3,128 research outputs found
Cohomology of the space of commuting n-tuples in a compact Lie group
Consider the space Hom(Z^n,G) of pairwise commuting n-tuples of elements in a
compact Lie group G. This forms a real algebraic variety, which is generally
singular. In this paper, we construct a desingularization of the generic
component of Hom(Z^n,G), which allows us to derive formulas for its ordinary
and equivariant cohomology in terms of the Lie algebra of a maximal torus in G
and the action of the Weyl group. This is an application of a general theorem
concerning G-spaces for which every element is fixed by a maximal torus.Comment: 11 pages Changes made: Implemented referee recommendations, in
particular to use the Vietoris mapping theorem to generalize results and
simplify argument
Top-Contact Lateral Organic Photodetectors for Deep Ultraviolet Applications
Organic semiconductors are very attractive for thin film Organic Photodetectors (OPDs) since they possess a number of desirable attributes for optical sensing including high absorption coefficients over visible and ultraviolet wavelengths and compatibility with large-area deposition processes such as ink-jet, screen printing, and solution processing.
OPDs, in general, utilize a vertical device architecture where the photoactive organic semiconductor layers are sandwiched between top and bottom electrodes that provide electrical contact. More recently, an interest in utilizing a lateral device architecture instead of the vertical one, has emerged. In this architecture, the two contacts are positioned on the two sides of the photoactive material with respect to the direction of the incoming signal, separated by a small gap. However, the factors governing lateral OPDs’ photo-response are still not well understood.
In this thesis, we fabricate top-contact lateral OPDs using a thermal evaporation only fabrication process. We study the factors governing both the dark and photo currents of lateral OPDs. The effect of the wide gap between the two electrodes on the current-voltage characteristics is discussed and the role of space charge limited conduction is investigated. The contributions in the photoresponse of light scattering through the active layers as well as the back reflection of light at the metallic contacts are emphasized.
The reproducibility over repeated operation cycles of both dark and photo currents values is explored. Exposure to light of the lateral OPD is found to lead to a significant increase in the dark current. The role of the conductivity enhancement in the channel due to light-induced trap filling is investigated.
External quantum efficiency and detectivity estimates are given for deep ultraviolet lateral (DUV) OPDs. A comparison with vertical DUV OPDs performances is provided.
Finally, the use of a phosphorescent sensitizer doped in the absorbing bottom layer to improve top-contact lateral OPDs efficiency is discussed
Isohemagglutinins of Graft Origin after ABO-Unmatched Liver Transplantation
THE increasing success of liver transplantation in recent years has provided an experimental model to study and document the hepatic synthesis of many plasma proteins.12345 The normal hepatobiliary tract has not been regarded as a major source of antibody,6,7 aside from the enteric IgA secreted from plasma into the biliary tree.8 Liver transplantation affords the opportunity to study the production of antibody to red cells. Recipient ABO incompatibility to the donor (a mismatched transplant, e.g., a group A liver transplanted into a group B recipient), although not absolutely contraindicated in liver transplantation, is avoided when possible. However, ABO-unmatched transplants (defined. © 1984, Massachusetts Medical Society. All rights reserved
Global Spinors and Orientable Five-Branes
Fermion fields on an M-theory five-brane carry a representation of the double
cover of the structure group of the normal bundle. It is shown that, on an
arbitrary oriented Lorentzian six-manifold, there is always an Sp(2) twist that
allows such spinors to be defined globally. The vanishing of the arising
potential obstructions does not depend on spin structure in the bulk, nor does
the six-manifold need to be spin or spin-C. Lifting the tangent bundle to such
a generalised spin bundle requires picking a generalised spin structure in
terms of certain elements in the integral and modulo-two cohomology of the
five-brane world-volume in degrees four and five, respectively.Comment: 18 pages, LaTeX; v2: version to appear in JHE
Isometry groups of non-positively curved spaces: discrete subgroups
We study lattices in non-positively curved metric spaces. Borel density is
established in that setting as well as a form of Mostow rigidity. A converse to
the flat torus theorem is provided. Geometric arithmeticity results are
obtained after a detour through superrigidity and arithmeticity of abstract
lattices. Residual finiteness of lattices is also studied. Riemannian symmetric
spaces are characterised amongst CAT(0) spaces admitting lattices in terms of
the existence of parabolic isometries.Comment: This is the second of a pair of two articles, originally posted on
September 2, 2008 as arXiv:0809.0457v1 For the first part, see
arXiv:0809.0457v
Special metrics and Triality
We investigate a new 8-dimensional Riemannian geometry defined by a generic
closed and coclosed 3-form with stabiliser PSU(3), and which arises as a
critical point of Hitchin's variational principle. We give a Riemannian
characterisation of this structure in terms of invariant spinor-valued 1-forms,
which are harmonic with respect to the twisted Dirac operator \Db on
. We establish various obstructions to the existence of
topological reductions to PSU(3). For compact manifolds, we also give
sufficient conditions for topological PSU(3)-structures that can be lifted to
topological SU(3)-structures. We also construct the first known compact example
of an integrable non-symmetric PSU(3)-structure. In the same vein, we give a
new Riemannian characterisation for topological quaternionic K\"ahler
structures which are defined by an -invariant self-dual
4-form. Again, we show that this form is closed if and only if the
corresponding spinor-valued 1-form is harmonic for \Db and that these
equivalent conditions produce constraints on the Ricci tensor.Comment: 41 pages. v2: typos corrected, presentation improved v3: final
versio
Grating-assisted superresolution of slow waves in Fourier space
We present a far-field optical technique allowing measurements of the dispersion relation of electromagnetic fields propagating under the light cone in photonic nanostructures. It relies on the use of a one-dimensional grating to probe the evanescent tail of the guided field in combination with a high-numerical-aperture Fourier- space imaging setup. A high-resolution spectroscopy of the far-field emission diagram allows us to accurately and efficiently determine the dispersion curve and the group-index dispersion of planar photonic crystal waveguides operating in the slow-light regime
Indications for liver transplantation in the cyclosporine era
One hundred seventy orthotopic liver transplants were performed under conventional immunosuppression with azathioprine and steroids with 1- and 5-year survivals of 32.9% and 20.0%, respectively. Since the introduction of cyclosporine-prednisone therapy in March 1980, 313 primary orthotopic liver transplants have been performed. Actuarial survivals at 1 and 5 years have improved to 69.7% and 62.8%, respectively. Biliary atresia is now the most common indication for liver replacement. In adults, primary biliary cirrhosis and sclerosing cholangitis have become more common indications for transplantation, and alcoholic cirrhosis and primary liver malignancy as indications have declined. Early enthusiasm for liver transplantation in patients with hepatic cancer has been tempered by the finding that recurrence is both common and rapid. An increasing number of patients with inborn errors of metabolism originating in the liver are receiving transplants, including patients with Wilson's disease, tyrosinemia, alpha-1-antitrypsin deficiency, glycogen storage disease, familial hypercholesterolemia, and hemochromatosis. Survival in this group of patients has been excellent (74.4% at 1 and 5 years). A hemophiliac who received a transplant for postnecrotic cirrhosis has survived and may have been cured of his hemophilia. About 20% of patients require retransplantation for rejection, technical failure, or primary graft failure. Only 4 of the patients receiving retransplants under conventional immunosuppression survived beyond 6 months, and all died within 14 months of retransplantation. Sixty-eight patients have received retransplants under cyclosporine-prednisone. Thirty-one patients are surviving, all for at least 1 year. Six of the 12 patients requiring a third transplant are alive 2 to 3 years after the primary operation. An aggressive approach to retransplantation in the patient with a failed graft is justified
The Steenrod problem of realizing polynomial cohomology rings
In this paper we completely classify which graded polynomial R-algebras in
finitely many even degree variables can occur as the singular cohomology of a
space with coefficients in R, a 1960 question of N. E. Steenrod, for a
commutative ring R satisfying mild conditions. In the fundamental case R = Z,
our result states that the only polynomial cohomology rings over Z which can
occur, are tensor products of copies of H^*(CP^\infty;Z) = Z[x_2],
H^*(BSU(n);Z) = Z[x_4,x_6,...,x_{2n}], and H^*(BSp(n):Z) =
Z[x_4,x_8,...,x_{4n}] confirming an old conjecture. Our classification extends
Notbohm's solution for R = F_p, p odd. Odd degree generators, excluded above,
only occur if R is an F_2-algebra and in that case the recent classification of
2-compact groups by the authors can be used instead of the present paper. Our
proofs are short and rely on the general theory of p-compact groups, but not on
classification results for these.Comment: 14 pages. v3: Final version. To appear in Journal of Topolog
Studies of hepatic synthesis in vivo of plasma proteins, including orosomucoid, transferrin, α-antitrypsin, C8, and factor B
Serum protein types were determined in eight recipients and donors in cases of hepatic homotransplantation. A change from recipient type to donor type was observed for factor B, C8, orosomucoid, haptoglobin, transferrin, α1-antitrypsin, C3 and C6, but not for Gm and Inv immunoglobulin markers. The results indicate that all the proteins studied (except immunoglobulins) are produced primarily by the liver in vivo. © 1980
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