254,803 research outputs found
Hybrid power semiconductor
The voltage rating of a bipolar transistor may be greatly extended while at the same time reducing its switching time by operating it in conjunction with FETs in a hybrid circuit. One FET is used to drive the bipolar transistor while the other FET is connected in series with the transistor and an inductive load. Both FETs are turned on or off by a single drive signal of load power, the second FET upon ceasing conductions, rendering one power electrode of the bipolar transistor open. Means are provided to dissipate currents which flow after the bipolar transistor is rendered nonconducting
Symmetries and Lie algebra of the differential-difference Kadomstev-Petviashvili hierarchy
By introducing suitable non-isospectral flows we construct two sets of
symmetries for the isospectral differential-difference Kadomstev-Petviashvili
hierarchy. The symmetries form an infinite dimensional Lie algebra.Comment: 9 page
Rare-earth ions doped transparent oxyfluoride glass-ceramics
In recent years, rare-earth ions doped transparent oxyfluoride glass-ceramics have attracted great attentions for their low phonon energy environments of fluoride nanocrystals and high chemical and mechanical stabilities of oxide glassy matrix. In this chapter, firstly, the crystallization behaviors of the transparent glass ceramics containing CaF2 nanocrystals are presented to demonstrate the controllable microstructure evolution of nano-composites. Secondly, the optical properties of the newly developed transparent glass-ceramics containing β-YF3 nanocrystals are systematically reviewed. The rare-earth ions are inclined to partition into the YF3 nanocrystals after crystallization. Through variation of the rare-earth doping and control of the microstructures, the glass-ceramics could exhibit high-stimulated emission cross-section, broadband near infrared emission, high efficient ultraviolet upconversion emission and bright white light emission, indicating their potential multifunctional applications in solid state laser, upconversion, optical amplifier, three-dimensional display, and so on
Pseudo-Hermitian Hamiltonians Generating Waveguide Mode Evolution
We study the properties of Hamiltonians defined as the generators of transfer
matrices in quasi- one-dimensional waveguides. For single- or multi-mode
waveguides obeying flux conservation and time-reversal invariance, the
Hamiltonians defined in this way are non-Hermitian, but satisfy symmetry
properties that have previously been identified in the literature as "pseudo
Hermiticity" and "anti-PT symmetry". We show how simple one-channel and
two-channel models exhibit transitions between real, imaginary, and complex
eigenvalue pairs.Comment: 7 pages, 2 figure
Local molecular field theory for effective attractions between like charged objects in systems with strong Coulomb interactions
Strong short ranged positional correlations involving counterions can induce
a net attractive force between negatively charged strands of DNA, and lead to
the formation of ion pairs in dilute ionic solutions. But the long range of the
Coulomb interactions impedes the development of a simple local picture. We
address this general problem by mapping the properties of a nonuniform system
with Coulomb interactions onto those of a simpler system with short ranged
intermolecular interactions in an effective external field that accounts for
the averaged effects of appropriately chosen long ranged and slowly varying
components of the Coulomb interactions. The remaining short ranged components
combine with the other molecular core interactions and strongly affect pair
correlations in dense or strongly coupled systems. We show that pair
correlation functions in the effective short ranged system closely resemble
those in the uniform primitive model of ionic solutions, and illustrate the
formation of ion pairs and clusters at low densities. The theory accurately
describes detailed features of the effective attraction between two equally
charged walls at strong coupling and intermediate separations of the walls. New
analytical results for the minimal coupling strength needed to get any
attraction and for the separation where the attractive force is a maximum are
presented.Comment: 8 pages, 5 figures. To be published in PNA
The Angular Momentum Distribution within Halos in Different Dark Matter Models
We study the angular momentum profile of dark matter halos for a statistical
sample drawn from a set of high-resolution cosmological simulations of
particles. Two typical Cold Dark Matter (CDM) models have been analyzed, and
the halos are selected to have at least particles in order to
reliably measure the angular momentum profile. In contrast with the recent
claims of Bullock et al., we find that the degree of misalignment of angular
momentum within a halo is very high. About 50 percent of halos have more than
10 percent of halo mass in the mass of negative angular momentum . After the
mass of negative is excluded, the cumulative mass function follows
approximately the universal function proposed by Bullock et al., though we
still find a significant fraction of halos () which exhibit
systematic deviations from the universal function. Our results, however, are
broadly in good agreement with a recent work of van den Bosch et al.. We also
study the angular momentum profile of halos in a Warm Dark Matter (WDM) model
and a Self-Interacting Dark Matter (SIDM) model. We find that the angular
momentum profile of halos in the WDM is statistically indistinguishable from
that in the CDM model, but the angular momentum of halos in the SIDM is reduced
by the self-interaction of dark matter.Comment: 23 pages, 10 figures, 2 tables. Revised version, added a new table,
accepted for publication in MNRA
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Chinese peasant entrepreneurs: an examination of township and village
One of the greatest achievements in China's economic reform is the development of rural township and village enterprises (TVEs). Their importance in the national economy can be seen from the fact that by the end of 1993, TVEs accounted for nearly one third of the agricultural labour force, and one fifth of the total labour force nationwide. Despite some research attention being given to Chinese TVEs, the peasant entrepreneurs, owners and /or managers of these enterprises have remained largely unknown in the West. Who are they? Where are they from? What are their unique characteristics which distinguish them from both managers in the state-owned Chinese enterprises (SOEs) and small business owners /entrepreneurs in the West? What environmental factors contribute to the shaping of these characteristics? What are the problems faced by them and the possible solutions? This paper addresses these questions
Decay rate of a Wannier exciton in low dimensional systems
The superradiant decay rate of Wannier exciton in one dimensional system is
studied. The crossover behavior from 1D chain to 2D film is also examined. It
is found that the decay rate shows oscillatory dependence on channel width L.
When the quasi 1-D channel is embeded with planar microcavities, it is shown
that the dark mode exciton can be examined experimentally.Comment: 12 pages, 1 figur
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