9,920 research outputs found
Stable Large-Scale Perturbations in Interacting Dark-Energy Model
It is found that the evolutions of density perturbations on the super-Hubble
scales are unstable in the model with dark-sector interaction proportional
to the energy density of cold dark matter (CDM) and constant equation
of state parameter of dark energy . In this paper, to avoid the
instabilities, we suggest a new covariant model for the energy-momentum
transfer between DE and CDM. Then we show that the the large-scale
instabilities of curvature perturbations can be avoided in our model in the
universe filled only by DE and CDM. Furthermore, by including the additional
components of radiation and baryons, we calculate the dominant non-adiabatic
modes in the radiation era and find that the modes grow in the power law with
exponent at the order of unit.Comment: 14 pages, 2 figures. arXiv admin note: substantial text overlap with
arXiv:1110.180
Cogging torque and torque ripple reduction of a novel exterior-rotor geared motor
The reduction of cogging torque and torque ripple in permanent-magnet motors to suppress the vibration and acoustic noise is a major concern for motor designers. This study presents a novel exterior-rotor geared motor which integrates a brushless permanent-magnet (BLPM) motor with an epicyclic-type gear reducer to form a compact structural assembly without extra transmitting elements. One of the special features of the geared motor lies in the gear-teeth of the epicyclic-type gear reducer merged with the stator of the BLPM motor. The gear-teeth serve as the interfacial medium to connect the BLPM motor with the epicyclic-type gear reducer, which provides functions not only for transmission to achieve a desired speed ratio, but also effectively reduce the cogging torque and torque ripple of the geared motor. Five shape models of pole shoes with different values of the shoe depth and the shoe ramp are presented to effectively reduce the cogging torque and the torque ripple. With the aid of the finite-element analysis, shape model III of the geared motor performs better than the existing BLPM motor on the cogging torque with 87 % decreasing and the torque ripple with 23 % decreasing. Such a unique characteristic of the geared motor is of benefit to the widely applications on accurate motion and position control systems
New Interaction between Dark Energy and Dark Matter Changes Sign during Cosmological Evolution
It is found by Cai and Su that the interaction between dark energy and cold
dark matter is likely to change the sign during the cosmological evolution.
Motivated by this, we suggest a new form of interaction between dark energy and
dark matter, which changes from negative to positive as the expansion of our
universe changes from decelerated to accelerated. We find that the interacting
model is consistent with the second law of thermodynamics and the observational
constraints. And, we also discuss the unified adiabatic-squared sound speed of
the model.Comment: 16 pages, 3 figure, 1 table. Final version in PR
Degree correlation effect of bipartite network on personalized recommendation
In this paper, by introducing a new user similarity index base on the
diffusion process, we propose a modified collaborative filtering (MCF)
algorithm, which has remarkably higher accuracy than the standard collaborative
filtering. In the proposed algorithm, the degree correlation between users and
objects is taken into account and embedded into the similarity index by a
tunable parameter. The numerical simulation on a benchmark data set shows that
the algorithmic accuracy of the MCF, measured by the average ranking score, is
further improved by 18.19% in the optimal case. In addition, two significant
criteria of algorithmic performance, diversity and popularity, are also taken
into account. Numerical results show that the presented algorithm can provide
more diverse and less popular recommendations, for example, when the
recommendation list contains 10 objects, the diversity, measured by the hamming
distance, is improved by 21.90%.Comment: 9 pages, 3 figure
Non-Abelian Quantum Hall Effect in Topological Flat Bands
Inspired by recent theoretical discovery of robust fractional topological
phases without a magnetic field, we search for the non-Abelian quantum Hall
effect (NA-QHE) in lattice models with topological flat bands (TFBs). Through
extensive numerical studies on the Haldane model with three-body hard-core
bosons loaded into a TFB, we find convincing numerical evidence of a stable
bosonic NA-QHE, with the characteristic three-fold quasi-degeneracy of
ground states on a torus, a quantized Chern number, and a robust spectrum gap.
Moreover, the spectrum for two-quasihole states also shows a finite energy gap,
with the number of states in the lower energy sector satisfying the same
counting rule as the Moore-Read Pfaffian state.Comment: 5 pages, 7 figure
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