674 research outputs found
Conformal dimension and random groups
We give a lower and an upper bound for the conformal dimension of the
boundaries of certain small cancellation groups. We apply these bounds to the
few relator and density models for random groups. This gives generic bounds of
the following form, where is the relator length, going to infinity.
(a) 1 + 1/C < \Cdim(\bdry G) < C l / \log(l), for the few relator model,
and
(b) 1 + l / (C\log(l)) < \Cdim(\bdry G) < C l, for the density model, at
densities .
In particular, for the density model at densities , as the relator
length goes to infinity, the random groups will pass through infinitely
many different quasi-isometry classes.Comment: 32 pages, 4 figures. v2: Final version. Main result improved to
density < 1/16. Many minor improvements. To appear in GAF
Photometric stability analysis of the Exoplanet Characterisation Observatory
Photometric stability is a key requirement for time-resolved spectroscopic
observations of transiting extrasolar planets. In the context of the Exoplanet
Characterisation Observatory (EChO) mission design, we here present and
investigate means of translating spacecraft pointing instabilities as well as
temperature fluctuation of its optical chain into an overall error budget of
the exoplanetary spectrum to be retrieved. Given the instrument specifications
as of date, we investigate the magnitudes of these photometric instabilities in
the context of simulated observations of the exoplanet HD189733b secondary
eclipse.Comment: submitted to MNRA
Asymmetry of bipartite quantum discord
It is known from the analysis of the density matrix for bipartite systems
that the quantum discord (as a measure of quantum correlations) depends on the
particular subsystem chosen for the projective measurements. We study asymmetry
of the discord in a simple physical model of two spin-1/2 particles with the
dipole-dipole interaction governed by the XY Hamiltonian in the inhomogeneous
magnetic field. The dependence of the above discord asymmetry on the Larmour
frequencies at both T=0 (the ground state) and has been investigated. It
is demonstrated, in particular, that the asymmetry is negligible for high
temperatures but it may become significant with the decrease in temperature.Comment: 5 pages 3 figure
Quantum Dissension: Generalizing Quantum Discord for Three-Qubit States
We introduce the notion of quantum dissension for a three-qubit system as a
measure of quantum correlations. We use three equivalent expressions of
three-variable mutual information. Their differences can be zero classically
but not so in quantum domain. It generalizes the notion of quantum discord to a
multipartite system. There can be multiple definitions of the dissension
depending on the nature of projective measurements done on the subsystems. As
an illustration, we explore the consequences of these multiple definitions and
compare them for three-qubit pure and mixed GHZ and W states. We find that
unlike discord, dissension can be negative. This is because measurement on a
subsystem may enhance the correlations in the rest of the system. This approach
can pave a way to generalize the notion of quantum correlations in the
multiparticle setting.Comment: 9 pages 6 figures typo fixed and some arguments adde
EChOSim: The Exoplanet Characterisation Observatory software simulator
EChOSim is the end-to-end time-domain simulator of the Exoplanet
Characterisation Observatory (EChO) space mission. EChOSim has been developed
to assess the capability EChO has to detect and characterize the atmospheres of
transiting exoplanets, and through this revolutionize the knowledge we have of
the Milky Way and of our place in the Galaxy. Here we discuss the details of
the EChOSim implementation and describe the models used to represent the
instrument and to simulate the detection. Software simulators have assumed a
central role in the design of new instrumentation and in assessing the level of
systematics affecting the measurements of existing experiments. Thanks to its
high modularity, EChOSim can simulate basic aspects of several existing and
proposed spectrometers for exoplanet transits, including instruments on the
Hubble Space Telescope and Spitzer, or ground-based and balloon borne
experiments. A discussion of different uses of EChOSim is given, including
examples of simulations performed to assess the EChO mission
Magnetic excitations in a new anisotropic Kagom\'{e} antiferromagnet
The Nd-langasite compound contains planes of magnetic Nd3+ ions on a lattice
topologically equivalent to a kagom\'{e} net. The magnetic susceptibility does
not reveal any signature of long-range ordering down to 2 K but rather a
correlated paramagnetism with significant antiferromagnetic interactions
between the Nd and a single-ion anisotropy due to crystal field effect.
Inelastic neutron scattering on Nd-langasite powder and single-crystal allowed
to probe its very peculiar low temperature dynamical magnetic correlations.
They present unusual dispersive features and are broadly localized in
wave-vector Q revealing a structure factor associated to characteristics short
range-correlations between the magnetic atoms. From comparison with theoretical
calculations, these results are interpreted as a possible experimental
observation of a spin liquid state in an anisotropic kagom\'{e}
antiferromagnet.Comment: to appear in Physica
Параметрична логістична модель планування роботи різних видів транспорту
Запропонована логістична модель планування роботи різних видів транспорту вантажної станції за умов ефективного використання вантажно-розвантажувальних механізмів, вантажних фронтів та складської обробки вантажів.Предложенная логистическая модель планирования работы различных видов транспорта грузовой станции в условиях эффективного использования погрузочно-разгрузочных механизмов, грузовых фронтов и складской обработки грузов.The proposed model of logistic planning of transport modes cargo station for the effective use of material handling equipment, commercial fronts and warehouse handling
Order by Disorder and Energetic Selection of the Ground State in the XY Pyrochlore Antiferromagnet Er2Ti2O7. An Inelastic Neutron Scattering Study
Examples of materials where an "order by disorder" mechanism is at play to
select a particular ground state are scarce. It has recently been proposed,
however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most
convincing case of this mechanism. Observation of a spin gap at zone centers
has recently been interpreted as a corroboration of this physics. In this
paper, we argue, however, that the anisotropy generated by the
interaction-induced admixing between the crystal-field ground and excited
levels provides for an alternative mechanism. It especially predicts the
opening of a spin gap of about 15 micro-eV, which is of the same order of
magnitude as the one observed experimentally. We report new high resolution
inelastic neutron scattering data which can be well understood within this
scenario.Comment: This version corrects a mistake on the numerical values of the
exchange couplings in relation to the published versio
- …