3,636 research outputs found
Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K'-band observations of five Mira stars with the IOTA
interferometer. The interferograms were obtained with the FLUOR fiber optics
beam combiner, which provides high-accuracy visibility measurements in spite of
time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU
Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters
11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively.
Simultaneous photometric observations yielded the bolometric fluxes. The
derived angular Rosseland radii and the bolometric fluxes allowed the
determination of effective temperatures. For instance, the effective
temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland
radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland
radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas.
The observations were compared with theoretical Mira star models of Bessel et
al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type
Miras and the linear radius of R Aql indicate fundamental mode pulsation.Comment: 12 pages, 4 postscript figure
Reduction of quantum noise in optical interferometers using squeezed light
We study the photon counting noise in optical interferometers used for
gravitational wave detection. In order to reduce quantum noise a squeezed
vacuum state is injected into the usually unused input port. Here, we
specifically investigate the so called `dark port case', when the beam splitter
is oriented close to 90{\deg} to the incoming laser beam, such that nearly all
photons go to one output port of the interferometer, and only a small fraction
of photons is seen in the other port (`dark port'). For this case it had been
suggested that signal amplification is possible without concurrent noise
amplification [R.Barak and Y.Ben-Aryeh, J.Opt.Soc.Am.B25(361)2008]. We show
that by injection of a squeezed vacuum state into the second input port,
counting noise is reduced for large values of the squeezing factor, however the
signal is not amplified. Signal strength only depends on the intensity of the
laser beam.Comment: 8 pages, 1 figur
Shear band dynamics from a mesoscopic modeling of plasticity
The ubiquitous appearance of regions of localized deformation (shear bands)
in different kinds of disordered materials under shear is studied in the
context of a mesoscopic model of plasticity. The model may or may not include
relaxational (aging) effects. In the absence of relaxational effects the model
displays a monotonously increasing dependence of stress on strain-rate, and
stationary shear bands do not occur. However, in start up experiments transient
(although long lived) shear bands occur, that widen without bound in time. I
investigate this transient effect in detail, reproducing and explaining a t^1/2
law for the thickness increase of the shear band that has been obtained in
atomistic numerical simulations. Relaxation produces a negative sloped region
in the stress vs. strain-rate curve that stabilizes the formation of shear
bands of a well defined width, which is a function of strain-rate. Simulations
at very low strain-rates reveal a non-trivial stick-slip dynamics of very thin
shear bands that has relevance in the study of seismic phenomena. In addition,
other non-stationary processes, such as stop-and-go, or strain-rate inversion
situations display a phenomenology that matches very well the results of recent
experimental studies.Comment: 10 pages, 10 figure
Typical local measurements in generalised probabilistic theories: emergence of quantum bipartite correlations
What singles out quantum mechanics as the fundamental theory of Nature? Here
we study local measurements in generalised probabilistic theories (GPTs) and
investigate how observational limitations affect the production of
correlations. We find that if only a subset of typical local measurements can
be made then all the bipartite correlations produced in a GPT can be simulated
to a high degree of accuracy by quantum mechanics. Our result makes use of a
generalisation of Dvoretzky's theorem for GPTs. The tripartite correlations can
go beyond those exhibited by quantum mechanics, however.Comment: 5 pages, 1 figure v2: more details in the proof of the main resul
Many-Body Localization Implies that Eigenvectors are Matrix-Product States
The phenomenon of many-body localization has received a lot of attention
recently, both for its implications in condensed-matter physics of allowing
systems to be an insulator even at nonzero temperature as well as in the
context of the foundations of quantum statistical mechanics, providing
examples of systems showing the absence of thermalization following out-of-
equilibrium dynamics. In this work, we establish a novel link between
dynamical properties—a vanishing group velocity and the absence of
transport—with entanglement properties of individual eigenvectors. For systems
with a generic spectrum, we prove that strong dynamical localization implies
that all of its many-body eigenvectors have clustering correlations. The same
is true for parts of the spectrum, thus allowing for the existence of a
mobility edge above which transport is possible. In one dimension these
results directly imply an entanglement area law; hence, the eigenvectors can
be efficiently approximated by matrix-product states
Anomalous scaling due to correlations: Limit theorems and self-similar processes
We derive theorems which outline explicit mechanisms by which anomalous
scaling for the probability density function of the sum of many correlated
random variables asymptotically prevails. The results characterize general
anomalous scaling forms, justify their universal character, and specify
universality domains in the spaces of joint probability density functions of
the summand variables. These density functions are assumed to be invariant
under arbitrary permutations of their arguments. Examples from the theory of
critical phenomena are discussed. The novel notion of stability implied by the
limit theorems also allows us to define sequences of random variables whose sum
satisfies anomalous scaling for any finite number of summands. If regarded as
developing in time, the stochastic processes described by these variables are
non-Markovian generalizations of Gaussian processes with uncorrelated
increments, and provide, e.g., explicit realizations of a recently proposed
model of index evolution in finance.Comment: Through text revision. 15 pages, 3 figure
Systematic investigations of annealing and functionalization of carbon nanotube yarns
Carbon nanotube yarns (CNY) are a novel carbonaceous material and have received a great deal of interest since the beginning of the 21st century. CNY are of particular interest due to their useful heat conducting, electrical conducting, and mechanical properties. The electrical conductivity of carbon nanotube yarns can also be influenced by functionalization and annealing. A systematical study of this post synthetic treatment will assist in understanding what factors influences the conductivity of these materials. In this investigation, it is shown that the electrical conductivity can be increased by a factor of 2 and 5.5 through functionalization with acids and high temperature annealing respectively. The scale of the enhancement is dependent on the reducing of intertube space in case of functionalization. For annealing, not only is the highly graphitic structure of the carbon nanotubes (CNT) important, but it is also shown to influence the residual amorphous carbon in the structure. The promising results of this study can help to utilize CNY as a replacement for common materials in the field of electrical wiring
In-plane magnetic anisotropy of Fe atoms on BiSe(111)
The robustness of the gapless topological surface state hosted by a 3D
topological insulator against perturbations of magnetic origin has been the
focus of recent investigations. We present a comprehensive study of the
magnetic properties of Fe impurities on a prototypical 3D topological insulator
BiSe using local low temperature scanning tunneling microscopy and
integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the
BiSe surface, in the coverage range are heavily relaxed
into the surface and exhibit a magnetic easy axis within the surface-plane,
contrary to what was assumed in recent investigations on the opening of a gap.
Using \textit{ab initio} approaches, we demonstrate that an in-plane easy axis
arises from the combination of the crystal field and dynamic hybridization
effects.Comment: 5 pages, 3 figures, typos correcte
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