5,645 research outputs found
Coarse topology, enlargeability, and essentialness
Using methods from coarse topology we show that fundamental classes of closed
enlargeable manifolds map non-trivially both to the rational homology of their
fundamental groups and to the K-theory of the corresponding reduced
C*-algebras. Our proofs do not depend on the Baum--Connes conjecture and
provide independent confirmation for specific predictions derived from this
conjecture.Comment: 21 pages, 2 figures. Revised version. To appear in Ann. Sci. Ecole
Norm. Su
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
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Expression of natural killer receptor alleles at different Ly49 loci occurs independently and is regulated by major histocompatibility complex class I molecules.
Ly49 receptor genes are expressed by subsets of natural killer (NK) cells in an overlapping fashion, accounting for the capacity of NK subsets to attack host cells that have selectively downregulated self-major histocompatibility complex (MHC) class I molecules. It was shown previously that most NK cells express only one or the other allele of a given Ly49 gene, while a smaller population expresses both alleles. However, the methods used to detect monoallelic and biallelic cells were nonquantitative. Here, new allele-specific antibodies were used to provide the first quantitative examination of biallelic and monoallelic expression of Ly49A and Ly49G2. The results demonstrate conclusively that most Ly49A(+) and Ly49G2(+) NK cells express the corresponding gene in a monoallelic fashion, with a smaller subset expressing both alleles. Unexpectedly, biallelic Ly49A(+) NK cells were more numerous than predicted by completely independent allelic expression, suggesting some heterogeneity among NK progenitors in the potential to express a given Ly49 gene. The data also show that cells expressing one allele of Ly49G2 may express Ly49A from the same or opposite chromosome with equal likelihood, indicating that the expressed allele is chosen independently for different Ly49 genes. Finally, the data demonstrate that biallelic expression of Ly49A or Ly49G2 occurs least frequently in mice that express ligands for these receptors (H-2(d) mice), and most frequently in class I-deficient mice. Thus, biallelic expression of Ly49 genes is regulated by interactions of NK cell progenitors with MHC class I molecules
Transverse Emittance Measurement with the Three-Monitor-Method at the CERN Linac4
This report evaluates the applicability of the Three-Monitor-Method to determine the transverse emittance of the CERN Linac4 160 MeV H- -beam. The Three-Monitor-Method is a linear formalism allowing to calculate transverse emittance values from beam size measurements at three different positions along a beam line, assuming that the transfer matrix elements between these locations are known. It is planned to build two of these measurement systems, which should operate from 2013/14 immediately behind the exit of the linear accelerator in the dump line and close to the end of the transfer line to the PS Booster synchrotron in the LBE line. At first, the mathematical formalism and the simulation tools are briefly introduced. Then, the method is applied for both measurement lines. Results on measurement precisions and systematic errors are presented. Final conclusions are drawn at the end, and a summary of the equipment to be installed or modified will be given
Dynamical Properties of Two Coupled Hubbard Chains at Half-filling
Using grand canonical Quantum Monte Carlo (QMC) simulations combined with
Maximum Entropy analytic continuation, as well as analytical methods, we
examine the one- and two-particle dynamical properties of the Hubbard model on
two coupled chains at half-filling. The one-particle spectral weight function,
, undergoes a qualitative change with interchain hopping
associated with a transition from a four-band insulator to a two-band
insulator. A simple analytical model based on the propagation of exact rung
singlet states gives a good description of the features at large . For
smaller , is similar to that of the
one-dimensional model, with a coherent band of width the effective
antiferromagnetic exchange reasonably well-described by renormalized
spin-wave theory. The coherent band rides on a broad background of width
several times the parallel hopping integral , an incoherent structure
similar to that found in calculations on both the one- and two-dimensional
models. We also present QMC results for the two-particle spin and charge
excitation spectra, and relate their behavior to the rung singlet picture for
large and to the results of spin-wave theory for small .Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised
version with isotropic t_perp=t data include
Change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
One of the most remarkable properties of the high-temperature superconductors
is a pseudogap regime appearing in the underdoped cuprates above the
superconducting transition temperature T_c. The pseudogap continously develops
out of the superconducting gap. In this paper, we demonstrate by means of a
detailed comparison between theory and experiment that the characteristic
change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
can be understood as being due to phase fluctuations of the superconducting
order parameter. In particular, we show that within a phase fluctuation model
the characteristic back-turning BCS bands disappear above T_c whereas the gap
remains open. Furthermore, the pseudogap rather has a U-shape instead of the
characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts
closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at
the anti-nodal k=(pi,0) regions, yielding further support to the phase
fluctuation scenario.Comment: 6 pages, 4 eps-figure
Finding the optimum activation energy in DNA breathing dynamics: A Simulated Annealing approach
We demonstrate how the stochastic global optimization scheme of Simulated
Annealing can be used to evaluate optimum parameters in the problem of DNA
breathing dynamics. The breathing dynamics is followed in accordance with the
stochastic Gillespie scheme with the denaturation zones in double stranded DNA
studied as a single molecule time series. Simulated Annealing is used to find
the optimum value of the activation energy for which the equilibrium bubble
size distribution matches with a given value. It is demonstrated that the
method overcomes even large noise in the input surrogate data.Comment: 9 pages, 4 figures, iop article package include
Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy
Hexagonal boron nitride (h-BN) is a layered two-dimensional material with
properties that make it promising as a dielectric in various applications. We
report the growth of h-BN films on Ni foils from elemental B and N using
molecular beam epitaxy. The presence of crystalline h-BN over the entire
substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used
to examine the morphology and continuity of the synthesized films. A scanning
electron microscopy study of films obtained using shorter depositions offers
insight into the nucleation and growth behavior of h-BN on the Ni substrate.
The morphology of h-BN was found to evolve from dendritic, star-shaped islands
to larger, smooth triangular ones with increasing growth temperature
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