54,414 research outputs found

    Coherent Exciton Lasing in ZnSe/ZnCdSe Quantum Wells?

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    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

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    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 TcT_c. Our analysis suggests that both S wave states (J/ψJ/\psi and ηc\eta_c) and P wave states (χc0\chi_{c0} and χc1\chi_{c1}) disappear already at about 1.5Tc1.5 T_c. The charm diffusion coefficient is estimated through the Kubo formula and found to be compatible with zero below TcT_c and approximately 1/πT1/\pi T at 1.5TcT3Tc1.5 T_c\lesssim T\lesssim 3 T_c.Comment: 32 pages, 19 figures, typo corrected, discussions on isotropic vs anisotropic lattices expanded, published versio

    Cooperative Energy Harvesting Networks with Spatially Random Users

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    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?

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    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

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    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

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    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