Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} = 3~\mu$s, bound in the exciton.Comment: 17 pages, 16 figure

Influence of the heterointerface sharpness on exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap

The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons which are indirect in momentum-space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape and composition. The different exciton lifetimes, which result in photoluminescence with non-exponential decay obeying a power-law function, can be described by a phenomenological distribution function, which allows one to explain the photoluminescence decay with one fitting parameter only.Comment: 10 pages, 7 figure

Genetic analyses of archival specimens of the Atlantic sturgeon Acipenser sturio L., 1758

Se analizó la variabilidad genética en Acipenser sturio L., 1758 y Acipenser oxyrinchus oxyrinchus Mitchill, 1815 usando la variación en la región D-loop del ADN mitocondrial y en un número de microsatélites (marcadores nucleares). El material estudiado incluyó muestras de tejidos de: (1) 38 ejemplares almacenados de A. sturio colectados en diferentes museos de historia natural alemanes, suecos, daneses y franceses; (2) 27 A. sturio vivos correspondientes al stock de cría para recuperación de esta especie en aguas alemanas (Instituto de Ecología Dulceacuícola y Pescas Interiores, Berlín, Alemania); (3) 30 A. o. oxyrinchus silvestres capturados en el océano Atlántico cerca de la costa de Nueva Jersey y en el río Delaware (USA); y (4) 60 individuos de A. o. oxyrinchus de un stock reproducido artificialmente obtenido originalmente de diversos esturiones silvestres capturados en el río San Juan (Canadá). Se clonó y secuenció un fragmento de 250 pares de bases de la región D-loop del ADN mitocondrial. La longitud de una unidad repetida fue de 80 pares de bases en A. sturio y de 79 en A. o. oxyrinchus. Las unidades repetidas de A. sturio y A. o. oxyrinchus difirieron por 11 sustituciones y una delección o inserción, respectivamente. No se encontró heteroplasmia. Se observaron tres diferentes haplotipos de ADN mitocondrial en ambas especies. Cinco microsatélites presentaron patrones polimórficos de bandas. En A. sturio los análisis de microsatélites mostraron una disminución en números alélicos entre los años 1823 y 1992. Este declive tuvo como consecuencia la fijación de varios alelos. Para A. sturio, se observaron un haplotipo de ADN mitocondrial y siete alelos sólo en los ejemplares almacenados. Los cálculos de distancia genética mostraron una gran similitud genética entre las poblaciones del Gironda y del Mar del Norte, y una posición basal de las poblaciones de A. sturio del Mediterráneo y del Adriático. En A. o. oxyrinchus el número de alelos de microsatélite varió entre 14 (ríos Hudson y San Juan) y 22 (río Delaware). Los cálculos de distancia genética mostraron una gran similitud genética entre las subpoblaciones de A. o. oxyrinchus.Genetic variability was analysed in Acipenser sturio L., 1758 and Acipenser oxyrinchus oxyrinchus Mitchill, 1815 using variation in the D-loop region of mtDNA and a number of microsatellites (nuclear markers). The studied material included tissue samples from: (1) 38 A. sturio archival specimens collected in different German, Swedish, Danish, and French museums of natural history; (2) 27 live A. sturio representing a broodstock for restoration of this species in German waters (Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany); (3) 30 wild A. o. oxyrinchus caught in the Atlantic Ocean near the coast of New Jersey and in the Delaware River (USA); and (4) 60 individuals of A. o. oxyrinchus from an artificially reproduced stock originally obtained from several wild sturgeons captured in the St. John River (Canada). A 250-bp fragment of the D-loop region of mtDNA was cloned and sequenced. The length of a repeated unit was 80 bp in A. sturio and 79 bp in A. o. oxyrinchus. The repeated units of A. sturio and A. o. oxyrinchus differed by 11 substitutions and one deletion or insertion, respectively. No heteroplasmy was found. Three different haplotypes of mtDNA were observed in both species. Five microsatellites had polymorphic band patterns. In A. sturio, analyses of microsatellites showed a decrease in allelic numbers between the years 1823 and 1992. This decline resulted in a fixation of several alleles. For A. sturio, one mtDNA haplotype and seven alleles were observed only in archival samples. Genetic distance calculations showed a great genetic similarity between A. sturio populations in the Gironde River and the North Sea, and a basal position of the Mediterranean and Adriatic Sea A. sturio populations. In A. o. oxyrinchus, the number of microsatellite alleles ranged between 14 (Hudson and St. John rivers) and 22 (Delaware River). Genetic distance calculations showed a high genetic similarity between subpopulations of A. o. oxyrinchus.Instituto Español de Oceanografí

Tracing the first steps of American sturgeon pioneers in Europe

Background: A Baltic population of Atlantic sturgeon was founded ~1,200 years ago by migrants from North America, but after centuries of persistence, the population was extirpated in the 1960s, mainly as a result of over-harvest and habitat alterations. As there are four genetically distinct groups of Atlantic sturgeon inhabiting North American rivers today, we investigated the genetic provenance of the historic Baltic population by ancient DNA analyses using mitochondrial and nuclear markers. Results: The phylogeographic signal obtained from multilocus microsatellite DNA genotypes and mitochondrial DNA control region haplotypes, when compared to existing baseline datasets from extant populations, allowed for the identification of the region-of-origin of the North American Atlantic sturgeon founders. Moreover, statistical and simulation analyses of the multilocus genotypes allowed for the calculation of the effective number of individuals that originally founded the European population of Atlantic sturgeon. Our findings suggest that the Baltic population of A. oxyrinchus descended from a relatively small number of founders originating from the northern extent of the species' range in North America. Conclusion: These results demonstrate that the most northerly distributed North American A. oxyrinchus colonized the Baltic Sea ~1,200 years ago, suggesting that Canadian specimens should be the primary source of broodstock used for restoration in Baltic rivers. This study illustrates the great potential of patterns obtained from ancient DNA to identify population-of-origin to investigate historic genotype structure of extinct populations

Predominance of exotic and introduced species among sturgeons captured from the Baltic and North Seas and their watersheds, 1981-1999

Sturgeon catches (n = 256) from 1981-1999 reported mainly by commercial fishermen and anglers in German, Polish, and Dutch coastal waters and tributaries were analysed. During the study period, 20 % of catches were reported from coastal waters and 65 % from rivers and estuaries of large river systems, including the Odra, Elbe, Rhine and Weser. The data indicate that, from 1981- 1993, there was a major decline in the Atlantic sturgeon Acipenser sturio L., 1758, and an increase in the total catches of non-indigenous sturgeon species. The Siberian sturgeon Acipenser baerii Brandt, 1869, the Russian sturgeon Acipenser gueldenstaedtii Brandt & Ratzeberg, 1833, and various hybrids dominated. Occasional catches of the white sturgeon Acipenser transmontanus Richardson, 1836 and the sterlet Acipenser ruthenus L., 1758 were also reported. During the study period, significant changes in species composition and distribution of catches were observed. The predominance of non-indigenous sturgeon species is a result of the increasingly intensive sturgeon aquaculture activities in Germany, Poland, and the Netherlands. The most frequently reared species now dominate the catches. In addition to these escapees from fish farms, several intentional releases of sturgeons were reported. The results show that introduced exotic sturgeon species may thrive under certain natural conditions. Therefore, they may interfere with restoration efforts for the native A. sturio, competing for habitat and introducing diseases and hybridization.Se analizaron las capturas de esturiones (n = 256) entre 1981 y 1999 proporcionadas principalmente por pescadores profesionales y deportivos en las costas alemanas, polacas y holandesas, y en los sistemas fluviales que desembocan en ellas. En el periodo de estudio, el 20 % de las capturas correspondió a aguas litorales y el 65 % a ríos y estuarios de los grandes sistemas fluviales, incluidos los ríos Oder, Elba, Rin y Weser. Los datos indican que entre 1981 y 1993 tuvo lugar el mayor declive del esturión atlántico Acipenser sturio L., 1758 y un incremento en las capturas totales de las especies alóctonas de esturiones. Predominaron las capturas de esturión siberiano Acipenser baerii Brandt, 1869, esturión ruso Acipenser gueldenstaedtii Brandt & Ratzeberg, 1833 y varios híbridos. También se registraron capturas ocasionales de esturión blanco Acipenser transmontanus Richardson, 1836 y esterlete Acipenser ruthenus L., 1758. En el periodo de estudio se observaron cambios significativos en la composición de especies y en la distribución de las capturas. El predominio de las especies alóctonas de esturiones es el resultado de las crecientes actividades en la acuicultura intensiva de esturión en Alemania, Polonia y Países Bajos. Las especies cultivadas más frecuentemente dominan ahora las capturas. Además de estas fugas de las piscifactorías, se han registrado varias sueltas intencionadas. Los resultados muestran que las especies de esturiones exóticas introducidas pueden prosperar en ciertas condiciones naturales. Por esta razón, pueden interferir en los esfuerzos de restauración de la especie autóctona A. sturio, compitiendo por el hábitat e introduciendo enfermedades e hibridación.Instituto Español de Oceanografí

Spin-flip Raman scattering of the Γ\Gamma-X mixed exciton in indirect band-gap (In,Al)As/AlAs quantum dots

The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the $\Gamma$- and X-conduction band valleys, from which control of their spin states can be gained. This access is used here for studying the exciton spin-level structure by resonant spin-flip Raman scattering, allowing us to accurately measure the anisotropic hole and isotropic electron $g$ factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of $\Gamma$-X-valley mixing, as evidenced by both experiment and theory.Comment: 5 pages, 3 figure

Breeding biology, behaviour and foraging ecology of the Black Falcon Falco subniger near Tamworth, New South Wales

The breeding biology and behaviour of the Black Falcon Falco subniger were studied in the Tamworth district (northern inland New South Wales) through 146 hours of observation over 47 days in 2015 (one pair, pre-laying to early incubation) and 261 hours of observation over 69 days in 2016 (four pairs, pre-laying to fledging, with checks through the post-fledging period). Pellets were collected from under vacated nests. Aerial displays (e.g. agility, V-dives, 'undulatory roll,' 'high winnowing'), nest-site selection and occupation, courtship and mating are described. Adopted stick nests were high in tall or emergent riparian or paddock eucalypts; nearest-neighbour distances averaged 10.25 km (range 9-12 km). Eggs were laid in July, and the incubation period appeared to be 34 ± 1 days at one nest. Males took a minor share of incubation (1-3% of daylight) and brooding of hatchlings (1%). Interspecific conflict or nest-site defence was strongest against corvids in the pre-laying phase, and against Wedge-tailed Eagles Aquila audax during the nestling phase. Feeding rates and estimated biomass provision were 0.09-0.26 item/h and ~4–28 g/h at nests that failed during the incubation or hatchling phase, and 0.19 item/h and ~23 g/h to a single nestling that fledged, albeit underweight. Nest failure appeared to be related to cold, wet weather and poor hunting success around hatching time. Breeding productivity was 0.25 young per attempt in 2015-16, and 0.5-0.6 young per attempt for 10 nests since 2004, with up to half of fledglings failing to reach independence. The observed breeding diet was 98% birds and 2% rodents, although insects appeared in pellets. Hunting success on birds was 36% of observed attacks. Demographic and ecological research on this species is required. As the threatened and declining Black Falcon faces human-related impacts in the sheep-wheat belt, some possible management strategies are suggested (e.g. artificial nests)