5,625 research outputs found
Dissolving four-manifolds and positive scalar curvature
We prove that many simply connected symplectic four-manifolds dissolve after
connected sum with only one copy of .
For any finite group G that acts freely on the three-sphere we construct
closed smooth four-manifolds with fundamental group G which do not admit
metrics of positive scalar curvature, but whose universal covers do admit such
metrics.Comment: 13 pages; to appear in Mathematische Zeitschrif
Cycles, submanifolds, and structures on normal bundles
We give explicit examples of degree 3 cohomology classes not Poincare dual to
submanifolds, and discuss the realisability of homology classes by submanifolds
with Spin-C normal bundles.Comment: Several changes including an improvement of Theorem 1, our new
examples have torsion-free homolog
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
Recommended from our members
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
Comment on "Why is the DNA denaturation transition first order?"
In this comment we argue that while the conclusions in the original paper (Y.
Kafri, D. Mukamel and L. Peliti, Phys. Rev. Lett. 85, 4988 (2000)) are correct
for asymptotically long DNA chains, they do not apply to the chains used in
typical experiments. In the added last paragraph, we point out that for real
DNA the average distance between denatured loops is not of the order of the
persistence length of a single-stranded chain but much larger. This
corroborates our reasoning that the double helix between loops is quite rigid,
and thereby our conclusion.Comment: 1 page, REVTeX. Last paragraph adde
THE TIGHT-BINDING APPROACH TO THE DIELECTRIC RESPONSE IN THE MULTIBAND SYSTEMS
Starting from the random phase approximation for the weakly coupled multiband
tightly-bounded electron systems, we calculate the dielectric matrix in terms
of intraband and interband transitions. The advantages of this representation
with respect to the usual plane-wave decomposition are pointed out. The
analysis becomes particularly transparent in the long wavelength limit, after
performing the multipole expansion of bare Coulomb matrix elements. For
illustration, the collective modes and the macroscopic dielectric function for
a general cubic lattice are derived. It is shown that the dielectric
instability in conducting narrow band systems proceeds by a common softening of
one transverse and one longitudinal mode. Furthermore, the self-polarization
corrections which appear in the macroscopic dielectric function for finite band
systems, are identified as a combined effect of intra-atomic exchange
interactions between electrons sitting in different orbitals and a finite
inter-atomic tunneling.Comment: 20 pages, LaTeX, no figure
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
Binding of Holes to Magnetic Impurities in a Strongly Correlated System
The effect of a magnetic (S=1/2) impurity coupled to a 2D system of
correlated electrons (described by the t--J model) is studied by exact
diagonalisations. It is found that, if the exchange coupling of the impurity
with the neighboring spins is ferromagnetic or weakly antiferromagnetic, an
extra hole can form bound states of different spatial symmetries with the
impurity extending to a few lattice spacings. The binding energy is maximum
when the impurity is completely decoupled (vacancy) and vanishes for an
antiferromagnetic coupling exceeding . Several peaks appear in the
single hole spectral function below the lower edge of the quasiparticle band as
signatures of the d-, s- and p-wave boundstates.Comment: Latex 11 pages, postscript files in uuencoded form, report#
LPQTH-94/
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