Blurred Lines Between Competition and Parasitism

Accurately describing the ecological relationships between species is more than mere semantics-doing so has profound practical and applied implications, not the least of which is that inaccurate descriptions can lead to fundamentally incorrect predicted outcomes of community composition and functioning. Accurate ecological classifications are particularly important in the context of global change, where species interactions can change rapidly following shifts in species composition. Here, we argue that many common ecological interactions-particularly competition and parasitism-can be easily confused and that we often lack empirical evidence for the full reciprocal interaction among species. To make our case and to propose a theoretical framework for addressing this problem, we use the interactions between lianas and trees, whose outcomes have myriad implications for the ecology and conservation of tropical forests (e.g., Schnitzer et al. 2015)

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies excitonic and electron-hole plasma contributions. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute optically active exciton populations making it possible to extract a phase diagram for incoherent excitonic populations.Comment: 9 pages, 4 figure

Many-Body Dynamics and Exciton Formation Studied by Time-Resolved Photoluminescence

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.Comment: 11 pages, 5 figure

Influence of Coulomb and Phonon Interaction on the Exciton Formation Dynamics in Semiconductor Heterostructures

A microscopic theory is developed to analyze the dynamics of exciton formation out of incoherent carriers in semiconductor heterostructures. The carrier Coulomb and phonon interaction is included consistently. A cluster expansion method is used to systematically truncate the hierarchy problem. By including all correlations up to the four-point (i.e. two-particle) level, the fundamental fermionic substructure of excitons is fully included. The analysis shows that the exciton formation is an intricate process where Coulomb correlations rapidly build up on a picosecond time scale while phonon dynamics leads to true exciton formation on a slow nanosecond time scale.Comment: 18 pages, 7 figure

Gain spectroscopy of a type-II VECSEL chip

Using optical pump-white light probe spectroscopy the gain dynamics is investigated for a VECSEL chip which is based on a type-II heterostructure. The active region the chip consists of a GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs multiple quantum well. For this structure, a fully microscopic theory predicts a modal room temperature gain at a wavelength of 1170 nm, which is confirmed by experimental spectra. The results show a gain buildup on the type-II chip which is delayed relative to that of a type-I chip. This slower gain dynamics is attributed to a diminished cooling rate arising from reduced electron-hole scattering.Comment: 4 pages, 4 figure

Comparison between the Understanding Levels of Boys and Girls on the Concepts of Environmental Degradation, Meteorology and Climate Change in Tanzanian Secondary Schools

The study aimed to determine whether there was any significant difference in understanding levels between secondary school boys and girls on the concepts of environmental degradation, meteorology and climate change. Both structured survey and focus group discussions were used to collect information from 480 students, sampled randomly from 12 secondary schools in Morogoro region. The findings indicated that boys were significantly more knowledgeable than girls on the specified environmental concepts. This implies that girls lagged behind their counterparts in understanding important environmental concepts as specified in the school curriculum. Based on the findings of this study, it is recommended that girls should be inspired and encouraged to develop interest in natural science subjects which would enable them to broaden their understanding about environmental issues

Photoluminescence and Terahertz Emission from Femtosecond Laser-Induced Plasma Channels

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.Comment: 4 pages with 2 figures