1,207 research outputs found
Broadening of hot-spot response spectrum of superconducting NbN nanowire single-photon detector with reduced nitrogen content
The spectral detection efficiency and the dark count rate of superconducting
nanowire single-photon detectors (SNSPD) has been studied systematically on
detectors made from thin NbN films with different chemical compositions.
Reduction of the nitrogen content in the 4 nm thick NbN films results in a more
than two orders of magnitude decrease of the dark count rates and in a red
shift of the cut-off wavelength of the hot-spot SNSPD response. The observed
phenomena are explained by an improvement of uniformity of NbN films that has
been confirmed by a decrease of resistivity and an increase of the ratio of the
measured critical current to the depairing current. The latter factor is
considered as the most crucial for both the cut-off wavelength and the dark
count rates of SNSPD. Based on our results we propose a set of criteria for
material properties to optimize SNSPD in the infrared spectral region.Comment: 15 pages, 6 figure
A Berger type normal holonomy theorem for complex submanifolds
We prove a kind of Berger-Simons' Theorem for the normal holonomy group of a complex submanifold of the projective spac
Lp-cohomology of negatively curved manifolds
We compute the -cohomology spaces of some negatively curved manifolds.
We deal with two cases: manifolds with finite volume and sufficiently pinched
negative curvature, and conformally compact manifolds
Contracting automorphisms and L^p-cohomology in degree one
We characterize those Lie groups, and algebraic groups over a local field of
characteristic zero, whose first reduced L^p-cohomology is zero for all p>1,
extending a result of Pansu. As an application, we obtain a description of
Gromov-hyperbolic groups among those groups. In particular we prove that any
non-elementary Gromov-hyperbolic algebraic group over a non-Archimedean local
field of zero characteristic is quasi-isometric to a 3-regular tree. We also
extend the study to semidirect products of a general locally compact group by a
cyclic group acting by contracting automorphisms.Comment: 27 pages, no figur
Invariant four-forms and symmetric pairs
We give criteria for real, complex and quaternionic representations to define
s-representations, focusing on exceptional Lie algebras defined by spin
representations. As applications, we obtain the classification of complex
representations whose second exterior power is irreducible or has an
irreducible summand of co-dimension one, and we give a conceptual
computation-free argument for the construction of the exceptional Lie algebras
of compact type.Comment: 16 pages [v2: references added, last section expanded
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[no abstract available
Volume Comparison for Hypersurfaces in Lorentzian Manifolds and Singularity Theorems
We develop area and volume comparison theorems for the evolution of
spacelike, acausal, causally complete hypersurfaces in Lorentzian manifolds,
where one has a lower bound on the Ricci tensor along timelike curves, and an
upper bound on the mean curvature of the hypersurface. Using these results, we
give a new proof of Hawking's singularity theorem.Comment: 15 pages, LaTe
Polar foliations and isoparametric maps
A singular Riemannian foliation on a complete Riemannian manifold is
called a polar foliation if, for each regular point , there is an immersed
submanifold , called section, that passes through and that meets
all the leaves and always perpendicularly. A typical example of a polar
foliation is the partition of into the orbits of a polar action, i.e., an
isometric action with sections. In this work we prove that the leaves of
coincide with the level sets of a smooth map if is simply
connected. In particular, we have that the orbits of a polar action on a simply
connected space are level sets of an isoparametric map. This result extends
previous results due to the author and Gorodski, Heintze, Liu and Olmos, Carter
and West, and Terng.Comment: 9 pages; The final publication is available at springerlink.com
http://www.springerlink.com/content/c72g4q5350g513n1
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