5,296 research outputs found
Superconducting cosmic strings as sources of cosmological fast radio bursts
In this paper we calculate the radio burst signals from three kinds of
structures of superconducting cosmic strings. By taking into account the
observational factors including scattering and relativistic effects, we derive
the event rate of radio bursts as a function of redshift with the theoretical
parameters and of superconducting strings. Our analyses
show that cusps and kinks may have noticeable contributions to the event rate
and in most cases cusps would dominate the contribution, while the kink-kink
collisions tend to have secondary effects. By fitting theoretical predictions
with the normalized data of fast radio bursts, we for the first time constrain
the parameter space of superconducting strings and report that the parameter
space of and fit the observation well although the statistic
significance is low due to the lack of observational data. Moreover, we derive
two types of best fittings, with one being dominated by cusps with a redshift
, and the other dominated by kinks at the range of the maximal event
rate.Comment: 13 pages, 2 figures, 1 table; references update
Approximate Capacities of Two-Dimensional Codes by Spatial Mixing
We apply several state-of-the-art techniques developed in recent advances of
counting algorithms and statistical physics to study the spatial mixing
property of the two-dimensional codes arising from local hard (independent set)
constraints, including: hard-square, hard-hexagon, read/write isolated memory
(RWIM), and non-attacking kings (NAK). For these constraints, the strong
spatial mixing would imply the existence of polynomial-time approximation
scheme (PTAS) for computing the capacity. It was previously known for the
hard-square constraint the existence of strong spatial mixing and PTAS. We show
the existence of strong spatial mixing for hard-hexagon and RWIM constraints by
establishing the strong spatial mixing along self-avoiding walks, and
consequently we give PTAS for computing the capacities of these codes. We also
show that for the NAK constraint, the strong spatial mixing does not hold along
self-avoiding walks
Vectorized Circuit
In this report, a newly modified Newton algorithm (MNA) and a data structure for sparse matrix manipulation are presented for analyzing large-scale electronic circuits on the Cyber-205 supercomputer. The MNA is improved from the Multilevel Newton Algorithm (ML NA) developed by Rabbat Sanjiovanni-Vincentelli, and Hsieh (1979). The time complexity and convergence rate of MNA are analyzed. The computation steps are shown in detail by some example circuits. Scalar and vectorized simulation programs have been tested run on a VAX 11/780 Scalar machine and on the Cyber 205 vector processor at Purdue University. From the results obtained, we observe that the MNA results a speedup of about 100 on the Cyber-205 as compared with using a scalar computer to analyze an electronic circuit containing 500 identical subcircuits
Local fluctuations of vibrational polaritons monitored by two-dimensional infrared spectroscopy
We study the collective behavior of molecules placed in an infrared (IR)
microcavity, incorporating the local fluctuations, i.e., dynamical disorder.
The cooperative feature in vibrational polaritons is shown to be dynamically
eroded, due to intermolecule coherence. To further resolve such process, we
develop a two-dimensional infrared spectroscopy (2D-IR) for molecules
interacting with cavity modes. The cooperative feature in correspondence to the
spectroscopic signal is specified. The results reveal the dark states by the
cross peaks apart from the ones for polaritons, as a result of the breakdown of
cooperativity between molecules. We further show that the breakdown of
cooperativity profoundly connects to the localization of the vibrational
excitations whereas the polariton modes are extended wave over several
molecules. Besides, our work offers new physical insight for understanding the
recent 2D-IR experiments where the interaction between dark modes and bright
polaritons was evident.Comment: 11 pages, 6 figure
Performance of Joint Channel and Physical Network Coding Based on Alamouti STBC
This work considers the protograph-coded physical network coding (PNC) based
on Alamouti space-time block coding (STBC) over Nakagami-fading two-way relay
channels, in which both the two sources and relay possess two antennas. We
first propose a novel precoding scheme at the two sources so as to implement
the iterative decoder efficiently at the relay. We further address a simplified
updating rule of the log-likelihood-ratio (LLR) in such a decoder. Based on the
simplified LLR-updating rule and Gaussian approximation, we analyze the
theoretical bit-error-rate (BER) of the system, which is shown to be consistent
with the decoding thresholds and simulated results. Moreover, the theoretical
analysis has lower computational complexity than the protograph extrinsic
information transfer (PEXIT) algorithm. Consequently, the analysis not only
provides a simple way to evaluate the error performance but also facilitates
the design of the joint channel-and-PNC (JCNC) in wireless communication
scenarios.Comment: 6 pages, 4 figures, accpete
Hybrid AC/DC hub for integrating onshore wind power and interconnecting onshore and offshore DC networks
A hybrid AC/DC hub is proposed in this study, where a modular multilevel converter and a line-commutated converter are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. The hybrid AC/DC hub facilitates wind power integration and DC network interconnection with reduced converter ratings and power losses when compared with the ‘conventional’ approach using DC–DC converters. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection for the hybrid AC/DC hub is proposed and studied. Simulation results in PSCAD/EMTDC verify the feasibility and effectiveness of the proposed control and protection of the hybrid AC/DC hub under power flow change, AC and DC fault conditions
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