3,049 research outputs found
On Time-Bandwidth Product of Multi-Soliton Pulses
Multi-soliton pulses are potential candidates for fiber optical transmission
where the information is modulated and recovered in the so-called nonlinear
Fourier domain. While this is an elegant technique to account for the channel
nonlinearity, the obtained spectral efficiency, so far, is not competitive with
the classic Nyquist-based schemes. In this paper, we study the evolution of the
time-bandwidth product of multi-solitons as they propagate along the optical
fiber. For second and third order soliton pulses, we numerically optimize the
pulse shapes to achieve the smallest time-bandwidth product when the phase of
the spectral amplitudes is used for modulation. Moreover, we analytically
estimate the pulse-duration and bandwidth of multi-solitons in some practically
important cases. Those estimations enable us to approximate the time-bandwidth
product for higher order solitons.Comment: Accepted for ISIT 201
Plasmons in Two-Dimensional Topological Insulators
We analyze collective excitations in models of two-dimensional topological
insulators using the random phase approximation. In a two-dimensional extension
of the Su-Schrieffer-Heeger model, edge plasmonic excitations with induced
charge-density distributions localized at the boundaries of the system are
found in the topologically non-trivial phase, dispersing similarly as
one-dimensional bulk plasmons in the conventional Su-Schrieffer-Heeger chain.
For two-dimensional bulk collective modes, we reveal regimes of enhanced
inter-band wave function correlations, leading to characteristic hardening and
softening of inter- and intra-band bulk plasmonic branches, respectively. In
the two-dimensional Haldane Chern insulator model, chiral, uni-directional edge
plasmons in nano-ribbon architectures are observed, which can be characterized
by an effective Coulomb interaction cross section. Bulk collective excitations
in the two-dimensional Haldane model are shown to be originated by
single-particle band structure details in different topological phases
Control of Plasmons in Doped Topological Insulators via Basis Atoms
Collective excitations in topologically non-trivial systems have attracted
considerable attention in recent years. Here we study plasmons in the
Su-Schrieffer-Heeger model whose low-energy electronic band is only partially
filled, such that the system is metallic. Using the random phase approximation,
we calculate the intra- and inter-band polarization functions and determine the
bulk plasmonic dispersion from the dielectric function within the random phase
approximation. We find that the sub-lattice basis states strongly affect the
polarization functions and therefore control the system's plasmonic
excitations. By varying the real-space separation of these local orbitals, one
can thus selectively enhance or suppress the plasmonic energies via a tunable
trade-off between intra-band and inter-band screening processes. Specifically,
this mechanism can be used to stabilize undamped high energy plasmons that have
already been reported in related models. We propose scenarios on how to control
and observe these effects in experiments
Question Order Experiments in the German-European Context
In this paper, we investigate the context stability of questions on political issues in cross-national surveys. For this purpose, we conducted three replication studies (N1 = 213; N2 = 677; N3 = 1,489) based on eight split-ballot design experiments with undergraduate and graduate students to test for question order effects. The questions, which were taken from the Eurobarometer (2013), included questions on perceived performance and identification. Respondents were randomly assigned to one of two experimental groups which received the questions either in the original or the reversed order. In all three studies, respondents answered the questions about Germany and the European Union/Europe differently depending on whether the question was asked first or second in the question sequence. Specifically, when answering a subsequent question in a question sequence, the preceding question seems to have functioned as a standard of comparison. Our empirical findings also suggest that the likelihood of the occurrence of such context effects can be reduced by implementing informed questionnaire design strategies
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