94,136 research outputs found
Diffusion induced decoherence of stored optical vortices
We study the coherence properties of optical vortices stored in atomic
ensembles. In the presence of thermal diffusion, the topological nature of
stored optical vortices is found not to guarantee slow decoherence. Instead the
stored vortex state has decoherence surprisingly larger than the stored
Gaussian mode. Generally, the less phase gradient, the more robust for stored
coherence against diffusion. Furthermore, calculation of coherence factor shows
that the center of stored vortex becomes completely incoherent once diffusion
begins and, when reading laser is applied, the optical intensity at the center
of the vortex becomes nonzero. Its implication for quantum information is
discussed. Comparison of classical diffusion and quantum diffusion is also
presented.Comment: 5 pages, 2 figure
Coherent Graphene Devices: Movable Mirrors, Buffers and Memories
We theoretically report that, at a sharp electrostatic step potential in
graphene, massless Dirac fermions can obtain Goos-H\"{a}nchen-like shifts under
total internal reflection. Based on these results, we study the coherent
propagation of the quasiparticles along a sharp graphene \emph{p-n-p} waveguide
and derive novel dispersion relations for the guided modes. Consequently,
coherent graphene devices (e.g. movable mirrors, buffers and memories) induced
only by the electric field effect can be proposed.Comment: 12 pages, 5 figure
To be or not to be? – An empirical study on dual-class share structure of US listed Chinese companies
Dual-class share structure (DCSS) is prohibited in Mainland China currently with the OSOV (one share one vote) principle clearly written in both the Chinese company law and listing rules . As a result, those American stock exchanges become attractive to Mainland Chinese companies due to their tolerance of takeover defences, in particular, DCSS. To seek the soft regulation with the issuance of multiple voting shares, dozens of Chinese companies chose the American stock exchanges as their IPO (initial public offering) venues. In this paper, empirical research will be conducted to analyse those US-listed Mainland Chinese companies with DCSS. This paper aims to assess those transnational companies, their characteristics and corporate performances.
As to the range of data to be covered, only those companies listed after 2011 will be sampled in this paper. It is because some of those data of the Mainland Chinese companies on NYSE or NASDAQ listed prior to 2011 were not available or were inaccurate; counting them leads to misleading results. Furthermore, those data is dated for such a changing area of law. In order to reflect the up-to-date status accurately, this paper focuses its empirical study of data which date back to 2011
Calibrating the {\alpha} parameter of convective efficiency using observed stellar properties
Context. Synthetic model atmosphere calculations are still the most commonly
used tool when determining precise stellar parameters and stellar chemical
compositions. Besides three-dimensional models that consistently solve for
hydrodynamic processes, one-dimensional models that use an approximation for
convective energy transport play the major role.
Aims. We use modern Balmer-line formation theory as well as spectral energy
distribution (SED) measurements for the Sun and Procyon to calibrate the model
parameter {\alpha} that describes the efficiency of convection in our 1D
models. Convection was calibrated over a significant range in parameter space,
reaching from F-K along the main sequence and sampling the turnoff and giant
branch over a wide range of metallicities. This calibration was compared to
theoretical evaluations and allowed an accurate modeling of stellar
atmospheres.
Methods. We used Balmer-line fitting and SED fits to determine the convective
efficiency parameter {\alpha}. Both methods are sensitive to the structure and
temperature stratification of the deeper photosphere.
Results. While SED fits do not allow a precise determination of the
convective parameter for the Sun and Procyon, they both favor values
significantly higher than 1.0. Balmer-line fitting, which we find to be more
sensitive, suggests that the convective efficiency parameter {\alpha} is
2.0 for the main sequence and quickly decreases to 1.0 for
evolved stars. These results are highly consistent with predictions from 3D
models. While the values on the main sequence fit predictions very well,
measurements suggest that the decrease of convective efficiency as stars evolve
to the giant branch is more dramatic than predicted by models.Comment: 14 pages, 16 figures, accepted for publication in A&
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