928 research outputs found
Isosbestic Points: Theory and Applications
We analyze the sharpness of crossing ("isosbestic") points of a family of
curves which are observed in many quantities described by a function f(x,p),
where x is a variable (e.g., the frequency) and p a parameter (e.g., the
temperature). We show that if a narrow crossing region is observed near x* for
a range of parameters p, then f(x,p) can be approximated by a perturbative
expression in p for a wide range of x. This allows us, e.g., to extract the
temperature dependence of several experimentally obtained quantities, such as
the Raman response of HgBa2CuO4+delta, photoemission spectra of thin VO2 films,
and the reflectivity of CaCu3Ti4O12, all of which exhibit narrow crossing
regions near certain frequencies. We also explain the sharpness of isosbestic
points in the optical conductivity of the Falicov-Kimball model and the
spectral function of the Hubbard model.Comment: 12 pages, 11 figure
Commissioning and Alignment of the ATLAS Inner Detector using Cosmic Data
The ATLAS experiment is one of the two general purpose detectors at the LHC at CERN. ATLAS is equipped with a charged particle tracking system built on three sub-detectors, which provide high precision measurements made from a high detector granularity. The pixel and microstrip sub-detectors, which use the silicon technology, are complemented with the transition radiation tracker. The ATLAS detector is operational since 2008 and more than ten million cosmic tracks crossing the Inner Detector have been collected in 2008 and 2009. These data are used for the commissioning of the experiment. The alignment of the Inner Detector tracking system was performed using the 2008 cosmic data. The tracking performance obtained using this alignment is approaching the one obtained using Monte Carlo simulated with perfectly aligned geometry. The effect of systematic misalignments on physics measurements is being studied
The moduli space of hypersurfaces whose singular locus has high dimension
Let be an algebraically closed field and let and be integers with
and Consider the moduli space of
hypersurfaces in of fixed degree whose singular locus is
at least -dimensional. We prove that for large , has a unique
irreducible component of maximal dimension, consisting of the hypersurfaces
singular along a linear -dimensional subspace of . The proof
will involve a probabilistic counting argument over finite fields.Comment: Final version, including the incorporation of all comments by the
refere
Non-perturbative approaches to magnetism in strongly correlated electron systems
The microscopic basis for the stability of itinerant ferromagnetism in
correlated electron systems is examined. To this end several routes to
ferromagnetism are explored, using both rigorous methods valid in arbitrary
spatial dimensions, as well as Quantum Monte Carlo investigations in the limit
of infinite dimensions (dynamical mean-field theory). In particular we discuss
the qualitative and quantitative importance of (i) the direct Heisenberg
exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a
high spectral density near the band edges caused by an appropriate lattice
structure and/or kinetic energy of the electrons. We furnish evidence of the
stability of itinerant ferromagnetism in the pure Hubbard model for appropriate
lattices at electronic densities not too close to half-filling and large enough
. Already a weak direct exchange interaction, as well as band degeneracy, is
found to reduce the critical value of above which ferromagnetism becomes
stable considerably. Using similar numerical techniques the Hubbard model with
an easy axis is studied to explain metamagnetism in strongly anisotropic
antiferromagnets from a unifying microscopic point of view.Comment: 11 pages, Latex, and 6 postscript figures; Z. Phys. B, in pres
Directly Indecomposables in Semidegenerate Varieties of Connected po-Groupoids
We study varieties with a term-definable poset structure, "po-groupoids". It
is known that connected posets have the "strict refinement property" (SRP). In
[arXiv:0808.1860v1 [math.LO]] it is proved that semidegenerate varieties with
the SRP have definable factor congruences and if the similarity type is finite,
directly indecomposables are axiomatizable by a set of first-order sentences.
We obtain such a set for semidegenerate varieties of connected po-groupoids and
show its quantifier complexity is bounded in general
Fundamental groups of open K3 surfaces, Enriques surfaces and Fano 3-folds
We investigate when the fundamental group of the smooth part of a K3 surface
or Enriques surface with Du Val singularities, is finite. As a corollary we
give an effective upper bound for the order of the fundamental group of the
smooth part of a certain Fano 3-fold. This result supports Conjecture A below,
while Conjecture A (or alternatively the rational connectedness conjecture in
[KoMiMo] which is still open when the dimension is at least 4) would imply that
every log terminal Fano variety has a finite fundamental group (now a Theorem
of S. Takayama).Comment: Journal of Pure and Applied Algebra, to appear; 24 page
Tone-activated, remote, alert communication system
Pocket sized transmitter, frequency modulated by crystal derived tones, with integral loop antenna provides police with easy operating alert signal communicator which uses patrol car radio to relay signal. Communication channels are time shared by several patrol units
Commissioning and Alignment of the ATLAS Inner Detector Using Cosmic Data
The ATLAS experiment is one of the two general purpose detectors at the Large Hadron Collider. ATLAS is equipped with a charged particle tracking system built on three subdetectors, which provide high precision measurements made from a fine detector granularity. The pixel and microstrip subdetectors, which use the silicon technology, are complemented with the transition radiation tracker. The ATLAS detector is operational since 2008 and more than ten million cosmic tracks have been collected in 2008 and 2009. These data are used for the commissioning of the experiment. The current status of the Pixel, SCT and TRT detectors will be reviewed. We will report on the commissioning of the detector, including overviews on services, connectivity and observed problems. Alignment constants are calculated and the detector is calibrated. The required precision for the alignment of the most sensitive coordinates of the silicon sensors is just a few microns. The outline of the alignment algorithm and its implementation of the alignment software, its framework and the data flow will be discussed. The calibration of the silicon subdetectors, like the determination of the lorentz angle, is presented. The overall performance of the charged particle tracking system is studied
Telescopic actions
A group action H on X is called "telescopic" if for any finitely presented
group G, there exists a subgroup H' in H such that G is isomorphic to the
fundamental group of X/H'.
We construct examples of telescopic actions on some CAT[-1] spaces, in
particular on 3 and 4-dimensional hyperbolic spaces. As applications we give
new proofs of the following statements:
(1) Aitchison's theorem: Every finitely presented group G can appear as the
fundamental group of M/J, where M is a compact 3-manifold and J is an
involution which has only isolated fixed points;
(2) Taubes' theorem: Every finitely presented group G can appear as the
fundamental group of a compact complex 3-manifold.Comment: +higher dimension
Hopping on the Bethe lattice: Exact results for densities of states and dynamical mean-field theory
We derive an operator identity which relates tight-binding Hamiltonians with
arbitrary hopping on the Bethe lattice to the Hamiltonian with nearest-neighbor
hopping. This provides an exact expression for the density of states (DOS) of a
non-interacting quantum-mechanical particle for any hopping. We present
analytic results for the DOS corresponding to hopping between nearest and
next-nearest neighbors, and also for exponentially decreasing hopping
amplitudes. Conversely it is possible to construct a hopping Hamiltonian on the
Bethe lattice for any given DOS. These methods are based only on the so-called
distance regularity of the infinite Bethe lattice, and not on the absence of
loops. Results are also obtained for the triangular Husimi cactus, a recursive
lattice with loops. Furthermore we derive the exact self-consistency equations
arising in the context of dynamical mean-field theory, which serve as a
starting point for studies of Hubbard-type models with frustration.Comment: 14 pages, 9 figures; introduction expanded, references added;
published versio
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