681 research outputs found
Controlling quantum transport through a single molecule
We investigate multi-terminal quantum transport through single monocyclic
aromatic annulene molecules, and their derivatives, using the nonequilibrium
Green function approach in the self-consistent Hartree-Fock approximation. A
new device concept, the Quantum Interference Effect Transistor (QuIET) is
proposed, exploiting perfect destructive interference stemming from molecular
symmetry, and controlling current flow by introducing decoherence and/or
elastic scattering that break the symmetry. This approach overcomes the
fundamental problems of power dissipation and environmental sensitivity that
beset many nanoscale device proposals.Comment: 4 pages, 5 figure
Universality of Decay out of Superdeformed Bands in the 190 Mass Region
Superdeformed nuclei in the 190 mass region exhibit a striking universality
in their decay-out profiles. We show that this universality can be explained in
the two-level model of superdeformed decay as related to a strong separation of
energy scales: a higher scale related to the nuclear interactions, and a lower
scale caused by electromagnetic decay. Furthermore, we present the results of
the two-level model for all decays for which sufficient data are known,
including statistical extraction of the matrix element for tunneling through
the potential barrier.Comment: 7 pages, 3 figures. v2: some minor clarifications, minor correction
to Fig.
How to measure the spreading width for decay of superdeformed nuclei
A new expression for the branching ratio for the decay via the E1 process in
the normal-deformed band of superdeformed nuclei is given within a simple
two-level model. Using this expression, the spreading or tunneling width
Gamma^downarrow for superdeformed decay can be expressed entirely in terms of
experimentally known quantities. We show how to determine the tunneling matrix
element V from the measured value of Gamma^downarrow and a statistical model of
the energy levels. The accuracy of the two-level approximation is verified by
considering the effects of the other normal-deformed states.Comment: 4 pages, 4 figure
The Space Density of Compton-thick AGN
We constrain the number density and evolution of Compton-thick Active
Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray
background. In the local Universe we use the wide area surveys from the Swift
and INTEGRAL satellites, while for high redshifts we explore candidate
selections based on mid-IR parameters. We present the properties of a sample of
211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South
(ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR
ratios for these sources are significantly larger than that of star-forming
galaxies and ~2 orders of magnitude smaller than for the general AGN
population, suggesting column densities of NH>5x10^24 cm^-2. The space density
of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a
strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.Comment: 4 pages, 3 figures, to appear in proceedings for 'X-ray Astronomy
2009', Bologna 09/2009, AIP Conference Series, Eds. A. Comastri, M. Cappi, L.
Angelini; author list fixe
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