3,564 research outputs found
Twisting type-N vacuum fields with a group
We derive the equations corresponding to twisting type-N vacuum gravitational
fields with one Killing vector and one homothetic Killing vector by using the
same approach as that developed by one of us in order to treat the case with
two non-commuting Killing vectors. We study the case when the homothetic
parameter takes the value -1, which is shown to admit a reduction to a
third-order real ordinary differential equation for this problem, similar to
that previously obtained by one of us when two Killing vectors are present.Comment: LaTeX, 11 pages. To be published in Classical and Quantum Gravit
Optically Probing Spin and Charge Interactions in an Tunable Artificial Molecule
We optically probe and electrically control a single artificial molecule
containing a well defined number of electrons. Charge and spin dependent
inter-dot quantum couplings are probed optically by adding a single
electron-hole pair and detecting the emission from negatively charged exciton
states. Coulomb and Pauli blockade effects are directly observed and
hybridization and electrostatic charging energies are independently measured.
The inter-dot quantum coupling is confirmed to be mediated predominantly by
electron tunneling. Our results are in excellent accord with calculations that
provide a complete picture of negative excitons and few electron states in
quantum dot molecules.Comment: shortened version: 6 pages, 3 figures, 1 table, to appear in Phys.
Rev. Let
Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule
We probe acoustic phonon mediated relaxation between tunnel coupled exciton
states in an individual quantum dot molecule in which the inter-dot quantum
coupling and energy separation between exciton states is continuously tuned
using static electric field. Time resolved and temperature dependent optical
spectroscopy are used to probe inter-level relaxation around the point of
maximum coupling. The radiative lifetimes of the coupled excitonic states can
be tuned from ~2 ns to ~10 ns as the spatially direct and indirect character of
the wavefunction is varied by detuning from resonance. Acoustic phonon mediated
inter-level relaxation is shown to proceed over timescales comparable to the
direct exciton radiative lifetime, indicative of a relaxation bottleneck for
level spacings in the range $\Delta E\$ ~3-6 meV.Comment: 6 pages, 4 figures, submitted for publicatio
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The Feasibility of Locating a Texas Salt Test Facility
Differences in the geology of dome salt and bedded salt dictate that data and mining experience gained in one environment will not be fully applicable to the other. A test facility in a salt dome could be mined through a relatively thin section of elastic, a cap rock of evaporite minerals that may contain highly porous zones, and possibly 300 m (1,000 ft) of salt. The salt will probably be quite pure except for a few percent anhydrite, increasing in concentration toward the top of the salt stock (Balk, 1949; Muehlberger, 1959; and Dutton and Kreitler, 1980). Infrequent inclusions of the surrounding sediments may be found in dome salt that has been highly contorted and whose crystals show translational gliding as a result of salt dome emplacement (Muehlberger, 1959; Clabaugh, 1962; and Hofrichter, 1968).
Access to bedded salt will require mining through a sequence of predominantly terrigenous elastics and evaporites. The degree of consolidation, porosity, and permeability of these sediments will vary between horizons. The objective interval will be a required thickness of relatively pure salt. Crystallographic properties, water content, and elastic inclusions will be results of the original depositional environment rather than emplacement of a salt stock.
The feasibility of locating a Salt Test Facility in Texas has been studied and evaluated. Measured parameters are summarized in table 1. For primarily technical reasons, the most feasible Texas sites are the Gyp Hill salt dome in Brooks County or bedded salt in Loving County.Bureau of Economic Geolog
The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?
The X-ray source 2S0114+650=LSI+65 010 is a binary system containing a B-type
primary and a low mass companion believed to be a neutron star. The system has
three reported periodicities: the orbital period, P{orb}~11.6 d, X-ray flaring
with P{flare}~2.7 hr, and a "superorbital" X-ray periodicity P{super}~30.7 d.
The objective of this paper is to show that the puzzling periodicities in the
system may be explained in the context of scenarios in which tidal interactions
drive oscillations in the B-supergiant star. We calculate the solution of the
equations of motion for one layer of small surface elements distributed along
the equator of the star, as they respond to the forces due to gas pressure,
centrifugal, coriolis, viscous forces, and the gravitational forces of both
stars. This calculation provides variability timescales that can be compared
with the observations. In addition, we use observational data obtained at the
Observatorio Astron\'omico Nacional en San Pedro M\'artir (OAN/SPM) between
1993-2004 to determine which of the periodicities may be present in the optical
region. We suggest that the tidal oscillations lead to a structured stellar
wind which, when fed to the neutron star, produces the X-ray modulations. The
connection between the stellar oscillations and the modulation of the mass
ejection may lie in the shear energy dissipation generated by the tangential
motions that are produced by the tidal interaction, particularly in the tidal
bulge region. The tidal oscillation scenario weakens the case for 2S0114+650
containing a magnetar descendent.Comment: 12 pages, 14 figure
BeppoSAX observations of the three Gamma-ray pulsars PSR B0656+14, PSR B1055-52 and PSR B1706-44
We report the results of the observations of the three gamma-ray pulsars PSR
B0656+14, PSR B1055-52 and PSR B1706-44 performed with BeppoSAX. We detected a
pulsed emission only for PSR B1055-52: in the range 0.1-6.5 keV the pulse
profile is sinusoidal and the statistical significance is 4.5 sigma. The pulsed
fraction was estimated 0.64+/-0.17. This pulsation was detected also at
energies greater than 2.5 keV suggesting either a non-thermal origin or a quite
high temperature region on the neutron star surface. Spectral analysis showed
that only the X-ray spectrum of PSR B1706-44 can be fitted by a single
power-law component, while that of PSR B1055-52 requires also a blackbody
component (kT = 0.075 keV) and that of PSR B0656+14 two blackbody components
(kT_1 = 0.059, kT_2 = 0.12 keV).Comment: 8 pages, 5 figures. Accepted for publication in A&
Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule
We report the direct observation of quantum coupling in individual quantum
dot molecules and its manipulation using static electric fields. A pronounced
anti-crossing of different excitonic transitions is observed as the electric
field is tuned. Comparison of our experimental results with theory shows that
the observed anti-crossing occurs between excitons with predominant spatially
\emph{direct} and \emph{indirect} character. The electron component of the
exciton wavefunction is shown to have molecular character at the anti-crossing
and the quantum coupling strength is deduced optically. In addition, we
determine the dependence of the coupling strength on the inter-dot separation
and identify a field driven transition of the nature of the molecular ground
state.Comment: 11 pages, 4 figures submitted to Physical Review Letter
New first integral for twisting type-N vacuum gravitational fields with two non-commuting Killing vectors
A new first integral for the equations corresponding to twisting type-N
vacuum gravitational fields with two non-commuting Killing vectors is
introduced. A new reduction of the problem to a complex second-order ordinary
differential equation is given. Alternatively, the mentioned first integral can
be used in order to provide a first integral of the second-order complex
equation introduced in a previous treatment of the problem.Comment: 7 pages, LaTeX, uses ioplppt.sty and iopl12.sty; to be published in
Class. Quantum Gra
Asymmetric optical nuclear spin pumping in a single uncharged quantum dot
A highly asymmetric dynamic nuclear spin pumping is observed in a single self
assembled InGaAs quantum dot subject to resonant optical pumping of the neutral
exciton transition leading to a large maximum polarization of 54%. This dynamic
nuclear polarization is found to be much stronger following pumping of the
higher energy Zeeman state. Time-resolved measurements allow us to directly
monitor the buildup of the nuclear spin polarization in real time and to
quantitatively study the dynamics of the process. A strong dependence of the
observed dynamic nuclear polarization on the applied magnetic field is found,
with resonances in the pumping efficiency being observed for particular
magnetic fields. We develop a model that fully accounts for the observed
behaviour, where the pumping of the nuclear spin system is due to
hyperfine-mediated spin flip transitions between the states of the neutral
exciton manifold.Comment: published version; 4+ pages, 3 figures (eps
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