714 research outputs found
Torsion nonminimally coupled to the electromagnetic field and birefringence
In conventional Maxwell--Lorentz electrodynamics, the propagation of light is
influenced by the metric, not, however, by the possible presence of a torsion
T. Still the light can feel torsion if the latter is coupled nonminimally to
the electromagnetic field F by means of a supplementary Lagrangian of the type
l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific
nonminimal term of this nature. We evaluate the spacetime relation of Preuss in
the background of a general O(3)-symmetric torsion field and prove by
specifying the optical metric of spacetime that this can yield birefringence in
vacuum. Moreover, we show that the nonminimally coupled homogeneous and
isotropic torsion field in a Friedmann cosmos affects the speed of light.Comment: Revtex, 12 pages, no figure
On Loop Quantum Gravity Phenomenology and the Issue of Lorentz Invariance
A simple model is constructed which allows to compute modified dispersion
relations with effects from loop quantum gravity. Different quantization
choices can be realized and their effects on the order of corrections studied
explicitly. A comparison with more involved semiclassical techniques shows that
there is agreement even at a quantitative level.
Furthermore, by contrasting Hamiltonian and Lagrangian descriptions we show
that possible Lorentz symmetry violations may be blurred as an artifact of the
approximation scheme. Whether this is the case in a purely Hamiltonian analysis
can be resolved by an improvement in the effective semiclassical analysis.Comment: 16 pages, RevTeX
Comparison of Plume Dynamics for Laser Ablated Metals: Al and Ti
Emissive plumes from pulsed laser ablation of bulk Ti and Al from KrF laser irradiation at laser fluence up to 3.5 J/cm2 and argon background pressures of 0–1 Torr have been observed using gated intensified charged-coupled device imagery. Mass loss for Ti increases from 0.1 to 0.8 μg/pulse as pulse energy increase from 174 to 282 mJ/pulse (35–170 photons/atom) and decreases by ∼30% as pressure increases from vacuum to 1 Torr. Early plume energies are described by the free expansion velocities of 1.57 ± 0.02 and of 1.81 ± 0.07 cm/μs for Ti and Al, respectively, and up to 90% of the incoming laser energy can be attributed to the Al shock front in the mid-field. The ablation thresholds of 90 ± 27 mJ (1.12 ± 0.34 J/cm2) for Ti and 126 ± 13 mJ (1.58 ± 0.16 J/cm2) for Al also represent 30%–70% of the incident laser energy. The decrease in mass loss at higher pressures is attributed to plasma shielding of the target surface
A 727/JT8D-100 series engine exhaust system propulsion performance model test
The results are presented from testing one-eighth scale models of the Pratt and Whitney aircraft reference and Boeing nozzles for the JT8D-100 series mixed flow engines. The objective of the test was to obtain the nozzle velocity and flow coefficients for the reference configurations and compare these with the Boeing configurations which incorporated a longer splitter between the fan and primary flows. A further comparison was made between the JT8D-100 series nozzles and the Boeing JT8D-9/727 production nozzle performance. A statistical analysis was used to compare configurations which showed the performance (velocity coefficient) of the reference and the Boeing configuration was the same for the JT8D-109. It also showed no difference between reference and the Boeing configuration for the JT8D-115 and no difference for the JT8D-117 nozzles. Bypass ratio (match) was shown to be equally dependent on splitter position as on nozzle area within the range investigated. The nozzles were very similar in flow coefficient within an engine family. Excellent profile data was recorded. The effects of swirl on the nozzle performance was examined and found to degrade the velocity and flow coefficients
Enhanced critical current density of YBa2Cu3Ox films grown on Nd1/3Eu1/3Gd1/3Ba2Cu3Ox with nano-undulated surface morphology
We report a simple and easily controllable method where a nano-undulated
surface morphology of Nd1/3Eu1/3Gd1/3Ba2Cu3Ox (NEG) films leads to a
substantial increase in the critical current density in superconducting
YBa2Cu3Ox (YBCO) films deposited by pulsed laser deposition on such NEG layers.
The enhancement is observed over a wide range of fields and temperatures.
Transmission electron microscopy shows that such YBCO films possess a high
density of localized areas, typically 20 x 20 nm2 in size, where distortion of
atomic planes give rotational (2 to 5 degrees) moire patterns. Their
distribution is random and uniform, and expected to be the origin of the
enhanced flux pinning. Magneto-optical imaging shows that these films have
excellent macroscopic magnetic uniformity.Comment: 4 pages, 4 figure
An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853
The coupling of the electromagnetic field directly with gravitational gauge
fields leads to new physical effects that can be tested using astronomical
data. Here we consider a particular case for closer scrutiny, a specific
nonminimal coupling of torsion to electromagnetism, which enters into a
metric-affine geometry of space-time. We show that under the assumption of this
nonminimal coupling, spacetime is birefringent in the presence of such a
gravitational field. This leads to the depolarization of light emitted from
extended astrophysical sources. We use polarimetric data of the magnetic white
dwarf to set strong constraints on the essential coupling
constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in
Phys.Rev.
Quantum spin Hall effect in two-dimensional metals without spin-orbit coupling
The quantum spin Hall effect has been observed in topological insulators
using spin-orbit coupling as the probe, but it has not yet been observed in a
metal. An experiment is proposed to measure the quantum spin Hall effect of an
electron or hole in a two-dimensional (2D) metal by using the previously
unexplored but relativistically generated 2D quantum spin Hall Hamiltonian, but
without using spin-orbit coupling. A long cylindrical solenoid lies normally
through the inner radius of a 2D metallic Corbino disk. The current
surrounding the solenoid produces an azimuthal magnetic vector potential but no
magnetic field in the disk. In addition, a radial electric field is generated
across the disk by imposing either (a) a potential difference or (b)
a radial charge current across its inner and outer radii. Combined
changes in and in either or generate spontaneously
quantized azimuthal charge and spin currents. The experiment is designed to
measure these quantized azimuthal charge and spin currents in the disk
consistently. The quantum Hamiltonians for experiments (a) and (b) are both
solved exactly. A method to control the Joule heating is presented, which could
potentially allow the quantum spin Hall measurements to be made at room
temperature. Extensions of this design to an array of thermally-managed
solenoids, each surrounded by thermally-managed stacks of 2D metallic Corbino
disks, could function as a quantum computer that could potentially operate at
room temperature.Comment: 13 pages, 7 figure
Testing the Equivalence Principle by Lamb shift Energies
The Einstein Equivalence Principle has as one of its implications that the
non-gravitational laws of physics are those of special relativity in any local
freely-falling frame. We consider possible tests of this hypothesis for systems
whose energies are due to radiative corrections, i.e. which arise purely as a
consequence of quantum field theoretic loop effects. Specifically, we evaluate
the Lamb shift transition (as given by the energy splitting between the
and atomic states) within the context of violations of
local position invariance and local Lorentz invariance, as described by the formalism. We compute the associated red shift and time dilation
parameters, and discuss how (high-precision) measurements of these quantities
could provide new information on the validity of the equivalence principle.Comment: 40 pages, latex, epsf, 1 figure, final version which appears in
Physical Review
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