1,921 research outputs found
Representations of reductive normal algebraic monoids
The rational representation theory of a reductive normal algebraic monoid
(with one-dimensional center) forms a highest weight category, in the sense of
Cline, Parshall, and Scott. This is a fundamental fact about the representation
theory of reductive normal algebraic monoids. We survey how this result was
obtained, and treat some natural examples coming from classical groups.Comment: 10 pages. To appear in a volume of the Fields Communications Series:
"Algebraic Monoids, Group Embeddings, and Algebraic Combinatorics," edited by
Mahir Can, Zhenheng Li, Benjamin Steinberg, and Qiang Wan
Hole Spin Mixing in InAs Quantum Dot Molecules
Holes confined in single InAs quantum dots have recently emerged as a
promising system for the storage or manipulation of quantum information. These
holes are often assumed to have only heavy-hole character and further assumed
to have no mixing between orthogonal heavy hole spin projections (in the
absence of a transverse magnetic field). The same assumption has been applied
to InAs quantum dot molecules formed by two stacked InAs quantum dots that are
coupled by coherent tunneling of the hole between the two dots. We present
experimental evidence of the existence of a hole spin mixing term obtained with
magneto-photoluminescence spectroscopy on such InAs quantum dot molecules. We
use a Luttinger spinor model to explain the physical origin of this hole spin
mixing term: misalignment of the dots along the stacking direction breaks the
angular symmetry and allows mixing through the light-hole component of the
spinor. We discuss how this novel spin mixing mechanism may offer new spin
manipulation opportunities that are unique to holes.Comment: 13 pages, 9 figure
Photoluminescence Spectroscopy of the Molecular Biexciton in Vertically Stacked Quantum Dot Pairs
We present photoluminescence studies of the molecular neutral
biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum
dot pairs. We tune either the hole or the electron levels of the two dots into
tunneling resonances. The spectra are described well within a few-level,
few-particle molecular model. Their properties can be modified broadly by an
electric field and by structural design, which makes them highly attractive for
controlling nonlinear optical properties.Comment: 4 pages, 5 figures, (v2, revision based on reviewers comments,
published
Experiment K-6-03. Gravity and skeletal growth, part 1. Part 2: Morphology and histochemistry of bone cells and vasculature of the tibia; Part 3: Nuclear volume analysis of osteoblast histogenesis in periodontal ligament cells; Part 4: Intervertebral disc swelling pressure associated with microgravity
Bone area, bone electrophysiology, bone vascularity, osteoblast morphology, and osteoblast histogenesis were studied in rats associated with Cosmos 1887. The results suggest that the synchronous animals were the only group with a significantly larger bone area than the basal group, that the bone electrical potential was more negative in flight than in the synchronous rats, that the endosteal osteoblasts from flight rats had greater numbers of transitional Golgi vesicles but no difference in the large Golgi saccules or the alkaline phosphatase activity, that the perioteal vasculature in the shaft of flight rats often showed very dense intraluminal deposits with adjacent degenerating osteocytes as well as lipid accumulations within the lumen of the vessels and sometimes degeneration of the vascular wall (this change was not present in the metaphyseal region of flight animals), and that the progenitor cells decreased in flight rats while the preosteoblasts increased compared to controls. Many of the results suggest that the animals were beginning to recover from the effects of spaceflight during the two day interval between landing and euthanasia; flight effects, such as the vascular changes, did not appear to recover
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
Effects of clumping on temperature I: externally heated clouds
We present a study of radiative transfer in dusty, clumpy star-forming
regions. A series of self-consistent, 3-D, continuum radiative transfer models
are constructed for a grid of models parameterized by central luminosity,
filling factor, clump radius, and face-averaged optical depth. The temperature
distribution within the clouds is studied as a function of this
parameterization. Among our results, we find that: (a) the effective optical
depth is smaller in clumpy regions than in equivalent homogeneous regions; (b)
penetration of radiation is drive by the fraction of open sky (FOS) -- which
measures the fraction of solid angle which is devoid of clumps; (c) FOS
increases as clump radius increases and filling factor decreases; (d) for FOS >
0.6-0.8 the sky is sufficiently open that the temperature is relatively
insensitive to FOS; (e) the physical process by which radiation penetrates is
streaming between clumps; (f) filling factor dominates the temperature
distribution for large optical depths, and at small clump radii for small
optical depths; (g) at lower optical depths, the temperature distribution is
most sensitive to filling factors of 1-10 per cent, in accordance with many
observations; (h) direct shadowing can be important approximately one clump
radius behind a clump.Comment: 12 pages, 17 figures, accepted by MNRA
The Complexity of Fixed-Height Patterned Tile Self-Assembly
We characterize the complexity of the PATS problem for patterns of fixed
height and color count in variants of the model where seed glues are either
chosen or fixed and identical (so-called non-uniform and uniform variants). We
prove that both variants are NP-complete for patterns of height 2 or more and
admit O(n)-time algorithms for patterns of height 1. We also prove that if the
height and number of colors in the pattern is fixed, the non-uniform variant
admits a O(n)-time algorithm while the uniform variant remains NP-complete. The
NP-completeness results use a new reduction from a constrained version of a
problem on finite state transducers.Comment: An abstract version appears in the proceedings of CIAA 201
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
Physical-chemical modeling of the low-mass protostar IRAS 16293-2422
We present detailed gas-phase chemical models for the envelope of the
low-mass star-forming region IRAS 16293-2422. By considering both time- and
space-dependent chemistry, these models are used to study both the physical
structure proposed by Schoier et al. (2002), as well as the chemical evolution
of this region. A new feature of our study is the use of a detailed,
self-consistent radiative transfer model to translate the model abundances into
line strengths and compare them directly with observations of a total of 76
transitions for 18 chemical species, and their isotopes. The model can
reproduce many of the line strengths observed within 50%. The best fit is for
times in the range of 3e3 - 3e4 yrs, and requires only minor modifications to
our model for the high-mass star-forming region AFGL 2591. The ionization rate
for the source may be higher than previously expected -- either due to an
enhanced cosmic-ray ionization rate, or, more probably, to the presence of
X-ray induced ionization from the center. A significant fraction of the CO is
found to desorb in the temperature range of 15-40 K; below this temperature,
\~90% or more of the CO is frozen out. The inability of the model to explain
the HCS+, C2H, and OCS abundances suggests the importance of further laboratory
studies of basic reaction rates. Finally, predictions of the abundances and
spatial distributions of other species which could be observed by future
facilities (e.g., Herschel-HIFI, SOFIA, millimeter arrays) are provided.Comment: 15 pages, 11 Figures, accepted for publication by A&
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