11,564 research outputs found
Efficient Spatial Redistribution of Quantum Dot Spontaneous Emission from 2D Photonic Crystals
We investigate the modification of the spontaneous emission dynamics and
external quantum efficiency for self-assembled InGaAs quantum dots coupled to
extended and localised photonic states in GaAs 2D-photonic crystals. The
2D-photonic bandgap is shown to give rise to a 5-10 times enhancement of the
external quantum efficiency whilst the spontaneous emission rate is
simultaneously reduced by a comparable factor. Our findings are quantitatively
explained by a modal redistribution of spontaneous emission due to the modified
local density of photonic states. The results suggest that quantum dots
embedded within 2D-photonic crystals are suitable for practical single photon
sources with high external efficiency
Solutions of Penrose's Equation
The computational use of Killing potentials which satisfy Penrose's equation
is discussed. Penrose's equation is presented as a conformal Killing-Yano
equation and the class of possible solutions is analyzed. It is shown that
solutions exist in spacetimes of Petrov type O, D or N. In the particular case
of the Kerr background, it is shown that there can be no Killing potential for
the axial Killing vector.Comment: To appear in J. Math. Phy
The effects of lunar dust accumulation on the performance of photovoltaic arrays
Lunar base activity, particularly rocket launch and landing, will suspend and transport lunar dust. From preliminary models, the resulting dust accumulation can be significant, even as far as 2 km from the source. For example, at 2 km approximately 0.28 mg/sq cm of dust is anticipated to accumulate after only 10 surface missions with a 26,800 N excursion vehicle. The possible associated penalties in photovoltaic array performance were therefore the subject of experimental as well as theoretical investigation. To evaluate effects of dust accumulation on relative power output, current-voltage characteristics of dust-covered silicon cells were determined under the illumination of a Spectrolab X-25L solar simulator. The dust material used in these experiments was a terrestrial basalt which approximated lunar soil in particle size and composition. Cell short circuit current, an indicator of the penetrating light intensity, was found to decrease exponentially with dust accumulation. This was predicted independently by modeling the light occlusion caused by a growing layer of dust particles. Moreover, the maximum power output of dust-covered cells, derived from the I-V curves, was also found to degrade exponentially. Experimental results are presented and potential implications discussed
Superintegrable Systems in Darboux spaces
Almost all research on superintegrable potentials concerns spaces of constant
curvature. In this paper we find by exhaustive calculation, all superintegrable
potentials in the four Darboux spaces of revolution that have at least two
integrals of motion quadratic in the momenta, in addition to the Hamiltonian.
These are two-dimensional spaces of nonconstant curvature. It turns out that
all of these potentials are equivalent to superintegrable potentials in complex
Euclidean 2-space or on the complex 2-sphere, via "coupling constant
metamorphosis" (or equivalently, via Staeckel multiplier transformations). We
present tables of the results
Hydration and mobility of HO-(aq)
The hydroxide anion plays an essential role in many chemical and biochemical
reactions. But a molecular-scale description of its hydration state, and hence
also its transport, in water is currently controversial. The statistical
mechanical quasi-chemical theory of solutions suggests that HO[H2O]3- is the
predominant species in the aqueous phase under standard conditions. This result
is in close agreement with recent spectroscopic studies on hydroxide water
clusters, and with the available thermodynamic hydration free energies. In
contrast, a recent ab initio molecular dynamics simulation has suggested that
HO[H_2O]4- is the only dominant aqueous solution species. We apply adiabatic ab
initio molecular dynamics simulations, and find good agreement with both the
quasi-chemical theoretical predictions and experimental results. The present
results suggest a picture that is simpler, more traditional, but with
additional subtlety. These coordination structures are labile but the
tri-coordinate species is the prominent case. This conclusion is unaltered with
changes in the electronic density functional. No evidence is found for
rate-determining activated inter-conversion of a HO[H2O]4- trap structure to
HO[H2O]3-, mediating hydroxide transport. The view of HO- diffusion as the
hopping of a proton hole has substantial validity, the rate depending largely
on the dynamic disorder of the water hydrogen-bond network.Comment: 7 pages, 5 figures, additional results include
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