54,742 research outputs found
Coherent Exciton Lasing in ZnSe/ZnCdSe Quantum Wells?
A new mechanism for exciton lasing in ZnSe/ZnCdSe quantum wells is proposed.
Lasing, occurring below the lowest exciton line, may be associated with a
BCS-like condensed (coherent) exciton state. This state is most stable at low
temperatures for densities in the transition region separating the exciton Bose
gas and the coherent exciton state. Calculations show the gain region to lie
below the exciton line and to be separated from the absorption regime by a
transparency region of width, for example, about 80 meV for a 90 Angstrom
ZnSe/Zn_(0.75)Cd_(0.25)Se quantum well. Experimental observation of the
transparency region using differential spectroscopy would confirm this picture.Comment: 9 pages + 3 figs contained in 4 postscript files to appear Appl.
Phys. Lett. March 13, 199
Charmonium properties in hot quenched lattice QCD
We study the properties of charmonium states at finite temperature in
quenched QCD on large and fine isotropic lattices. We perform a detailed
analysis of charmonium correlation and spectral functions both below and above
. Our analysis suggests that both S wave states ( and )
and P wave states ( and ) disappear already at about . The charm diffusion coefficient is estimated through the Kubo formula and
found to be compatible with zero below and approximately at
.Comment: 32 pages, 19 figures, typo corrected, discussions on isotropic vs
anisotropic lattices expanded, published versio
Cooperative Energy Harvesting Networks with Spatially Random Users
This paper considers a cooperative network with multiple source-destination
pairs and one energy harvesting relay. The outage probability experienced by
users in this network is characterized by taking the spatial randomness of user
locations into consideration. In addition, the cooperation among users is
modeled as a canonical coalitional game and the grand coalition is shown to be
stable in the addressed scenario. Simulation results are provided to
demonstrate the accuracy of the developed analytical results
Energy Harvesting Cooperative Networks: Is the Max-Min Criterion Still Diversity-Optimal?
This paper considers a general energy harvesting cooperative network with M
source-destination (SD) pairs and one relay, where the relay schedules only m
user pairs for transmissions. For the special case of m = 1, the addressed
scheduling problem is equivalent to relay selection for the scenario with one
SD pair and M relays. In conventional cooperative networks, the max-min
selection criterion has been recognized as a diversity-optimal strategy for
relay selection and user scheduling. The main contribution of this paper is to
show that the use of the max-min criterion will result in loss of diversity
gains in energy harvesting cooperative networks. Particularly when only a
single user is scheduled, analytical results are developed to demonstrate that
the diversity gain achieved by the max-min criterion is only (M+1)/2, much less
than the maximal diversity gain M. The max-min criterion suffers this diversity
loss because it does not reflect the fact that the source-relay channels are
more important than the relay-destination channels in energy harvesting
networks. Motivated by this fact, a few user scheduling approaches tailored to
energy harvesting networks are developed and their performance is analyzed.
Simulation results are provided to demonstrate the accuracy of the developed
analytical results and facilitate the performance comparison.Comment: 30 pages, 7 figure
Hot Spots on the Fermi Surface of Bi2212: Stripes versus Superstructure
In a recent paper Saini et al. have reported evidence for a pseudogap around
(pi,0) at room temperature in the optimally doped superconductor Bi2212. This
result is in contradiction with previous ARPES measurements. Furthermore they
observed at certain points on the Fermi surface hot spots of high spectral
intensity which they relate to the existence of stripes in the CuO planes. They
also claim to have identified a new electronic band along Gamma-M1 whose one
dimensional character provides further evidence for stripes. We demonstrate in
this Comment that all the measured features can be simply understood by
correctly considering the superstructure (umklapp) and shadow bands which occur
in Bi2212.Comment: 1 page, revtex, 1 encapsulated postscript figure (color
Event-Based H∞ filter design for a class of nonlinear time-varying systems with fading channels and multiplicative noises
In this paper, a general event-triggered framework is developed to deal with the finite-horizon H∞ filtering problem for discrete time-varying systems with fading channels, randomly occurring nonlinearities and multiplicative noises. An event indicator variable is constructed and the corresponding event-triggered scheme is proposed. Such a scheme is based on the relative error with respect to the measurement signal in order to determine whether the measurement output should be transmitted to the filter or not. The fading channels are described by modified stochastic Rice fading models. Some uncorrelated random variables are introduced, respectively, to govern the phenomena of state-multiplicative noises, randomly occurring nonlinearities as well as fading measurements. The purpose of the addressed problem is to design a set of time-varying filter such that the influence from the exogenous disturbances onto the filtering errors is attenuated at the given level quantified by a H∞ norm in the mean-square sense. By utilizing stochastic analysis techniques, sufficient conditions are established to ensure that the dynamic system under consideration satisfies the H∞ filtering performance constraint, and then a recursive linear matrix inequality (RLMI) approach is employed to design the desired filter gains. Simulation results demonstrate the effectiveness of the developed filter design scheme
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