methodology of considering the geometric variation in the rotating non-spherical head in elastohydrodynamic lubrication solution was presented, which is applicable to all non-spherical head designs. The lubrication performance of a hip prosthesis using a specific non-spherical femoral head, Alpharabola, was analysed and compared with those of spherical bearing surfaces and a non-spherical Alpharabola cup investigated in previous studies. The sensitivity of the lubrication performance to the anteversion angle of the Alpharabola head was also investigated. Results showed that the non-spherical head introduced a large squeeze-film action and also led to a large variation in clearance within the loaded area. With the same equatorial clearance, the lubrication performance of the metal-on-metal hip prosthesis using an Alpharabola head was better than that of the conventional spherical bearings but worse than that of the metal-on-metal hip prosthesis using an Alpharabola cup. The reduction in the lubrication performance caused by the initial anteversion angle of the non-spherical head was small, compared with the improvement resulted from the non-spherical geometry
Gravity-Driven Acceleration of the Cosmic Expansion
It is shown here that a dynamical Planck mass can drive the scale factor of
the universe to accelerate. The negative pressure which drives the cosmic
acceleration is identified with the unusual kinetic energy density of the
Planck field. No potential nor cosmological constant is required. This suggests
a purely gravity driven, kinetic inflation. Although the possibility is not
ruled out, the burst of acceleration is often too weak to address the initial
condition problems of cosmology. To illustrate the kinetic acceleration, three
different cosmologies are presented. One such example, that of a bouncing
universe, demonstrates the additional feature of being nonsingular. The
acceleration is also considered in the conformally related Einstein frame in
which the Planck mass is constant.Comment: 23 pages, LaTex, figures available upon request, (revisions include
added references and comment on inflation) CITA-94-1
Unitary limit and quantum interference effect in disordered two-dimensional crystals with nearly half-filled bands
Based on the self-consistent -matrix approximation, the quantum
interference (QI) effect is studied with the diagrammatic technique in
weakly-disordered two-dimensional crystals with nearly half-filled bands. In
addition to the usual 0-mode cooperon and diffuson, there exist -mode
cooperon and diffuson in the unitary limit due to the particle-hole symmetry.
The diffusive -modes are gapped by the deviation from the exactly-nested
Fermi surface. The conductivity diagrams with the gapped -mode cooperon or
diffuson are found to give rise to unconventional features of the QI effect.
Besides the inelastic scattering, the thermal fluctuation is shown to be also
an important dephasing mechanism in the QI processes related with the diffusive
-modes. In the proximity of the nesting case, a power-law
anti-localization effect appears due to the -mode diffuson. For large
deviation from the nested Fermi surface, this anti-localization effect is
suppressed, and the conductivity remains to have the usual logarithmic
weak-localization correction contributed by the 0-mode cooperon. As a result,
the dc conductivity in the unitary limit becomes a non-monotonic function of
the temperature or the sample size, which is quite different from the
prediction of the usual weak-localization theory.Comment: 21 pages, 4 figure
Gemini Observations of Disks and Jets in Young Stellar Objects and in Active Galaxies
We present first results from the Near-infrared Integral Field Spectrograph
(NIFS) located at Gemini North. For the active galaxies Cygnus A and Perseus A
we observe rotationally-supported accretion disks and adduce the existence of
massive central black holes and estimate their masses. In Cygnus A we also see
remarkable high-excitation ionization cones dominated by photoionization from
the central engine. In the T-Tauri stars HV Tau C and DG Tau we see
highly-collimated bipolar outflows in the [Fe II] 1.644 micron line, surrounded
by a slower molecular bipolar outflow seen in the H_2 lines, in accordance with
the model advocated by Pyo et al. (2002).Comment: Invited paper presented at the 5th Stromlo Symposium. 9 pages, 7
figures. Accepted for publication in Astrophysics & Space Scienc
WMAP constraints on scalar-tensor cosmology and the variation of the gravitational constant
We present observational constraints on a scalar-tensor gravity theory by
test for CMB anisotropy spectrum. We compare the WMAP temperature
power spectrum with the harmonic attractor model, in which the scalar field has
its harmonic effective potential with curvature in the Einstein
conformal frame and the theory relaxes toward Einstein gravity with time. We
found that the present value of the scalar coupling, i.e. the present level of
deviation from Einstein gravity , is bounded to be smaller than
(), and () for . This constraint is much stronger than the bound from the solar
system experiments for large models, i.e., and 0.3 in
and limits, respectively. Furthermore, within the framework
of this model, the variation of the gravitational constant at the recombination
epoch is constrained as , and
.Comment: 7 page
Interacting new agegraphic viscous dark energy with varying
We consider the new agegraphic model of dark energy with a varying
gravitational constant, , in a non-flat universe. We obtain the equation of
state and the deceleration parameters for both interacting and noninteracting
new agegraphic dark energy. We also present the equation of motion determining
the evolution behavior of the dark energy density with a time variable
gravitational constant. Finally, we generalize our study to the case of viscous
new agegraphic dark energy in the presence of an interaction term between both
dark components.Comment: 12 pages, accepted for publication in IJTP (2010
Constraints from Inflation on Scalar-Tensor Gravity Theories
We show how observations of the perturbation spectra produced during
inflation may be used to constrain the parameters of general scalar-tensor
theories of gravity, which include both an inflaton and dilaton field. An
interesting feature of these models is the possibility that the curvature
perturbations on super-horizon scales may not be constant due to non-adiabatic
perturbations of the two fields. Within a given model, the tilt and relative
amplitude of the scalar and tensor perturbation spectra gives constraints on
the parameters of the gravity theory, which may be comparable with those from
primordial nucleosynthesis and post-Newtonian experiments.Comment: LaTeX (with RevTex) 19 pages, 8 uuencoded figures appended, also
available on WWW via http://star.maps.susx.ac.uk/index.htm
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