1,360 research outputs found
Melt-growth dynamics in CdTe crystals
We use a new, quantum-mechanics-based bond-order potential (BOP) to reveal
melt-growth dynamics and fine-scale defect formation mechanisms in CdTe
crystals. Previous molecular dynamics simulations of semiconductors have shown
qualitatively incorrect behavior due to the lack of an interatomic potential
capable of predicting both crystalline growth and property trends of many
transitional structures encountered during the melt crystal
transformation. Here we demonstrate successful molecular dynamics simulations
of melt-growth in CdTe using a BOP that significantly improves over other
potentials on property trends of different phases. Our simulations result in a
detailed understanding of defect formation during the melt-growth process.
Equally important, we show that the new BOP enables defect formation mechanisms
to be studied at a scale level comparable to empirical molecular dynamics
simulation methods with a fidelity level approaching quantum-mechanical method
Influence of UV radiation from a massive YSO on the chemistry of its envelope
We have studied the influence of far ultraviolet (UV) radiation from a
massive young stellar object (YSO) on the chemistry of its own envelope by
extending the models of Doty et al. (2002) to include a central source of UV
radiation. The models are applied to the massive star-forming region AFGL 2591
for different inner UV field strengths. Depth-dependent abundance profiles for
several molecules are presented and discussed. We predict enhanced column
densities for more than 30 species, especially radicals and ions. Comparison
between observations and models is improved with a moderate UV field incident
on the inner envelope, corresponding to an enhancement factor G0~10-100 at 200
AU from the star with an optical depth tau~15-17. Subtle differences are found
compared with traditional models of Photon Dominated Regions (PDRs) because of
the higher temperatures and higher gas-phase H2O abundance caused by
evaporation of ices in the inner region. In particular, the CN/HCN ratio is not
a sensitive tracer of the inner UV field, in contrast with the situation for
normal PDRs: for low UV fields, the extra CN reacts with H2 in the inner dense
and warm region and produces more HCN. It is found that the CH+ abundance is
strongly enhanced and grows steadily with increasing UV field. High-J lines of
molecules like CN and HCN are most sensitive to the inner dense region where UV
radiation plays a role. Thus, even though the total column density affected by
UV photons is small, comparison of high-J and low-J lines can selectively trace
and distinguish the inner UV field from the outer one. In addition, future
Herschel-HIFI observations of hydrides can sensitively probe the inner UV
field.Comment: Accepted for publication in A&A. 13 pages, 10 figure
Constructive Dimension and Turing Degrees
This paper examines the constructive Hausdorff and packing dimensions of
Turing degrees. The main result is that every infinite sequence S with
constructive Hausdorff dimension dim_H(S) and constructive packing dimension
dim_P(S) is Turing equivalent to a sequence R with dim_H(R) <= (dim_H(S) /
dim_P(S)) - epsilon, for arbitrary epsilon > 0. Furthermore, if dim_P(S) > 0,
then dim_P(R) >= 1 - epsilon. The reduction thus serves as a *randomness
extractor* that increases the algorithmic randomness of S, as measured by
constructive dimension.
A number of applications of this result shed new light on the constructive
dimensions of Turing degrees. A lower bound of dim_H(S) / dim_P(S) is shown to
hold for the Turing degree of any sequence S. A new proof is given of a
previously-known zero-one law for the constructive packing dimension of Turing
degrees. It is also shown that, for any regular sequence S (that is, dim_H(S) =
dim_P(S)) such that dim_H(S) > 0, the Turing degree of S has constructive
Hausdorff and packing dimension equal to 1.
Finally, it is shown that no single Turing reduction can be a universal
constructive Hausdorff dimension extractor, and that bounded Turing reductions
cannot extract constructive Hausdorff dimension. We also exhibit sequences on
which weak truth-table and bounded Turing reductions differ in their ability to
extract dimension.Comment: The version of this paper appearing in Theory of Computing Systems,
45(4):740-755, 2009, had an error in the proof of Theorem 2.4, due to
insufficient care with the choice of delta. This version modifies that proof
to fix the error
X-ray Chemistry in the Envelopes around Young Stellar Objects
We have studied the influence of X-rays from a massive young stellar object
(YSO) on the chemistry of its own envelope by extending the models of Doty et
al. (2002) and Staeuber et al. (2004). The models are applied to the massive
star-forming region AFGL 2591 for different X-ray luminosities and plasma
temperatures. Enhanced column densities for several species are predicted. In
addition we present first detections of CO+ and SO+ toward AFGL 2591. These
molecular ions are believed to be high-energy tracers. Herschel-HIFI will be
able to observe other tracers like CH and CH+ whereas ALMA is well suited to
measure the size and geometry of the emitting region.Comment: To appear in "The Dusty and Molecular Universe: A Prelude to Herschel
and ALMA", Eds.: A. Wilson. To be published in ESA Conference Serie
Large-area submillimeter resolution CdZnTe strip detector for astronomy
We report the first performance measurements of a sub-millimeter CdZnTe strip detector developed as a prototype for space-borne astronomical instruments. Strip detector arrays can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type and other candidate technologies are under investigation for the position-sensitive backplane detector for a coded-aperture telescope operating in the range of 30 - 300 keV. The prototype is a 1.4 mm thick, 64 multiplied by 64 stripe CdZnTe array of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. Pulse height spectra in both single and orthogonal stripe coincidence mode were recorded at several energies. The results are compared to slab- and pixel-geometry detector spectra. The room-temperature energy resolution is less than 10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio greater than 5:1. The response to photons with energies up to 662 keV appears to be considerably improved relative to that of previously reported slab and pixel detectors. We also show that strip detectors can yield spatial and energy resolutions similar to those of pixellated arrays with the same dimensions. Electrostatic effects on the pulse heights, read-out circuit complexity, and issues related to design of space borne instruments are also discussed
Electrically tunable g-factors in quantum dot molecular spin states
We present a magneto-photoluminescence study of individual vertically stacked
InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied
electric field tunes the relative energies of the two dots, we observe a strong
resonant increase or decrease in the g-factors of different spin states that
have molecular wavefunctions distributed over both quantum dots. We propose a
phenomenological model for the change in g-factor based on resonant changes in
the amplitude of the wavefunction in the barrier due to the formation of
bonding and antibonding orbitals.Comment: 5 pages, 5 figures, Accepted by Phys. Rev. Lett. New version reflects
response to referee comment
Spin Fine Structure in Optically Excited Quantum Dot Molecules
The interaction between spins in coupled quantum dots is revealed in distinct
fine structure patterns in the measured optical spectra of InAs/GaAs double
quantum dot molecules containing zero, one, or two excess holes. The fine
structure is explained well in terms of a uniquely molecular interplay of spin
exchange interactions, Pauli exclusion and orbital tunneling. This knowledge is
critical for converting quantum dot molecule tunneling into a means of
optically coupling not just orbitals, but spins.Comment: 10 pages, 7 figures, added material, (published
Water destruction by X-rays in young stellar objects
We study the H2O chemistry in star-forming environments under the influence
of a central X-ray source and a central far ultraviolet (FUV) radiation field.
The gas-phase water chemistry is modeled as a function of time, hydrogen
density and X-ray flux. To cover a wide range of physical environments,
densities between n_H = 10^4-10^9 cm^-3 and temperatures between T = 10-1000 K
are studied. Three different regimes are found: For T < 100 K, the water
abundance is of order 10^-7-10^-6 and can be somewhat enhanced or reduced due
to X-rays, depending on time and density. For 100 K < T < 250 K, H2O is reduced
from initial x(H2O) ~ 10^-4 following ice evaporation to x(H2O) ~ 10^-6 for F_X
> 10^-3 ergs s-1 cm^-2 (t = 10^4 yrs) and for F_X > 10^-4 ergs s^-1 cm^-2 (t =
10^5 yrs). At higher temperatures (T > 250 K) and hydrogen densities, water can
persist with x(H2O) ~ 10^-4 even for high X-ray fluxes. The X-ray and FUV
models are applied to envelopes around low-mass Class 0 and I young stellar
objects (YSOs). Water is destroyed in both Class 0 and I envelopes on
relatively short timescales (t ~ 5000 yrs) for realistic X-ray fluxes, although
the effect is less prominent in Class 0 envelopes due to the higher X-ray
absorbing densities there. FUV photons from the central source are not
effective in destroying water. The average water abundance in Class I sources
for L_X > 10^27 ergs s^-1 is predicted to be x(H2O) < 10^-6.Comment: 12 pages, 14 figures, Accepted for publication in A&
Development of an orthogonal-stripe CdZnTe gamma radiation imaging spectrometer
We report performance measurements of a sub-millimeter resolution CdZnTe strip detector developed as a prototype for astronomical instruments operating with good efficiency in the 30-300 keV photon energy range. The prototype is a 1.4 mm thick, 64×64 contact stripe CdZnTe array of 0.375 mm pitch in both dimensions. Pulse height spectra were recorded in orthogonal-stripe coincidence mode which demonstrate room-temperature energy resolution \u3c10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio \u3e5:1. Good response is also demonstrated at higher energies using a coplanar grid readout configuration. Spatial resolution capabilities finer than the stripe pitch are demonstrated. We present the image of a 133Ba source viewed through a collimator slit produced by a 4×4 stripe detector segment. Charge signals from electron and hole collecting contacts are also discussed
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