Electron capture in GaAs quantum wells via electron-electron and optic phonon scattering

Electron capture times in a separate confinement quantum well (QW) structure with finite electron density are calculated for electron-electron (e-e) and electron-polar optic phonon (e-pop) scattering. We find that the capture time oscillates as function of the QW width for both processes with the same period, but with very different amplitudes. For an electron density of 10^11 cm^-2 the e-e capture time is 10-1000 times larger than the e-pop capture time except for QW widths near the resonance minima, where it is only 2-3 times larger. With increasing electron density the e-e capture time decreases and near the resonance becomes smaller than the e-pop capture time. Our e-e capture time values are two-to-three orders of magnitude larger than previous results of Blom et al. [Appl. Phys. Lett. 62, 1490 (1993)]. The role of the e-e capture in QW lasers is therefore readdressed.Comment: 5 pages, standard LaTeX file + 5 PostScript figures (tarred, compressed and uuencoded) or by request from [email protected], accepted to Appl. Phys. Let

Distributed Branching Bisimulation Minimization by Inductive Signatures

We present a new distributed algorithm for state space minimization modulo branching bisimulation. Like its predecessor it uses signatures for refinement, but the refinement process and the signatures have been optimized to exploit the fact that the input graph contains no tau-loops. The optimization in the refinement process is meant to reduce both the number of iterations needed and the memory requirements. In the former case we cannot prove that there is an improvement, but our experiments show that in many cases the number of iterations is smaller. In the latter case, we can prove that the worst case memory use of the new algorithm is linear in the size of the state space, whereas the old algorithm has a quadratic upper bound. The paper includes a proof of correctness of the new algorithm and the results of a number of experiments that compare the performance of the old and the new algorithms

Expression of San Andreas Fault on Seasat Radar Image

On a Seasat image (23.5-cm wavelength) of the Durmid Hills in southern California, the San Andreas fault is expressed as a prominent southeast-trending tonal lineament that is bright on the southwest side and dark on the northeast side. Field investigation established that the bright signature corresponds to outcrops of the Borrego Formation, which weathers to a rough surface. The dark signature corresponds to sand and silt deposits of Lake Coahuila which are smooth at the wavelength of the Seasat radar. These signatures and field characteristics agree with calculations of the smooth and rough radar criteria. On Landsat and Skylab images of the Durmid Hills, the Borrego and Lake Coahuila surfaces have similar bright tones and the San Andreas fault is not detectable. On a side-looking airborne radar image (0.86-cm wavelength), both the Borrego and Lake Coahuila surfaces appear rough, which results in bright signatures on both sides of the San Andreas fault. Because of this lack of roughness contrast, the fault cannot be distinguished. The wavelength of the Seasat radar system is well suited for mapping geologic features in the Durmid Hills that are obscure on other remote sensing images

Antibiotic effects of three strains of chrysophytes (Ochromonas, Poterioochromonas) on freshwater bacterial isolates

We investigated the antibiotic effects of extracts of freeze-dried biomass and culture supernatants from the mixotrophic chrysophyte species Ochromonas danica, Poterioochromonas sp. strain DS, and Poterioochromonas malhamensis on bacterial strains isolated from lake water. Methanolic biomass extracts inhibited the growth of all tested strains, albeit to a different extent, whereas aqueous biomass extracts only affected bacteria of the genus Flectobacillus. The antibiotic action of supernatants from flagellate cultures could be mostly attributed to lipophilic substances, but the growth of bacteria affiliated with Flectobacillus and Sphingobium was also affected by hydrophilic compounds. A comparison of biomass extracts from light- and dark-adapted cultures of Poterioochromonas sp. strain DS showed that the growth-inhibiting factor was unrelated to chlorophyll derivatives. Supernatants from a dark-adapted, phagotrophically grown flagellate culture had stronger antibiotic effects and affected more bacterial strains than the supernatant from a light-adapted culture. Significant growth reduction of a Flectobacillus isolate was already induced by extremely low concentrations of lipophilic extracts from these supernatants. Our results show that metabolites of the studied flagellates − either released actively or during cell lysis − may selectively affect the growth of some aquatic bacteria even in very small doses and thus potentially affect microbial community composition. Moreover, the antibiotic potential of mixotrophic chrysophytes may change with their nutritional mod

Effects of age on the acquisition of agreement inflection

Grammaticality judgement tasks show that second language learners who started during childhood are significantly more accurate on judging inflection than learners who started after puberty [Johnson, J., & Newport, E. (1989). Cognitive Psychology, 21, 60-99; Johnson, J., & Newport, E. (1991). Cognition, 39, 215-258; McDonald, J. (2000). Applied Psycholinguistics, 21, 395-423. Production data confirmthat inflection is a bottleneck in adult language acquisition, and that they differ from child learners in this respect [Lardiere, D. (1998). Second Language Research, 14, 359-375; Prévost, P. (2003). Studies in Second Language Acquisition, 25, 65-97; Pre vost, P., & White, L. (2000). Second Language Research, 16(2), 103-133]. Although the observations suggest that the acquisition of inflection is influenced by age, there is no study that focuses on this particular issue nor is there an articulated explanation available for the observed age-related difference. In this contribution, we compare child L2 learners of Dutch to child L1 and adult L2 learners of Dutch in order to investigate effects of age on the acquisition of verbal and adjectival inflection. We hypothesize that adult agreement paradigms differ from child agreement paradigms, the reason being that adult learners cannot rely on syntactic cues, whereas children make reliable use of syntax in building paradigms. By effect, adult learners end up with non-targetlike small paradigms that contain underspecified suffixes. We focus on the types of errors in the three learner groups (child L1, child L2 and adult L2). Our empirical basis consists of results obtained in a series of production experiments

Articles, adjectives and age of onset: the acquisition of Dutch grammatical gender

International audienceA comparison of the error profiles of monolingual (child L1) learners of Dutch, Moroccan children (child L2) and Moroccan adults (adult L2) learning Dutch as their L2 shows that participants in all groups massively overgeneralize [—neuter] articles to [+neuter] contexts. In all groups, the reverse gender mistake infrequently occurs. Gender expressed by Dutch attributive adjectives reveals an age-related asymmetry between the three groups, however. Whereas participants in the child groups overgeneralize one particular suffix (namely the schwa), adult participants use both adjectival forms, the schwa-adjective and the bare adjective, incorrectly. It is argued that the asymmetry observed in adjectives reflects that adult learners exploit an input-based, lexical learning route, whereas children rely on grammar-based representations. The similarity in article selection between all groups follows from the assumption that adults, like children, make use of lexical frames. Crucially, lexical frames can successfully describe the distribution of gender-marked articles, but they cannot account for gender in adjectives