2,386 research outputs found
Fermi-Edge Superfluorescence from a Quantum-Degenerate Electron-Hole Gas
We report on the observation of spontaneous bursts of coherent radiation from
a quantum-degenerate gas of nonequilibrium electron-hole pairs in semiconductor
quantum wells. Unlike typical spontaneous emission from semiconductors, which
occurs at the band edge, the observed emission occurs at the quasi-Fermi edge
of the carrier distribution. As the carriers are consumed by recombination, the
quasi-Fermi energy goes down toward the band edge, and we observe a
continuously red-shifting streak. We interpret this emission as cooperative
spontaneous recombination of electron-hole pairs, or superfluorescence, which
is enhanced by Coulomb interactions near the Fermi edge. This novel many-body
enhancement allows the magnitude of the spontaneously developed macroscopic
polarization to exceed the maximum value for ordinary superfluorescence, making
electron-hole superfluorescence even more "super" than atomic
superfluorescence.Comment: 10 pages, 5 figure
The Influence of the Free Troposphere on Boundary Layer Ozone Mixing Ratios over Europe
In an effort to determine cost-effective emission control strategies to reduce surface ozone concentrations in Europe, an integrated assessment model for boundary layer ozone is being developed at IIASA. In its current version, the ozone formation and transport module of this integrated assessment model predicts ozone mixing ratios based on annual emissions of nitrogen oxides and volatile organic compounds, both of which play major roles in the ozone problem. One source of ozone that is not currently included as an explicit predictor within this model, however, is the transport of ozone and its precursors from the free troposphere to the boundary layer.
The purpose of this paper is to study the influence of the free troposphere on boundary layer ozone mixing ratios and to determine whether or not this free tropospheric influence is significant enough to be included in the model
An Assessment of the Academic Impact of Shock Society Members
Professional society membership enhances career development and productivity by offering opportunities for networking and learning about recent advances in the field. The quality and contribution of such societies can be measured in part through the academic productivity, career status, and funding success rates of their members. Here, using Scopus, NIH RePORTER, and departmental websites, we compare characteristics of the Shock Society membership to those of the top 55 NIH-funded American university and hospital-based departments of surgery. Shock Society members' mean number of publications, citations and H-indices were all significantly higher than those of non-members in surgery departments (P < 0.001). A higher percentage of members also have received funding from the NIH (42.5% vs. 18.5%, P < 0.001). Regression analysis indicated that members were more likely to have NIH funding compared with non-members (OR 1.46, 95% CI 1.12-1.916). Trauma surgeons belonging to the Shock Society had a higher number of publications and greater NIH funding than those who did not (130.4 vs. 42.7, P < 0.001; 40.4% vs. 8.5%, P < 0.001). Aggregate academic metrics from the Shock Society were superior to those of the Association for Academic Surgery and generally for the Society of University Surgeons as well. These data indicate that the Shock Society represents a highly academic and productive group of investigators. For surgery faculty, membership is associated with greater academic productivity and career advancement. While it is difficult to ascribe causation, certainly the Shock Society might positively influence careers for its members
Dispersive diffusion controlled distance dependent recombination in amorphous semiconductors
The photoluminescence in amorphous semiconductors decays according to power
law at long times. The photoluminescence is controlled by
dispersive transport of electrons. The latter is usually characterized by the
power of the transient current observed in the time-of-flight
experiments. Geminate recombination occurs by radiative tunneling which has a
distance dependence. In this paper, we formulate ways to calculate reaction
rates and survival probabilities in the case carriers execute dispersive
diffusion with long-range reactivity. The method is applied to obtain tunneling
recombination rates under dispersive diffusion. The theoretical condition of
observing the relation is obtained and theoretical
recombination rates are compared to the kinetics of observed photoluminescence
decay in the whole time range measured.Comment: To appear in Journal of Chemical Physic
Ghosts- and Tachyon-Free Regions of the Randall-Sundrum Model Parameter Space
Model building within the Randall-Sundrum (RS) framework generally involves
placing the Standard Model fields in the bulk. Such fields may possess non-zero
values for their associated brane-localized kinetic terms (BLKTs) in addition
to possible bulk mass parameters. In this paper we clearly identify the regions
of the RS model parameter space where the presence of bulk mass terms and BLKTs
yield a setup which is free from both ghost and tachyon instabilities. Such
physically acceptable parameter space regions can then be used to construct
realistic and phenomenologically viable RS models.Comment: Latex, 30 pages, 2 figure
Ground operation of robotics on Space Station Freedom
This paper reflects work carried out on Ground Operated Telerobotics (GOT) in 1992 to refine further the ideas, procedures, and technologies needed to test the procedures in a high latency environment, and to integrate GOT into Space Station Freedom operations. Space Station Freedom (SSF) will be in operation for 30 years, and will depend on robots to carry out a significant part of the assembly, maintenance, and utilization workload. Current plans call for on-orbit robotics to be operated by on-board crew members. This approach implies that on-orbit robotics operations use up considerable crew time, and that these operations cannot be carried out when SSF is unmanned. GOT will allow robotic operations to be operated from the ground, with on-orbit crew interventions only when absolutely required. The paper reviews how GOT would be implemented, how GOT operations would be planned and supported, and reviews GOT issues, critical success factors, and benefits
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