35,529 research outputs found
Deployment/retraction ground testing of a large flexible solar array
The simulated zero-gravity ground testing of the flexible fold-up solar array consisting of eighty-four full-size panels (.368 m x .4 m each) is addressed. Automatic, hands-off extension, retraction, and lockup operations are included. Three methods of ground testing were investigated: (1) vertical testing; (2) horizontal testing, using an overhead water trough to support the panels; and (3) horizontal testing, using an overhead track in conjunction with a counterweight system to support the panels. Method 3 was selected as baseline. The wing/assembly vertical support structure, the five-tier overhead track, and the mast-element support track comprise the test structure. The flexible solar array wing assembly was successfully extended and retracted numerous times under simulated zero-gravity conditions
A Possible Resolution of the Black Hole Information Puzzle
The problem of information loss is considered under the assumption that the
process of black hole evaporation terminates in the decay of the black hole
interior into a baby universe. We show that such theories can be decomposed
into superselection sectors labeled by eigenvalues of the third-quantized baby
universe field operator, and that scattering is unitary within each
superselection sector. This result relies crucially on the quantum-mechanical
variability of the decay time. It is further argued that the decay rate in the
black hole rest frame is necessarily proportional to , where
is the total entropy produced during the evaporation process,
entailing a very long-lived remnant.Comment: 15 pages, 3 uuencoded figures. Revised version contains some
notational simplification
Progress toward the development of dual junction GaAs/Ge solar cells
Large area GaAs/Ge cells offer substantial promise for increasing the power output from existing silicon solar array designs and for providing an enabled technology for missions hitherto impossible using silicon. Single junction GaAs/Ge cells offer substantial advantages in both size, weight, and cost compared to GaAs cells but the efficiency is limited to approximately 19.2 to 20 percent AMO. The thermal absorptance of GaAs/Ge cells is also worse than GaAs/GaAs cells (0.88 vs 0.81 typ.) due to the absorption in the Ge substrate. On the other hand dual junction GaAs/Ge cells offer efficiencies up to ultimately 24 percent AMO in sizes up to 8 x 8 cm but there are still technological issues remaining to achieve current matching in the GaAs and Ge cells. This can be achieved through tuned antireflection (AR) coatings, improved quality of the GaAs growth, improved quality Ge wafers and the use of a Back Surface Field (BSF)/Back Surface Reflector (BSR) in the Ge cell. Although the temperature coefficients of efficiency and voltage are higher for dual junction GaAs/Ge cells, it has been shown elsewhere that for typical 28 C cell efficiencies of 22 percent (dual junction) vs 18.5 percent (single junction) there is a positive power tradeoff up to temperatures as high as 120 C. Due to the potential ease of fabrication of GaAs/Ge dual junction cells there is likely to be only a small cost differential compared to single junction cells
Topological Quantum Computing with p-Wave Superfluid Vortices
It is shown that Majorana fermions trapped in three vortices in a p-wave
superfluid form a qubit in a topological quantum computing (TQC). Several
similar ideas have already been proposed: Ivanov [Phys. Rev. Lett. {\bf 86},
268 (2001)] and Zhang {\it et al.} [Phys. Rev. Lett. {\bf 99}, 220502 (2007)]
have proposed schemes in which a qubit is implemented with two and four
Majorana fermions, respectively, where a qubit operation is performed by
exchanging the positions of Majorana fermions. The set of gates thus obtained
is a discrete subset of the relevant unitary group. We propose, in this paper,
a new scheme, where three Majorana fermions form a qubit. We show that
continuous 1-qubit gate operations are possible by exchanging the positions of
Majorana fermions complemented with dynamical phase change. 2-qubit gates are
realized through the use of the coupling between Majorana fermions of different
qubits.Comment: 5 pages, 2 figures. Two-qubit gate implementation is added
On the gravitational production of superheavy dark matter
The dark matter in the universe can be in the form of a superheavy matter
species (WIMPZILLA). Several mechanisms have been proposed for the production
of WIMPZILLA particles during or immediately following the inflationary epoch.
Perhaps the most attractive mechanism is through gravitational particle
production, where particles are produced simply as a result of the expansion of
the universe. In this paper we present a detailed numerical calculation of
WIMPZILLA gravitational production in hybrid-inflation models and
natural-inflation models. Generalizing these findings, we also explore the
dependence of the gravitational production mechanism on various models of
inflation. We show that superheavy dark matter production seems to be robust,
with Omega_X h^2 ~ (M_X / (10^11 GeV))^2 (T_RH / (10^9 GeV)), so long as M_X <
H_I, where M_X is the WIMPZILLA mass, T_RH is the reheat temperature, and H_I
is the expansion rate of the universe during inflation.Comment: 26 pages, 7 figures; LaTeX; submitted to Physical Review D; minor
typographical error correcte
Quintessential Kination and Leptogenesis
Thermal leptogenesis induced by the CP-violating decay of a right-handed
neutrino (RHN) is discussed in the background of quintessential kination, i.e.,
in a cosmological model where the energy density of the early Universe is
assumed to be dominated by the kinetic term of a quintessence field during some
epoch of its evolution. This assumption may lead to very different
observational consequences compared to the case of a standard cosmology where
the energy density of the Universe is dominated by radiation. We show that,
depending on the choice of the temperature T_r above which kination dominates
over radiation, any situation between the strong and the super--weak wash--out
regime are equally viable for leptogenesis, even with the RHN Yukawa coupling
fixed to provide the observed atmospheric neutrino mass scale ~ 0.05 eV. For M<
T_r < M/100, i.e., when kination stops to dominate at a time which is not much
later than when leptogenesis takes place, the efficiency of the process,
defined as the ratio between the produced lepton asymmetry and the amount of CP
violation in the RHN decay, can be larger than in the standard scenario of
radiation domination. This possibility is limited to the case when the neutrino
mass scale is larger than about 0.01 eV. The super--weak wash--out regime is
obtained for T_r << M/100, and includes the case when T_r is close to the
nucleosynthesis temperature ~ 1 MeV. Irrespective of T_r, we always find a
sufficient window above the electroweak temperature T ~ 100 GeV for the
sphaleron transition to thermalize, so that the lepton asymmetry can always be
converted to the observed baryon asymmetry.Comment: 13 pages, 8 figure
Indium segregation during III-V quantum wire and quantum dot formation on patterned substrates
We report a model for metalorganic vapor-phase epitaxy on non-planar
substrates, specifically V-grooves and pyramidal recesses, which we apply to
the growth of InGaAs nanostructures. This model, based on a set of coupled
reaction-diffusion equations, one for each facet in the system, accounts for
the facet-dependence of all kinetic processes (e.g., precursor decomposition,
adatom diffusion, and adatom lifetimes) and has been previously applied to
account for the temperature, concentration, and temporal-dependence of AlGaAs
nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses.
In the present study, the growth of InGaAs quantum wires at
the bottom of V-grooves is used to determine a set of optimized kinetic
parameters. Based on these parameters, we have modeled the growth of
InGaAs nanostructures formed in pyramidal site-controlled
quantum-dot systems, successfully producing a qualitative explanation for the
temperature-dependence of their optical properties, which have been reported in
previous studies. Finally, we present scanning electron and cross-sectional
atomic force microscopy images which show previously unreported facetting at
the bottom of the pyramidal recesses that allow quantum dot formation.Comment: 9 pages, 8 figure
Quantum criticality in Kondo quantum dot coupled to helical edge states of interacting 2D topological insulators
We investigate theoretically the quantum phase transition (QPT) between the
one-channel Kondo (1CK) and two-channel Kondo (2CK) fixed points in a quantum
dot coupled to helical edge states of interacting 2D topological insulators
(2DTI) with Luttinger parameter . The model has been studied in Ref. 21,
and was mapped onto an anisotropic two-channel Kondo model via bosonization.
For K<1, the strong coupling 2CK fixed point was argued to be stable for
infinitesimally weak tunnelings between dot and the 2DTI based on a simple
scaling dimensional analysis[21]. We re-examine this model beyond the bare
scaling dimension analysis via a 1-loop renormalization group (RG) approach
combined with bosonization and re-fermionization techniques near weak-coupling
and strong-coupling (2CK) fixed points. We find for K -->1 that the 2CK fixed
point can be unstable towards the 1CK fixed point and the system may undergo a
quantum phase transition between 1CK and 2CK fixed points. The QPT in our model
comes as a result of the combined Kondo and the helical Luttinger physics in
2DTI, and it serves as the first example of the 1CK-2CK QPT that is accessible
by the controlled RG approach. We extract quantum critical and crossover
behaviors from various thermodynamical quantities near the transition. Our
results are robust against particle-hole asymmetry for 1/2<K<1.Comment: 17 pages, 9 figures, more details added, typos corrected, revised
Sec. IV, V, Appendix A and
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