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  • American Institute of Physics (AIP)  (72.674)
  • Wiley-Blackwell  (66.579)
  • American Meteorological Society (AMS)
  • 2005-2009  (46)
  • 2000-2004  (26.976)
  • 1990-1994  (112.231)
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  • 1
    Publikationsdatum: 2021-06-22
    Beschreibung: In this work, we analyse continuous measurements of microseisms to assess the reliability of the fundamental resonance frequency estimated by means of the horizontal-to-vertical (H/V) spectral ratio within the 0.1–1 Hz frequency range, using short-period sensors (natural period of 1 s). We apply the H/V technique to recordings of stations installed in two alluvial basins with different sedimentary cover thicknesses—the Lower Rhine Embayment (Germany) and the Gubbio Plain (Central Italy). The spectral ratios are estimated over the time–frequency domain, and we discuss the reliability of the results considering both the variability of the microseism activity and the amplitude of the instrumental noise. We show that microseisms measured by short period sensors allow the retrieval of fundamental resonance frequencies greater than about 0.1–0.2 Hz, with this lower frequency bound depending on the relative amplitude of themicroseism signal and the self-noise of the instruments. In particular,we show an examplewhere the considered short-period sensor is connected to instruments characterized by an instrumental noise level which allows detecting only fundamental frequencies greater than about 0.4 Hz. Since the frequency at which the peak of the H/V spectral ratio is biased depends upon the seismic signal-to-instrument noise ratio, the power spectral amplitude of instrumental self-noise should be always considered when interpreting the frequency of the peak as the fundamental resonance frequency of the investigated site.
    Beschreibung: Published
    Beschreibung: 175-184
    Beschreibung: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Beschreibung: JCR Journal
    Beschreibung: open
    Schlagwort(e): site effects ; fourier analysis ; 04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 2
    Publikationsdatum: 2017-04-04
    Beschreibung: Geochemical investigations have shown that there is a considerable inflow of gas into both crater lakes of Monticchio, Southern Italy. These lakes are located in two maars that formed 140 000 years ago during Mt. Vulture volcano s last eruptive activity. Isotopic analyses suggest that CO2 and helium are of magmatic origin; the latter displays 3He ⁄ 4He isotope ratios similar to those measured in olivines of the maar ejecta. In spite of the fact that the amount of dissolved gases in the water is less than that found in Lake Nyos (Cameroon), both the results obtained and the historical reports studied indicate that these crater lakes could be highly hazardous sites, even though they are located in a region currently considered inactive. This could be of special significance in very popular tourist areas such as the Monticchio lakes, which are visited by about 30 000 people throughout the summer, for the most part on Sundays.
    Beschreibung: Published
    Beschreibung: 83-87
    Beschreibung: 2.4. TTC - Laboratori di geochimica dei fluidi
    Beschreibung: JCR Journal
    Beschreibung: reserved
    Schlagwort(e): volcanic gases ; gas hazard ; crater lakes ; 04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry ; 04. Solid Earth::04.08. Volcanology::04.08.01. Gases ; 04. Solid Earth::04.08. Volcanology::04.08.08. Volcanic risk
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 3
    Publikationsdatum: 2017-04-04
    Beschreibung: On 2009 April 6, the Central Apennines were hit by an Mw = 6.3 earthquake. The region had been shaken since 2008 October by seismic activity that culminated in two foreshocks with Mw 〉 4, 1 week and a few hours before the main shock. We computed seismic moment tensors for 26 events with Mw between 3.9 and 6.3, using the Regional Centroid Moment Tensor (RCMT) scheme. Most of these source parameters have been computed within 1 hr after the earthquake and rapidly revised successively. The focal mechanisms are all extensional, with a variable and sometimes significant strike-slip component. This geometry agrees with the NE–SW extensional deformation of the Apennines, known from previous seismic and geodetic observations. Events group into three clusters. Those located in the southern area have larger centroid depths and a wider distribution of T-axis directions. These differences suggest that towards south a different fault systemwas activated with respect to the SW-dipping normal faults beneath L’Aquila and more to the north.
    Beschreibung: In press
    Beschreibung: 1.1. TTC - Monitoraggio sismico del territorio nazionale
    Beschreibung: JCR Journal
    Beschreibung: open
    Schlagwort(e): moment tensor ; seismotectonics ; L'Aquila ; 04. Solid Earth::04.07. Tectonophysics::04.07.04. Plate boundaries, motion, and tectonics
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 4
    Publikationsdatum: 2017-04-04
    Beschreibung: The strong motion data of 2009 April 6 L’Aquila (Central Italy) earthquake (Mw = 6.3) and of 12 aftershocks (4.1 ≤ Mw ≤ 5.6) recorded by 56 stations of the Italian strong motion network are spectrally analysed to estimate the source parameters, the seismic attenuation, and the site amplification effects. The obtained source spectra for S wave have stress drop values ranging from 2.4 to 16.8 MPa, being the stress drop of the main shock equal to 9.2 MPa. The spectral curves describing the attenuation with distance show the presence of shoulders and bumps, mainly around 50 and 150 km, as consequence of significant reflected and refracted arrivals from crustal interfaces. The attenuation in the first 50 km is well described by a quality factor equal to Q( f ) = 59 f 0.56 obtained by fixing the geometrical spreading exponent to 1. Finally, the horizontal-to-vertical spectral ratio provides unreliable estimates of local site effects for those stations showing large amplifications over the vertical component of motion.
    Beschreibung: Published
    Beschreibung: 1573–1579
    Beschreibung: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Beschreibung: JCR Journal
    Beschreibung: open
    Schlagwort(e): Generalized inversion ; strong-motion ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository-Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Materialart: article
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  • 5
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    Wiley-Blackwell
    In:  Journal of Zoology, 179 (3). pp. 291-295.
    Publikationsdatum: 2020-07-17
    Beschreibung: The cartilaginous tubercles of the mantle of the squid Cranchia scabra have been examined with the scanning electron microscope. Some tubercles are small, simple nodules whereas others are large with a complex Maltese cross form. The varying shapes and sizes probably represent a developmental sequence. The possible role of the tubercles is discussed.
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  • 6
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    Wiley-Blackwell
    In:  Journal of Quaternary Science, 25 (5). pp. 633-650.
    Publikationsdatum: 2017-12-19
    Beschreibung: Two cores were recovered in the southeastern part of Lake Shkodra (Montenegro and Albania) and sampled for identification of tephra layers. The first core (SK13, 7.8 m long) was recovered from a water depth of 7 m, while the second core (SK19, 5.8 m long) was recovered close to the present-day shoreline (water depth of 2 m). Magnetic susceptibility investigations show generally low values with some peaks that in some cases are related to tephra layers. Naked-eye inspection of the cores allowed the identification of four tephra layers in core SK13 and five tephra layers in core SK19. Major element analyses on glass shards and mineral phases allowed correlation of the tephra layers between the two cores, and their attribution to six different Holocene explosive eruptions of southern Italy volcanoes. Two tephra layers have under-saturated composition of glass shards (foiditic and phonolitic) and were correlated to the AD 472 and the Avellino (ca. 3.9 cal. ka BP) eruptions of Somma-Vesuvius. One tephra layer has benmoreitic composition and was correlated to the FL eruption of Mount Etna (ca. 3.4 cal. ka BP). The other three tephra layers have trachytic composition and were correlated to Astroni (ca. 4.2 cal. ka BP), Agnano Monte Spina (ca. 4.5 cal. ka BP) and Agnano Pomici Principali (ca. 12.3 cal. ka BP) eruptions of Campi Flegrei. The ages of tephra layers are in broad agreement with eight 14C accelerator mass spectrometric measurements carried out on plant remains and charcoal from the lake sediments at different depths along the two cores. The recognition of distal tephra layers from Italian volcanoes allowed the physical link of the Holocene archive of Lake Shkodra to other archives located in the central Mediterranean area and the Balkans (i.e. Lake Ohrid). Five of the recognised tephra layers were recognised for the first time in the Balkans area, and this has relevance for volcanic hazard assessment and for ash dispersal forecasting in case of renewed explosive activity from some of the southern Italy volcanoes. Copyright © 2009 John Wiley & Sons, Ltd.
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  • 7
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    Wiley-Blackwell
    In:  Journal of Quaternary Science, 24 (5). pp. 437-449.
    Publikationsdatum: 2018-05-15
    Beschreibung: This paper presents a temperature reconstruction of the past 1000 years for Central Europe, based on chronological records. The advantages and limitations of this hermeneutic, text-based approach are discussed and the statistic methodology is introduced. Historical documents represent direct observation of weather and atmospheric conditions with highest temporal resolution available and precise dating. A major advantage of these extensive data is that they allow the reconstruction of large numbers of variables such as winter temperature, precipitation, pressure patterns or climate extremes as well as floods or storms. Within this hermeneutic climatological research approach, even human impacts and social dimensions of climate development can be examined. In order to quantify the historical information, statistical methods are applied, based on an index approach.
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  • 8
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    Wiley-Blackwell
    In:  Journal of Zoology, 170 (4). pp. 451-462.
    Publikationsdatum: 2020-07-17
    Beschreibung: The dry weight and the crest length of the upper and lower beak, the length of the radula ribbon, the average width of the base of the six proximal and distal rachidian teeth as well as the total number of these teeth have all been related to the live body weight of octopuses between 1.1 and 4440 g. From any one of these parameters it is possible to estimate the size and approximate age of the animal.
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  • 9
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    Wiley-Blackwell
    In:  Journal of Zoology, 181 (4). pp. 527-559.
    Publikationsdatum: 2020-07-17
    Beschreibung: Features of the brain of this oceanic squid have been investigated and related, as far as possible, to its habits and mode of life. The body and arms are much vacuolated for buoyancy and the animal probably lives with the head upwards. The very long whip-like tentacles are not vacuolated and perhaps hang downwards. They are covered by numerous minute pedunculated suckers, perhaps providing a sticky surface. A special nerve running outside the brain carries signals from the arms and tentacles to the magnocellular lobe, which is very large and of complex structure. However, there are no giant cells and the mantle is weak. Propulsion is mainly by the large fins, which are controlled from the magnocellular lobe, presumably using the information from the arms and tentacles.
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  • 10
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    Wiley-Blackwell
    In:  Journal of Zoology, 158 (4). pp. 475-483.
    Publikationsdatum: 2020-07-17
    Beschreibung: An automatic food dispenser was designed for use with Octopus vulgaris Lamarck. One live crab was delivered each time the octopus pulled a white shape attached to the dispenser. The apparatus provided a continuous record of the time and frequency of feeding over periods of up to 15 days.
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  • 11
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    Wiley-Blackwell
    In:  Journal of Zoology, 188 (1). pp. 53-67.
    Publikationsdatum: 2020-07-17
    Beschreibung: The movements of the isolated buccal mass of Octopus vulgaris have been investigated. The beaks undergo rhythmic cycles of activity in the absence of applied stimulation and after electrical stimulation of the inter-buccal connective. Initial opening, closing, retraction and re-opening phases of movement are described. This cycle of movements is taken to resemble those in the intact animal. Anatomical and electrical evidence identifies the superior mandibular muscle as being partly responsible for the closing and retraction phases of movement. The inferior buccal ganglion determines the sequence of these buccal movements, but modification by sensory feed-back from the musculature is also implied. The preparation will allow a closer comparison of the control of movement in cephalopods and gastropods.
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  • 12
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    Wiley-Blackwell
    In:  Journal of Zoology, 179 (1). pp. 19-83.
    Publikationsdatum: 2020-07-17
    Beschreibung: Taonius megalops is a neutrally buoyant oceanic squid, very different in form when young and old. The young, has a round, sac-like mantle and relatively long tentacles, while the adult has an elongated cone-like mantle and relatively short tentacles. The transition in shape and form is gradual and has been followed in animals of between 3 and 180 mm dorsal mantle length. Statistical tests on various parameters investigated, both external and internal, revealed good correlation with the dorsal mantle length and confirmed the descriptions of the development of the chromatophores and subocular light organs with growth. It was concluded that these animals, captured in the Atlantic Ocean, all belonged to the species T. megalops Prosch 1849. This study has permitted us to suggest a tentative outline of the life cycle, although no adults were present in the material available.
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  • 13
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    Wiley-Blackwell
    In:  Journal of Zoology, 202 (3). pp. 441-447.
    Publikationsdatum: 2020-07-17
    Beschreibung: A crab which has been captured and paralysed by an octopus but retrieved 1 1/2 min later cannot at first be pulled apart by the experimenter: 27 min later it can be dismembered easily. This demonstrates that there is external digestion when Octopus vulgaris feeds upon crabs. However, it is strictly limited at this stage to the arthrodial membrane and the musculo-skeletal attachment mechanisms as the exoskeleton separates at the joints allowing the muscles to be drawn out of the appendages. And yet, two hours after capture, pieces of crab meat are still recognizable in the octopus's stomach. The process of paralysing and cleaning a crab was noticeably slowed after the surgical removal of the radula, salivary papilla or the lateral buccal palps.
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  • 14
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    Wiley-Blackwell
    In:  Journal of Zoology, 150 (1). pp. 1-9.
    Publikationsdatum: 2020-07-17
    Beschreibung: The changes in body weight of 12 octopuses, fed on fish or crabs, were followed under laboratory conditions for periods of 1 to 7 1/2 months. The food intake was estimated and compared with the changes in body weight of the octopuses; 25 to 55% of the total intake of food appeared to be incorporated. The range of the average increase in weight over the whole observation period of each of the animals was 1.9 to 7.7g per day (1 to 7 1/2 months); the mean value was 4.8g per day. The effect of changing the diet of small octopuses (fish or crab)was followed for four weeks but there was no evidence that alteration of the diet affected the rate of changes in body weight of animals of more than 47g initial body weight.
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  • 15
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    Wiley-Blackwell
    In:  Journal of Zoology, 186 (1). pp. 95-108.
    Publikationsdatum: 2020-07-17
    Beschreibung: Reproductive mechanisms in the seven species of the thecosomatous pteropod genus Limacina are described and compared. All species are protandrous hermaphrodites. Five species–L. bulimoides, L. helicina, L. lesueuri, L. retroversa and L. trochiformis–have a similar reproductive anatomy in which the gonoduct leading from the gonad to the common genital pore functions as a seminal vesicle in the male and is elaborated into mucous and albumen glands in the female. The male system consists of a prostate gland and penis connected to the common genital pore by an external ciliary tract. All five species have a free-swimming veliger stage which hatches from free-floating egg masses. Limacina helicoides has the same reproductive anatomy but is ovoviviparous, with embryos retained in capsules in the mucous gland until they are juveniles of 50 mm in shell diameter. Limacina inflata lacks mucous and albumen glands and a penis; a spermatophore formed by the prostate gland is used in aphallic sperm transfer. This species exhibits brood protection with un-encapsulated embryos retained in the mantle cavity until they are released as veligers measuring 0067 mm in diameter. L. inflata is the most abundant of the seven species despite lowered fecundity; reasons for its ecological success are discussed.
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  • 16
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    Wiley-Blackwell
    In:  In: Biofouling. , ed. by Dürr, S. and Thomason, J. Wiley-Blackwell, Weinheim, pp. 73-86. ISBN 978-1-4051-6926-4
    Publikationsdatum: 2012-02-23
    Materialart: Book chapter , PeerReviewed
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  • 17
    Publikationsdatum: 2020-07-27
    Beschreibung: Understanding the influence of growth temperature and carbon dioxide (CO2) on seed quality in terms of seed composition, subsequent seedling emergence and early seedling vigour is important under present and future climates. The objective of this study was to determine the combined effects of elevated temperature and CO2 during seed-filling of parent plants on seed composition, subsequent seedling emergence and seedling vigour of red kidney bean (Phaseolus vulgaris). Plants of cultivar ‘Montcalm’, were grown at daytime maximum/nighttime minimum sinusoidal temperature regimes of 28/18 and 34/24 °C at ambient CO2 (350 μmol mol−1) and at elevated CO2 (700 μmol mol−1) from emergence to maturity. Seed size and seed composition at maturity and subsequent per cent emergence, early seedling vigour (rate of development) and seedling dry matter production were measured. Elevated CO2 did not influence seed composition, emergence, or seedling vigour of seeds produced either at 28/18 or 34/24 °C. Seed produced at 34/24 °C had smaller seed size, decreased glucose concentration, but significantly increased concentrations of sucrose and raffinose compared to 28/18 °C. Elevated growth temperatures during seed production decreased the subsequent per cent emergence and seedling vigour of the seeds and seedling dry matter production of seed produced either at ambient or elevated CO2.
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  • 18
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    Wiley-Blackwell
    In:  Journal of Fish Biology, 75 (5). pp. 960-996.
    Publikationsdatum: 2015-09-14
    Beschreibung: Absolute barriers to dispersal are not common in marine systems, and the prevalence of planktonic larvae in marine taxa provides potential for gene flow across large geographic distances. These observations raise the fundamental question in marine evolutionary biology as to whether geographic and oceanographic barriers alone can account for the high levels of species diversity observed in marine environments such as coral reefs, or whether marine speciation also operates in the presence of gene flow between diverging populations. In this respect, the ecological hypothesis of speciation, in which reproductive isolation results from divergent or disruptive natural selection, is of particular interest because it may operate in the presence of gene flow. Although important insights into the process of ecological speciation in aquatic environments have been provided by the study of freshwater fishes, comparatively little is known about the possibility of ecological speciation in marine teleosts. In this study, the evidence consistent with different aspects of the ecological hypothesis of speciation is evaluated in marine fishes. Molecular approaches have played a critical role in the development of speciation hypotheses in marine fishes, with a role of ecology suggested by the occurrence of sister clades separated by ecological factors, rapid cladogenesis or the persistence of genetically and ecologically differentiated species in the presence of gene flow. Yet, ecological speciation research in marine fishes is still largely at an exploratory stage. Cases where the major ingredients of ecological speciation, namely a source of natural divergent or disruptive selection, a mechanism of reproductive isolation and a link between the two have been explicitly documented are few. Even in these cases, specific predictions of the ecological hypothesis of speciation remain largely untested. Recent developments in the study of freshwater fishes illustrate the potential for molecular approaches to address specific questions related to the ecological hypothesis of speciation such as the nature of the genes underlying key ecological traits, the magnitude of their effect on phenotype and the mechanisms underlying their differential expression in different ecological contexts. The potential provided by molecular studies is fully realized when they are complemented with alternative (e.g. ecological, theoretical) approaches.
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  • 19
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    Wiley-Blackwell
    In:  In: Biofouling. , ed. by Dürr, S. and Thomason, J. Wiley-Blackwell, Weinheim, pp. 100-108. ISBN 978-1-4051-6926-4
    Publikationsdatum: 2012-02-23
    Materialart: Book chapter , PeerReviewed
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  • 20
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    Wiley-Blackwell
    In:  Journal of Quaternary Science, 24 (7). pp. 664-676.
    Publikationsdatum: 2016-06-29
    Beschreibung: Quantitative X-ray diffraction analysis of the 〈2mm sediment fraction was carried out on 1257 samples (from the seafloor and 16 cores) from the Iceland shelf west of 188 W. All but one core (B997-347PC) were from transects along troughs on theNW to N-central shelf, an area that in modern and historic times has been affected by drift ice. The paper focuses on the non-clay mineralogy of the sediments (excluding calcite and volcanic glass). Quartz and potassium feldspars occupy similar positions in an R-mode principal component analysis, and oligoclase feldspar tracks quartz; these minerals are used as a proxy for ice-rafted detritus (IRD). Accordingly, the sum of these largely foreign minerals (Q&K) (to Icelandic bedrock) is used as a proxy for drift ice. A stacked, equi-spaced 100 a record is developed which shows both low-frequency trends and higher-frequency events. The detrended stacked record compares well with the flux of quartz (mg cm-2 a-1) at MD99-2269 off N Iceland. The multi-taper method indicated that there are three significant frequencies at the 95% confidence level with periods of ca. 2500, 445 and 304 a. Regime shift analysis pinpoints intervals when there was a statistically significant shift in the average Q&K weight %, and identifies four IRDrich events separated by intervals with lower inputs. There is some association between peaks of IRD input, less dense surface waters (from d18O data on planktonic foraminifera) and intervals of moraine building.
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  • 21
    Publikationsdatum: 2017-09-14
    Beschreibung: An experimental study was performed to disentangle parental and environmental effects on the growth of Atlantic cod Gadus morhua larvae and juveniles. Eggs were collected during the spawning season from spawning pairs (families) kept separately in specially designed spawning compartments. Newly hatched larvae were released simultaneously into two mesocosms of 2500 and 4400 m3. Larval growth was monitored by sampling over a 10 week period, after which juveniles were transferred to on-growing tanks, where they were tagged and kept for up to 2 years. Maternal origin was determined by individual microsatellite genotyping of the larvae (n = 3949, 24 families) and juveniles (n = 600). The results showed significant positive correlations between egg size and larval size during the whole mesocosm period. Correlations, however, weakened with time and were no longer significant at the first tank-rearing sampling at an age of 9 months. Significant family-specific differences in growth were observed. The coefficient of variation (c.v.) was calculated in order to examine variation in standard length of larvae during the mesocosm period. Inter-family c.v. was on average 69% of intra-family c.v. Differences in zooplankton densities between the two mesocosms were reflected in larval growth, condition factor and c.v. Low food abundance appeared to reduce c.v. and favour growth of larvae that showed relatively slow growth at high food abundance. It is suggested that genetically determined variation in growth potential is maintained by environmental variability.
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  • 22
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    Wiley-Blackwell
    In:  International Journal of Climatology, 29 (12). pp. 1731-1744.
    Publikationsdatum: 2020-03-19
    Beschreibung: The annual cycle of extreme 1-day precipitation events across the UK is investigated by developing a statistical model and fitting it to data from 689 rain gauges. A generalized extreme-value distribution (GEV) is fit to the time series of monthly maxima, across all months of the year simultaneously, by approximating the annual cycles of the location and scale parameters by harmonic functions, while keeping the shape parameter constant throughout the year. We average the shape parameter of neighbouring rain gauges to decrease parameter uncertainties, and also interpolate values of all model parameters to give complete coverage of the UK. The model reveals distinct spatial patterns for the estimated parameters. The annual mean of the location and scale parameter is highly correlated with orography. The annual cycle of the location parameter is strong in the northwest UK (peaking in late autumn or winter) and in East Anglia (where it peaks in late summer), and low in the Midlands. The annual cycle of the scale parameter exhibits a similar pattern with strongest amplitudes in East Anglia. The spatial patterns of the annual cycle phase suggest that they are linked to the dominance of frontal precipitation for generating extreme precipitation in the west and convective precipitation in the southeast of the UK. The shape parameter shows a gradient from positive values in the east to negative values in some areas of the west. We also estimate 10-year and 100-year return levels at each rain gauge, and interpolated across the UK.
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  • 23
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    Wiley-Blackwell
    In:  Journal of Zoology, 210 (1). pp. 137-147.
    Publikationsdatum: 2020-07-17
    Beschreibung: Epipelagic pterotracheid heteropods and young cranchiid squids rely primarily on transparency for concealment; yet they have opaque structures, the eyes and visceral organs that compose the visceral nuclei, which can only be camouflaged in other ways. These two groups have achieved convergent solutions to this problem. The visceral nucleus has a narrow and conical shape and a covering layer of iridophores that lies parallel to the surface of the organ. The eyes also have iridophore layers and tapered shapes. A minimal ventral silhouette results when the long axes of the visceral nucleus and eyes are oriented vertically, with the narrowest ends directed downward. In pterotracheids, this is actively achieved by tilting the nucleus and eyes and flexing the body and proboscis. In cranchiids, tilting of the organs alone suffices and adjustments are accomplished much more rapidly than in the pterotracheids.
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  • 24
    Publikationsdatum: 2016-07-28
    Beschreibung: Coccoliths were studied from the ODP Hole 1002C and core PL07-39PC in the Cariaco Basin. Increases in Emiliania huxleyi are synchronous with decreases of Gephyrocapsa oceanica and vice versa. A new index (GEX) based on the relative abundances of these two taxa is proposed, and correlates with various other proxies. It is shown that GEX can serve as upwelling proxy. This confirms that the Intertropical Convergence Zone shifted north during the Bølling/Allerød, south during the Younger Dryas and back north during the Preboreal. The upwelling proxy shows few discrepancies with the terrigenous record.
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  • 25
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    Wiley-Blackwell
    In:  Journal of Zoology, 207 (4). pp. 511-519.
    Publikationsdatum: 2020-07-17
    Beschreibung: A study of the diet of Sepia officinalis and S. elegans in the Ria de Vigo has shown that crustaceans are the most abundant prey in both species, followed by fish. Changes in the food composition of both species occur with growth. The type of prey eaten by the two sexes of these species is very similar. The possibility of trophic competition between juveniles of S. qficinalis and S. elegans is discussed.
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  • 26
    Publikationsdatum: 2019-09-11
    Beschreibung: To investigate and disentangle the role of major histocompatibility complex (MHC)‐based ‘good genes' and ‘compatible genes' in mate choice, three‐spined sticklebacks Gasterosteus aculeatus with specific MHC IIB genotypes were allowed to reproduce in an outdoor enclosure system. Here, fish were protected from predators but encountered their natural parasites. Mate choice for an intermediate genetic distance between parental MHC genotypes was observed, which would result in intermediate diversity in the offspring, but no mate choice based on good genes was found under the current semi‐natural conditions. Investigation of immunological variables revealed that the less‐specific innate immune system was more active in individuals with a genetically more divergent MHC allele repertoire. This suggests the need to compensate for an MHC‐diminished T‐cell repertoire and potentially explains the observed mate choice for intermediate MHC genetic distance. The present findings support a general pattern of mate choice for intermediate MHC diversity (i.e. compatible genes). In addition, the potentially dynamic role of MHC good genes in mate choice under different parasite pressures is discussed in the light of present and previous results.
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  • 27
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    Wiley-Blackwell
    In:  Journal of Fish Biology, 75 . pp. 290-294.
    Publikationsdatum: 2017-09-14
    Beschreibung: Individual behaviour of Atlantic cod Gadus morhua in the presence of hypoxic water was measured in situ in the vertically stratified Bornholm Basin of the Baltic Sea. Considering all recaptured individuals, the use of hypoxic habitat was comparable to data derived by traditional survey data, but some G. morhua had migrated towards the centre of the c.100 m deep basin and spent about a third of their time at oxygen saturation 〈50%, possibly to forage on zoobenthos. Maximal residence time per visit in such hypoxic water was limited to a few hours, allowing for the digestion of consumed prey items in waters with sufficient dissolved oxygen.
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  • 28
    Publikationsdatum: 2020-03-20
    Beschreibung: Speciation and the maintenance of recently diverged species has been subject of intense research in evolutionary biology for decades. Although the concept of ecological speciation has been accepted, its mechanisms and genetic bases are still under investigation. Here, we present a mechanism for speciation that is orchestrated and strengthened by parasite communities acting on polymorphic genes of the immune system. In vertebrates, these genes have a pleiotropic role with regard to parasite resistance and mate choice. In contrasting niches, parasite communities differ and thus the pools of alleles of the adapted major histocompatibility complex (MHC) also differ between niches. Mate choice for the best‐adapted MHC genotype will favour local adaptations and will accelerate separation of both populations: thus immune genes act as pleiotropic speciation genes –‘magic traits’. This mechanism should operate not only in sympatric populations but also under allopatry or parapatry. Each individual has a small subset of the many MHC alleles present in the population. If all individuals could have all MHC alleles from the pool, MHC‐based adaptation is neither necessary nor possible. However, the typically small optimal individual number of MHC loci thus enables MHC‐based speciation. Furthermore, we propose a new mechanism selecting against species hybrids. Hybrids are expected to have super‐optimal individual MHC diversity and should therefore suffer more from parasites in all habitats.
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  • 29
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    Wiley-Blackwell
    In:  Journal of Quaternary Science, 23 (1). pp. 3-20.
    Publikationsdatum: 2017-04-06
    Beschreibung: Investigations indicate that the Iceland Ice Sheet was reduced in size during MIS 3 but readvanced to the shelf break at the LGM. Retreat occurred very rapidly around 15 k–16 k cal. yr BP. By contrast, the margin of the ice sheet on the East Greenland shelf, north of the Denmark Strait, was at or close to the shelf break during MIS 3 and 2 and retreat starting ∼17 k cal. yr BP. Quantitative X-ray diffraction analysis of the 〈2 mm sediment fraction was undertaken on 161 samples from Iceland and East Greenland diamictons, and from cores on the slopes and margins of the Denmark Strait. Weight% mineralogical data are used in a principal component analysis to differentiate sediments derived from the two margins. The first two PC axes explain 52% of the variance. These associations are used to characterise sediments as being affiliated with (a) Iceland, (b) East Greenland or (c) mixed. The contribution from Iceland becomes prominent during MIS 2. The extensive outcrop of early Tertiary basalts on East Greenland between 68° and 71° N is an alternative source for basaltic clasts and North Atlantic sediments with εNd(0) values close to ±0.
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  • 30
    Publikationsdatum: 2020-07-24
    Beschreibung: Myrionecta rubra, a ubiquitous planktonic ciliate, has received much attention due to its wide distribution, occurrence as a red tide organism, and unusual cryptophyte endosymbiont. Although well studied in coastal waters, M. rubra is poorly examined in the open ocean. In the Irminger Basin, North Atlantic, the abundance of M. rubra was 0–5 cells/ml, which is low compared with that found in coastal areas. Distinct patchiness (100 km) was revealed by geostatistical analysis. Multiple regression indicated there was little relationship between M. rubra abundance and a number of environmental factors, with the exception of temperature and phytoplankton biomass, which influenced abundance in the spring. We also improve on studies that indicate distinct size classes of M. rubra; we statistically recognise four significantly distinct width classes (5–16, 12–23, 18–27, 21–33 μm), which decrease in abundance with increasing size. A multinomial logistic regression revealed the main variable correlated with this size distribution was ambient nitrate concentration. Finally, we propose a hypothesis for the distribution of sizes, involving nutrients, feeding, and dividing of the endosymbiont.
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  • 31
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    Wiley-Blackwell
    In:  Oikos, 117 . pp. 399-405.
    Publikationsdatum: 2017-01-31
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  • 32
    Publikationsdatum: 2016-05-27
    Beschreibung: A visible tephra horizon in the NGRIP ice core has been identified by geochemical analysis as the Fugloyarbanki Tephra, a widespread marker horizon in marine cores from the Faroe Islands area and the northern North Atlantic. An age of 26 740 ± 390 yr b2k (1s uncertainty) is derived for this tephra according to the new Greenland Ice Core Chronology (GICC05) based on multiparameter counting of annual layers. Detection of this tephra for the first time within the NGRIP ice core provides a key tie-point between marine and ice-core records during the transition between MIS 3 and 2. Identification of this volcanic event within the Greenland records demonstrates the future potential of using tephrochronology to precisely correlate palaeoarchives in widely separated localities that span the last glacial period, as well as providing a potential method for examining the extent of the radiocarbon marine reservoir effect at this time.
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  • 33
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    Wiley-Blackwell
    In:  In: Microbial Ecology of the Oceans. Wiley-Blackwell, Hoboken, NJ, pp. 383-441. 2. Edition ISBN 978-0-470-04344-8
    Publikationsdatum: 2019-12-06
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  • 34
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    Wiley-Blackwell
    In:  Fish and Fisheries, 9 (4). pp. 450-461.
    Publikationsdatum: 2016-12-13
    Beschreibung: Archived scales and otoliths constitute a unique source of DNA that potentially enables extension of the temporal scale of genetic studies of fish populations by decades and even centuries. We review recent insights into fish population and conservation genetics obtained using analysis of DNA from archived samples. This involves both new knowledge about demographic parameters and population structure in wild populations and insights into consequences of anthropogenic pressure resulting from over-harvesting, habitat degradation and stocking. We show that the latter category of studies have led to significant changes of management practices. Ongoing improvement of genetic methods will undoubtedly further expand the ability to utilize historical DNA samples. We envisage that temporal comparisons of large numbers of coding genes will lead to novel insights into selective responses of fish populations to anthropogenic challenges, particularly fisheries-induced selection and global warming. However, both acquisition and storage of historical DNA samples can be hurdles to temporal genetic analyses, while degradation and low copy number in historical DNA samples render genetic data from such sources prone to technical artefacts. We summarize recommendations for storage of samples and DNA extraction and provide checklists for validation of genotyping results. Finally, we stress that validation procedures also involve documentation of the time and population of origin of historical samples, and the inferences drawn should account for the technical and statistical uncertainties associated with historical DNA analysis.
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  • 35
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    Wiley-Blackwell
    In:  Oikos, 117 (5). pp. 754-762.
    Publikationsdatum: 2019-09-23
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  • 36
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    Wiley-Blackwell
    In:  Marine Ecology, 28 (1). pp. 152-159.
    Publikationsdatum: 2020-07-14
    Beschreibung: Meiobenthos densities and higher taxon composition were studied in an active gas seepage area at depths from 182 to 252 m in the submarine Dnieper Canyon located in the northwestern part of the Black Sea. The meiobenthos was represented by Ciliata, Foraminifera, Nematoda, Polychaeta, Bivalvia, Gastropoda, Amphipoda, and Acarina. Also present in the sediment samples were juvenile stages of Copepoda and Cladocera which may be of planktonic origin. Nematoda and Foraminifera were the dominant groups. The abundance of the meiobenthos varied between 2397 and 52,593 ind.·m−2. Maximum densities of Nematoda and Foraminifera were recorded in the upper sediment layer of a permanent H2S zone at depths from 220 to 250 m. This dense concentration of meiobenthos was found in an area where intense methane seeps were covered by methane-oxidizing microbial mats. Results suggest that methane and its microbial oxidation products are the factors responsible for the presence of a highly sulfidic and biologically productive zone characterized by specially adapted benthic groups. At the same time, an inverse correlation was found between meiofauna densities and methane concentrations in the uppermost sediment layers. The hypothesis is that the concentration of Nematoda and Foraminifera within the areas enriched with methane is an ecological compromise between the food requirements of these organisms and their adaptations to the toxic H2S.
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  • 37
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    Wiley-Blackwell
    In:  Magnetic Resonance in Chemistry, 45 (12). pp. 1072-1075.
    Publikationsdatum: 2020-07-15
    Beschreibung: Diosmetin, 5,7,3′-trihydroxy-4′-methoxyflavone shows chemopreventive, antimutagenic, and antiallergic effects. On the other hand, chrysoeriol, 5,7,4′-trihydroxy-3′-methoxyflavone induced nodABC-lacZ in Rhizobium meliloti. Both of them belong to hydroxymethoxy- flavones. One major difference between diosmetin and chrysoeriol is the substituted position of hydroxyl and methoxyl groups. In order to elucidate the relationships between their structures and activity, one of the first things to be done is the determination of their structures. However, most flavones occur widely in nature, and thus it is difficult to obtain in sufficient amounts from natural sources to identify their structures. Assignments of NMR data of several hydroxymethoxyflavones may help us to identify novel flavonoid compounds isolated from natural sources based on their NMR experiments. Therefore, we report here the complete assignments of 1H and 13C NMR data of 13 hydroxymethoxyflavones.
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  • 38
    Publikationsdatum: 2015-09-01
    Beschreibung: Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a high spatial resolution analytical method which has been applied to the analysis of silicic tephras. With current instrumentation, around 30 trace elements can be determined from single glass shards as small as ∼ 40 µm, separated from tephra deposits. As a result of element fractionation during the ablation process using a 266 nm laser, a relatively complex calibration strategy is required. Nonetheless, such a strategy gives analyses which are accurate (typically within ±5%) and have an analytical precision which varies from ∼ ±2% at 100 ppm, to ∼ ±15% at 1 ppm. Detection limits for elements used in correlation and discrimination studies are well below 1 ppm. Examples of the application of trace element analysis by LA-ICP-MS in tephra studies are presented from the USA, New Zealand and the Mediterranean. Improvements in instrumental sensitivity in recent years have the potential to lower detection limits and improve analytical precision, thus allowing the analysis of smaller glass shards from more distal tephras. Laser systems operating at shorter wavelengths (e.g. 193 nm) are now more widely available, and produce a much more controllable ablation in glasses than 266 nm lasers. Crater sizes of 〈10 µm are easily achieved, and at 193 nm many of the elemental fractionation issues which mar longer wavelengths are overcome. By coupling a short wavelength laser to a modern ICP-MS it should be possible to determine the trace element composition of glass shards as small as 20 µm and, providing sample preparation issues can be overcome, the determination of the more abundant trace elements in glass shards as small as 10 µm is within instrumental capabilities. This will make it possible to chemically fingerprint tephra deposits which are far from their sources, and will greatly extend the range over which geochemical correlation of tephras can be undertaken.
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  • 39
    Publikationsdatum: 2020-04-28
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  • 40
    Publikationsdatum: 2017-10-05
    Beschreibung: Natural communities are constantly changing due to a variety of interacting external processes and the temporal occurrence and intensity of these processes can have important implications for the diversity and structure of marine sessile assemblages. In this study, we investigated the effects of temporal variation in a disturbance regime, as well as the specific timing of events within different regimes, on the composition and diversity of marine subtidal fouling assemblages. We did this in a multi-factorial experiment using artificial settlement tiles deployed at two sites on the North East coast of England. We found that although there were significant effects of disturbances on the composition of assemblages, there were no effects of either the variation in the disturbance regime or the specific timing of events on the diversity or assemblage composition at either site. In contrast to recent implications we conclude that in marine fouling assemblages, the variability in disturbance regimes (as a driving force) is unimportant, while disturbance itself is an important force for structuring robust ecosystems.
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  • 41
    Publikationsdatum: 2017-12-19
    Beschreibung: This paper presents an event stratigraphy based on data documenting the history of vegetation cover, lake-level changes and fire frequency, as well as volcanic eruptions, over the Last Glacial–early Holocene transition from a terrestrial sediment sequence recovered at Lake Accesa in Tuscany (north-central Italy). On the basis of an age–depth model inferred from 13 radiocarbon dates and six tephra horizons, the Oldest Dryas–Bølling warming event was dated to ca. 14 560 cal. yr BP and the Younger Dryas event to ca. 12 700–11 650 cal. yr BP. Four sub-millennial scale cooling phases were recognised from pollen data at ca. 14 300–14 200, 13 900–13 700, 13 400–13 100 and 11 350–11 150 cal. yr BP. The last three may be Mediterranean equivalents to the Older Dryas (GI-1d), Intra-Allerød (GI-1b) and Preboreal Oscillation (PBO) cooling events defined from the GRIP ice-core and indicate strong climatic linkages between the North Atlantic and Mediterranean areas during the last Termination. The first may correspond to Intra-Bølling cold oscillations registered by various palaeoclimatic records in the North Atlantic region. The lake-level record shows that the sub-millennial scale climatic oscillations which punctuated the last deglaciation were associated in central Italy with different successive patterns of hydrological changes from the Bølling warming to the 8.2 ka cold reversal. Copyright © 2006 John Wiley & Sons, Ltd.
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  • 42
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    Wiley-Blackwell
    In:  Geofluids, 6 . pp. 241-250. Date online first: 2006
    Publikationsdatum: 2017-08-02
    Beschreibung: Groundwater seeps are known to occur in Eckernförde Bay, Baltic Sea. Their discharge rate and dispersion were investigated with a new schlieren technique application, which is able to visualize heterogeneous water parcels with density anomalies down to Drt ¼ 0.049 on the scale of millimeters. With the use of an inverted funnel, discharged fluids can be captured and the outflow velocity can be determined. Overall, 46 stations could be categorized by three different cases: active vent sites, seep-influenced sites, and non-seep sites. New seep locations were discovered, even at shallow near-shore sites, lacking prominent sediment depression, which indicate submarine springs. The detection of numerous seeps was possible and the groundwater-influenced area was defined to be approximately 6.3 km2. Flow rates of between 0.05 and 0.71 l m)2 min)1 were measured. A single focused fluid plume, which was not disturbed by the funnel was recorded and revealed a flux of 59.6 ± 20 ml cm)2 min)1 and it was calculated that this single focused plume would be strong enough to produce a flow rate through the funnel of 1.32 ± 0.44 l m)2 min)1. The effect of different seep-meter funnel sizes is discussed.
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  • 43
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    Wiley-Blackwell
    In:  Journal of Quaternary Science, 21 (6). pp. 645-675.
    Publikationsdatum: 2018-10-22
    Beschreibung: Intra‐plate volcanism in western Europe shows statistically significant episodicity during the Quaternary period. By comparing the known ages for eruptions in France and Germany, which are compiled here, with a composite oxygen isotope record, we have investigated the link between this episodic volcanism and the climate record over the last two million years. We show that increased volcanism between 415–400 ka and 17–5 ka correlates with warming phases at the end of the last Weichselian (Devensian) and earlier Elsterian (Anglian) glacial stages. The three significant caldera explosions in the eastern Eifel, Germany, are all associated with warming phases at the onset of interglacials. The growth and decay of nearby ice sheets suggest that surface changes in continental mass distribution during glacial Milankovich cycles could provide a mechanism for this correlation by means of the distal effects of flexural loading on the lithosphere.
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  • 44
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    Wiley-Blackwell
    In:  Journal of Fish Biology, 67 . pp. 1585-1602.
    Publikationsdatum: 2017-09-14
    Beschreibung: The seasonal growth trajectories of wild Atlantic salmon Salmo salar juveniles by age group within the Margaree River, Canada, are described. Circuli counts from scales were used to infer growth rates at different ages and these were used to predict the proportions of age 2‐ and 3‐year old smolts from different portions of the watershed. In the wild Atlantic salmon juveniles from the Margaree River, there was no bimodality in fork length frequencies and no 1 year old smolts were produced. Water temperature differences during the growing season were insufficient to explain the differences in growth rates and size at age among the sites sampled. There was a positive association between the growth rate in the first year and the subsequent age at smoltification. In the Margaree River, differences in tributary specific growth rates and size at age were expected to produce important differences in the relative ages at smoltification.
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  • 45
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    Wiley-Blackwell
    In:  Journal of Fish Biology, 66 . pp. 882-887.
    Publikationsdatum: 2017-09-14
    Beschreibung: An analysis of mass (M) and standard length (LS) data for larval, juvenile and adult sprat (Sprattus sprattus; Clupeidae) revealed marked changes in the allometric scaling factor (b in inline image). For sprat 〈44 mm LS, b was 5·0, whereas in larger juveniles and adults, b was c. 3·4 indicating a relatively protracted metamorphic period for this species.
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  • 46
    Publikationsdatum: 2017-01-31
    Beschreibung: Human activities have differentially altered biogeochemical cycling at local, regional and global scales. We propose that a stoichiometric approach, examining the fluxes of multiple elements and the ratio between them, may be a useful tool for better understanding human effects on ecosystem processes and services. The different scale of impacts of the elements carbon, nitrogen and phosphorus and the different nature of their biogeochemical cycles, imply a large variation of their stoichiometric ratios in space and time and thus divergent impacts on biota. In this paper, we examine the effects of anthropogenic perturbations on nutrient ratios in ecosystems in two examples and one case study. Altered stoichiometry in agricultural systems (example 1) can affect not only crop yield and quality but also the interactions between plants and their pollinators, pests and pathogens. Human activities have also altered stoichiometry in coastal ecosystems (example 2). Increased N loading has especially lead to increased N:P and reduced Si:N ratios, with detrimental effects on ecosystem services derived from coastal pelagic food webs, such as fish yield and water quality. The terrestrial–aquatic linkage in stoichiometric alterations is illustrated with a case study, the Mississippi River watershed, where anthropogenic activities have caused stoichiometric changes that have propagated through the watershed into the northern Gulf of Mexico. Coupled with altered stoichiometric nutrient inputs are the inherent differences in variation and sensitivity of different ecosystems to anthropogenic disturbance. Furthermore, the connections among the components of a watershed may result in downstream cascades of disrupted functioning. Applying a multiple element perspective to understanding and addressing societal needs is a new direction for both ecological stoichiometry and sustainability.
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  • 47
    Publikationsdatum: 2020-04-28
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  • 48
    Publikationsdatum: 2019-01-22
    Beschreibung: This contribution aims to report the reflections we had with the scientific community during two international workshops on reference materials for stable isotopes in Davos (2002) and Nice (2003). After evaluating the isotopic homogeneity of some existing reference materials, based on either certificates, literature data or specific inter-laboratory rounds, we confirm these as primary reference materials or propose new ones relative to which stable isotope compositions should be reported. We propose DSM-3 for Mg, NIST SRM 915a for Ca, L-SVEC for Li and NBS28 for Si. Cadmium does not yet have a well identified delta zero material, although three commercial mono-elemental Cd solutions have yielded the same isotopic composition relative to one another. In order to scale the linearity of any mass spectrometer, some secondary reference materials are also proposed: Cambridge-1 solution for Mg, the “Münster-Cd” and JEPPIM Cd solutions for Cd and the “Big Batch” silicate for Si. The team from Nancy propose to prepare a mixed spike solution for Li isotopes. Well-characterised natural samples such as ocean or continental waters, diatoms, sponges, rocks and minerals are needed to validate the entire analytical procedure, particularly to take into account the effect of sample mineralisation and of chemical manipulations for elemental separation prior to analysis.
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  • 49
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    Wiley-Blackwell
    In:  Oikos, 106 . pp. 93-104.
    Publikationsdatum: 2016-05-26
    Beschreibung: Ecological stoichiometry describes the biochemical constraints of trophic interactions emerging from the different nutrient content and nutrient demand of producers and consumers, respectively. Most research on this topic originates from well-mixed pelagic food webs, whereas the idea has received far less attention in spatially structured habitats. Here, we test how light as well as grazing and nutrient regeneration by consumers affects growth and biomass of benthic primary producers. In the first laboratory experiment, we manipulated grazer presence (two different snail species plus ungrazed control), in the second experiment we factorially combined manipulation of grazer presence and light intensity. We monitored snail and periphyton biomass as well as dissolved and particulate nutrients (nitrogen and phosphorus) over time. Grazers significantly reduced algal biomass in both experiments. Grazers affected periphyton nutrient content depending on the prevailing nutrient limitation and their own body stoichiometry. In the nitrogen (N-) limited first experiment, grazers increased N both in the periphyton and in the water column. The effect was stronger for grazers with lower N-content. In the phosphorus (P-) limited second experiment, grazers increased the P-content of the periphyton, but the grazer with lower N-content had additionally positive effects on algal N. Light reduction did not affect periphyton biomass, but increased chlorophyll-, N- and P-content of the periphyton. These experiments revealed that the indirect effects of grazers on periphyton were bound by stoichiometric constraints of nutrient incorporation and excretion.
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  • 50
    Publikationsdatum: 2019-09-23
    Beschreibung: Ecosystem resistance to a single stressor relies on tolerant species that can compensate for sensitive competitors and maintain ecosystem processes, such as primary production. We hypothesize that resistance to additional stressors depends increasingly on species tolerances being positively correlated (i.e. positive species co-tolerance). Initial exposure to a stressor combined with positive species co-tolerance should reduce the impacts of other stressors, which we term stress-induced community tolerance. In contrast, negative species co-tolerance is expected to result in additional stressors having pronounced additive or synergistic impacts on biologically impoverished functional groups, which we term stress-induced community sensitivity. Therefore, the sign and strength of the correlation between species sensitivities to multiple stressors must be considered when predicting the impacts of global change on ecosystem functioning as mediated by changes in biodiversity.
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  • 51
    Publikationsdatum: 2017-01-31
    Beschreibung: Recent experiments, mainly in terrestrial environments, have provided evidence of the functional importance of biodiversity to ecosystem processes and properties. Compared to terrestrial systems, aquatic ecosystems are characterised by greater propagule and material exchange, often steeper physical and chemical gradients, more rapid biological processes and, in marine systems, higher metazoan phylogenetic diversity. These characteristics limit the potential to transfer conclusions derived from terrestrial experiments to aquatic ecosystems whilst at the same time provide opportunities for testing the general validity of hypotheses about effects of biodiversity on ecosystem functioning. Here, we focus on a number of unique features of aquatic experimental systems, propose an expansion to the scope of diversity facets to be considered when assessing the functional consequences of changes in biodiversity and outline a hierarchical classification scheme of ecosystem functions and their corresponding response variables. We then briefly highlight some recent controversial and newly emerging issues relating to biodiversity-ecosystem functioning relationships. Based on lessons learnt from previous experimental and theoretical work, we finally present four novel experimental designs to address largely unresolved questions about biodiversity-ecosystem functioning relationships. These include (1) investigating the effects of non-random species loss through the manipulation of the order and magnitude of such loss using dilution experiments; (2) combining factorial manipulation of diversity in interconnected habitat patches to test the additivity of ecosystem functioning between habitats; (3) disentangling the impact of local processes from the effect of ecosystem openness via factorial manipulation of the rate of recruitment and biodiversity within patches and within an available propagule pool; and (4) addressing how non-random species extinction following sequential exposure to different stressors may affect ecosystem functioning. Implementing these kinds of experimental designs in a variety of systems will, we believe, shift the focus of investigations from a species richness-centred approach to a broader consideration of the multifarious aspects of biodiversity that may well be critical to understanding effects of biodiversity changes on overall ecosystem functioning and to identifying some of the potential underlying mechanisms involved.
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  • 52
    facet.materialart.
    Unbekannt
    Wiley-Blackwell
    In:  Fish and Fisheries, 5 (2). pp. 131-140.
    Publikationsdatum: 2016-11-25
    Beschreibung: Marine and fisheries scientists are increasingly using metapopulation concepts to better understand and model their focal systems. Consequently, they are considering what defines a metapopulation. One perspective on this question emphasizes the importance of extinction probability in local populations. This view probably stems from the focus on extinction in Levins' original metapopulation model, but places unnecessary emphasis on extinction–recolonization dynamics. Metapopulation models with more complex structure than Levins' patch-occupancy model and its variants allow a broader range of population phenomena to be examined, such as changes in population size, age structure and genetic structure. Analyses along these lines are critical in fisheries science, where presence–absence resolution is far too coarse to understand stock dynamics in a meaningful way. These more detailed investigations can, but need not, aim to assess extinction risk or deal with extinction-prone local populations. Therefore, we emphasize the coupling of spatial scales as the defining feature of metapopulations. It is the degree of demographic connectivity that characterizes metapopulations, with the dynamics of local populations strongly dependent upon local demographic processes, but also influenced by a nontrivial element of external replenishment. Therefore, estimating rates of interpopulation exchange must be a research priority. We contrast metapopulations with other spatially structured populations that differ in the degree of local closure of their component populations. We conclude with consideration of the implications of metapopulation structure for spatially explicit management, particularly the design of marine protected area networks.
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  • 53
    facet.materialart.
    Unbekannt
    Wiley-Blackwell
    In:  Fish and Fisheries, 5 (1). pp. 86-91.
    Publikationsdatum: 2017-02-02
    Beschreibung: Three simple fisheries indicators are presented: (i) percentage of mature fish in catch, with 100% as target; (ii) percent of specimens with optimum length in catch, with 100% as target; and (iii) percentage of ‘mega-spawners‘ in catch, with 0% as target, and 30–40% as representative of reasonable stock structure if no upper size limit exists. Application of these indicators to stocks of Gadus morhua, Sardinella aurita and Epinephelus aeneus demonstrate their usefulness. It is argued that such simple indicators have the potential to allow more stakeholders such as fishers, fish dealers, supermarket managers, consumers and politicians to participate in fisheries management and eventually hold and reverse the global pattern of convenience overfishing, which is defined here as deliberate overfishing sanctioned by official bodies who find it more convenient to risk eventual collapse of fish stocks than to risk social and political conflicts.
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  • 54
    Publikationsdatum: 2020-04-28
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  • 55
    Publikationsdatum: 2020-04-28
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  • 56
    facet.materialart.
    Unbekannt
    Wiley-Blackwell
    In:  Oikos (100). pp. 592-600.
    Publikationsdatum: 2017-01-31
    Beschreibung: Conceptual models predict counteractive effects of herbivores and nutrient enrichment on plant diversity and reversed effects of grazers under different nutrient regimes. I tested these hypotheses in 11 field experiments with periphyton communities in three different aquatic habitats (a highly eutrophic lake, an meso-eutrophic lake, and an meso-eutrophic part of the Baltic Sea coast) and in different seasons. Grazer access and nutrient supply were manipulated in a factorial design. Species richness and evenness were chosen as response variables. Both manipulated factors had significant and contrasting effects on diversity, with variable effect strength between sites and seasons. From the two aspects of diversity, evenness well reflected the changes in community composition. Fertilization tended to increase the dominance of few species and thus to decrease evenness, whereas grazers counteracted these effects by removing dominant life forms. The response of species richness was not as expected, since grazers decreased richness throughout, whereas nutrients had weaker effects but tended to increase richness. Species richness rather reflected changes in periphyton architecture. Grazers reduced algal richness presumably by co-consumption of rare species in the tightly connected periphyton assemblages, whereas enrichment may increase richness by providing more structure via increased dominance of filamentous species. Although grazer and nutrient effects on richness and evenness were opposing, there was no change in the effect of one factor by manipulation of the other.
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  • 57
    facet.materialart.
    Unbekannt
    Wiley-Blackwell
    In:  Journal of Fish Biology, 62 . pp. 253-276.
    Publikationsdatum: 2017-09-08
    Beschreibung: A set of histological characteristics to judge ovarian development was established and used to elaborate morphological criteria of 10 maturity stages of Baltic cod Gadus morhua sampled throughout the annual cycle to represent different macroscopic maturity stages. The applied characteristics confirmed most stages of the macroscopic scale, but the separation of late immature and resting mature females remained imprecise. Atretic vitellogenic oocytes or encapsulated residual eggs identified the resting condition morphologically, but not all ovaries with visible signs of previous spawning showed such features. One ovarian stage that was previously classified as ‘ripening’ was changed to ‘spawning’, owing to the prevalence of hydrated eggs and empty follicles. Ovaries with malfunctions were defined by a separate stage. Macroscopic criteria were revised by comparing the gross anatomy of ovaries with their histology. Female length and gonado-somatic index supported stage definitions, but substantial variation in Fulton's condition factor and the hepato-somatic index rendered these of little use for this purpose. The time of sampling influenced staging accuracy. A female spawner probability function based on the proportion of ripening and ripe specimens in early spring seems to be the most appropriate method to estimate spawner biomass and reproductive potential.
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  • 58
    facet.materialart.
    Unbekannt
    Wiley-Blackwell
    In:  Journal of Fish Biology, 63 . pp. 280-299.
    Publikationsdatum: 2017-09-13
    Beschreibung: During peak spawning of sprat Sprattus sprattus in the Baltic Sea in May–June egg specific gravity averaged ±s.d. 1·00858 ± 0·00116 g cm−3 but was significantly higher in the beginning and significantly lower towards the end of the spawning season. A close relationship was found between egg diameter and egg specific gravity (r2 = 0·71). This relationship, however, changed during the spawning season indicating that some other factor was involved causing the decrease in specific gravity during the spawning period. The vertical egg distribution changed during the spawning season: eggs were distributed mainly in the deep layers early in the season, occurred in and above the permanent halocline during peak spawning, and above the halocline towards the end of the spawning season. Consequently, poor oxygen conditions in the deep layers and low temperatures in layers between the halocline and the developing thermocline may affect egg development. Thus, opportunities for egg development vary over the spawning season and among spawning areas, and depending on frequency of saline water inflows into the Baltic Sea and severity of winters, between years
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  • 59
    Publikationsdatum: 2019-01-22
    Beschreibung: A compilation of δ44/40Ca (δ44/40Ca) data sets of different calcium reference materials is presented, based on measurements in three different laboratories (Institute of Geological Sciences, Bern; Centre de Géochimie de la Surface, Strasbourg; GEOMAR, Kiel) to support the establishment of a calcium isotope reference standard. Samples include a series of international and internal Ca reference materials, including NIST SRM 915a, seawater, two calcium carbonates and a CaF2 reference sample. The deviations in δ44/40Ca for selected pairs of reference samples have been defined and are consistent within statistical uncertainties in all three laboratories. Emphasis has been placed on characterising both NIST SRM 915a as an internationally available high purity Ca reference sample and seawater as representative of an important and widely available geological reservoir. The difference between δ44/40Ca of NIST SRM 915a and seawater is defined as -1.88 O.O4%o (δ44/42CaNISTSRM915a/Sw= -0.94 0.07%o). The conversion of values referenced to NIST SRM 915a to seawater can be described by the simplified equation δ44/40CaSa/Sw=δ44/40CaSa/NIST SRM 915a - 1.88 (δ44/42CaSa/Sw=δ44/42CaSa/NIST SRM 915a - 0.94). We propose the use of NIST SRM 915a as general Ca isotope reference standard, with seawater being defined as the major reservoir with respect to oceanographic studies.
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  • 60
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5840-5852 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In this study, micro-Raman spectroscopy has been used to investigate the vibrational properties of laterally epitaxial overgrown (LEO) GaN. The LEO GaN films were grown by metal organic chemical vapor deposition on a 2 in. sapphire substrate with SiN mask. Photoluminescence and polarized Raman scattering measurements have been performed in the two regions of GaN growth (wing and window regions). Raman scattering results are consistent with the lateral growth of GaN in the overgrown region. We have observed second-order Raman scattering in the wing and window regions of GaN. The observations of longitudinal optical phonon plasmon modes in the overgrown region demonstrate that LEO GaN is doped. We have carried out micro-Raman mapping of the local strain and free carrier concentration in the LEO GaN. Anharmonicity due to temperature in LEO GaN has also been investigated. The anharmonicity was found to increase with increasing temperature, and such temperature-induced anharmonicity introduces changes in the linewidth and line center position of the Raman active phonons. The phonon lifetimes in GaN are estimated in the LEO region as well as in the coherently grown region (window region). © 2002 American Institute of Physics.
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  • 61
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5853-5857 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Microstructural observations of gallium arsenide single crystals irradiated with a few tens of MeV C60 incident clusters (fullerenes) were performed. Normal and grazing incidences were investigated. Similar to in the case of silicon and germanium, cylindrical amorphous tracks whose diameters vary as a function of the projectile energy were found. However, for a given energy of the clusters, the track diameters are slightly different from one material to another. Also depending on the fullerene, energy is the length of the amorphous cylinder that formed along the projectile's path. The recrystallization process under an electron beam during transmission electron microscopy observation was analyzed and a higher growth rate for gallium arsenide compared to that of germanium was seen. © 2002 American Institute of Physics.
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  • 62
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5858-5866 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: This article presents an analysis of the response of silicon carbide to high velocity impact. This includes a wide range of loading conditions that produce large strains, high strain rates, and high pressures. Experimental data from the literature are used to determine constants for the Johnson–Holmquist constitutive model for brittle materials (JH-1). It is possible to directly determine the strength and pressure response of the intact material from test data in the literature. After the ceramic has failed, however, there are not adequate experimental data to directly determine the response of the failed material. Instead, the response is inferred from a comparison of computational results to ballistic penetration test results. After the constants have been obtained for the JH-1 model, a wide range of computational results are compared to experimental data in the literature. Generally, the computational results are in good agreement with the experimental results. Included are computational results that model interface defeat, which occurs when a high velocity projectile impacts a ceramic target and then dwells on the surface of the ceramic with no significant penetration. © 2002 American Institute of Physics.
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  • 63
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5867-5874 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The influence of ion-beam-produced lattice defects as well as H, B, C, N, O, and Si, introduced by ion implantation, on the luminescence properties of wurtzite GaN is studied by cathodoluminescence spectroscopy. Results indicate that intrinsic lattice defects produced by ion bombardment mainly act as nonradiative recombination centers and do not give rise to the yellow luminescence (YL) of GaN. Experimental data unequivocally shows that C is involved in the defect-impurity complex responsible for YL. In addition, C-related complexes appear to act as efficient nonradiative recombination centers. Implantation of H produces a broad luminescent peak which is slightly blueshifted with respect to the C-related YL band in the case of high excitation densities. The position of this H-related YL peak exhibits a blueshift with increasing excitation density. Based on this experimental data and results reported previously, the chemical origin of the YL band is discussed. © 2002 American Institute of Physics.
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  • 64
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5875-5881 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: InGaAs strained quantum well (SQW) samples with lattice matched InGaAsP quaternary barriers are grown on GaAs substrates by metalorganic vapor phase epitaxy. These SQW samples are characterized using photoluminescence, photomodulated reflectance, surface photovoltage spectroscopy and high-resolution x-ray diffraction techniques. The results based on numerical calculations are used to identify the various transitions seen in the spectra. The effect of growth temperature on the indium content of the InGaAs SQW with lattice matched InGaAsP quaternary barriers is studied. Contrary to the reported higher value of indium incorporation in InGaAs SQW with GaAs (In-free) barriers when the growth is performed at low temperatures, we find that the indium content of the InGaAs SQW with InGaAsP (In-based) quaternary barriers decreases if the SQW is grown at lower growth temperatures. A possible explanation for this behavior is provided. © 2002 American Institute of Physics.
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  • 65
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5882-5886 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: B1–NaCl-structure CrN(001) layers were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. X-ray diffraction analyses establish the epitaxial relationship as cube-on-cube, (001)CrN(parallel)(001)MgO with [100]CrN(parallel)[100]MgO, while temperature-dependent measurements show that the previously reported phase transition to the orthorhombic Pnma structure is, due to epitaxial constraints, absent in our layers. The resistivity increases with decreasing temperature, from 0.028 Ω cm at 400 K to 271 Ω cm at 20 K, indicating semiconducting behavior with hopping conduction. Optical absorption is low (α〈2×104 cm−1) for photon energies below 0.7 eV and increases steeply at higher energies. In situ ultraviolet photoelectron spectra indicate that the density of states vanishes at the Fermi level. The overall results provide evidence for CrN exhibiting a Mott–Hubbard type band gap. © 2002 American Institute of Physics.
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  • 66
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5887-5891 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We present a simple model for quantum well photodetectors for simultaneous middle infrared and near infrared or visible dual-band detection. We derive analytical formulas for the responsivity and detectivity as functions of the material and structural parameters. It is shown that the characteristics of near infrared and visible radiation detection can be sensitive to parameters of the hole transport, capture into quantum wells, and reflection from the electron emitting contact. We demonstrate that a properly designed photodetector can exhibit comparable performance for both middle infrared and near infrared (or visible) detection. The obtained results can be used to optimize the photodetector design and characteristics in both spectral ranges. © 2002 American Institute of Physics.
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  • 67
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5892-5895 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We investigated through ab initio total energy calculations the interaction of arsenic impurities with the core of a 30° partial dislocation in silicon. It was found that when an arsenic atom sits in a crystalline position near the dislocation core, there is charge transfer from the arsenic towards the dislocation core. As a result, the arsenic becomes positively charged and the core negatively charged. The results indicate that the structural changes around the impurity are very small in both environments, namely, the crystal and the dislocation core. In this scenario, the interaction between arsenic and the core is essentially electrostatic, which eventually leads to arsenic segregation. The segregation energy was found to be as large as 0.5 eV/atom. Additionally, it was found that arsenic pairing inside the core is not energetically favorable. © 2002 American Institute of Physics.
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  • 68
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5896-5901 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report on inelastic electron tunneling spectroscopy of a tunneling metal–oxide–semiconductor (MOS) device over an extended energy range compared to previous results. We have clearly observed the vibrations of the hydrogen-passivated (111)Si Pb center in this extended energy range. The assignment of this mode has been confirmed by a comparison with infrared experiments. Capabilities and limitations of the technique to detect and observe molecular vibrations in tunneling MOS devices are discussed. © 2002 American Institute of Physics.
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  • 69
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5902-5908 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report on the effect of rapid thermal annealing (RTA) on the photoluminescence (PL) properties of GaNxAs1−x/GaAs structures. In particular, a blueshift of the PL peak energy is observed when annealing the samples. The results are examined as a consequence of a RTA-induced nitrogen diffusion inside the GaNxAs1−x material rather than diffusion out of the alloy, which homogenizes initial nitrogen composition fluctuations. We propose a simple model that describes the RTA-induced blueshift of the low temperature PL peak energy. This model is in good agreement with experimental results and is consistent with recent studies in which lateral composition fluctuations in the GaNAs alloy were reported. © 2002 American Institute of Physics.
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  • 70
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5909-5914 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A quadratic dependence of the band gap energy on the alloy composition x was quantified for CuAl(SxSe1−x)2 films grown by low-pressure metalorganic vapor phase epitaxy, by means of photoreflectance and photoluminescence excitation spectroscopies. The bowing parameter for the A-exciton energy was estimated to be 0.20 eV. Several high-quality films grown on GaAs(001) substrates exhibited excitonic photoluminescence peaks in the blue to ultraviolet spectral ranges. The flow rate of the Al precursor was found to affect the incorporation ratio of S/Se, indicating that the Al–S compound plays a key role in controlling x. All films grown on GaAs(001) showed c(001) orientation. Conversely, the epitaxitial orientation of the films on GaP(001) changed from a(100) to c(001) with an increase in x. The critical value of x was around 0.5. The preferred orientations were explained by the natural selection rule under which the lattice strain in the epilayer is minimized. The residual strain in the 0.5-μm-thick epilayers on GaAs(001) was nearly constant for all x, although the lattice mismatch between the epilayer and the GaAs substrate varied from 0.62% to 5.39% with an increase in x. Consequently the strain was attributed to thermal stress. © 2002 American Institute of Physics.
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  • 71
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3556-3561 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The design and manufacture of diode lasers for gas analysis or multilayer thin-film optical devices used at low-temperature require the refractive index and the temperature coefficient of IV–VI compound over a significant temperature range. In this article, the refractive index and the absorption coefficient of Pb0.94Ge0.06Te thin film have been determined from transmission spectra measured at temperature between 80 and 300 K in the spectral range of 2.5–8.5 μm by fitting based on a Lorentz-oscillator model. It is found that the maximum refractive index occurs at 150 K, which corresponds to the structural phase transition from rocksalt to rhombohedrally distorted structure and reflects an increase of lattice polarizability. The value of the index of refraction is 5.350–6.000 in the spectral range of 4.0–8.5 μm for all measured temperatures, which reveals that Pb1−xGexTe is a highly refractive infrared material. The temperature coefficient of refractive index, dn/dT, is found to be −0.006–0.002 K−1 in the spectral range of 3.0–8.5 μm for all measured temperature. An empirical formula that fits the temperature coefficient in the spectral range of 4.0-8.5 μm is presented. The dependence of the transmission and absorption spectra on decreasing temperature can be explained by the modification of the energy-band structure due to rhombohedral distortions. The conclusion can be drawn that anomalies corresponding to the ferroelectric phase transition occur in both refractive index and absorption coefficient of Pb1−xGexTe alloy. © 2002 American Institute of Physics.
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  • 72
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3569-3578 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: This article uses molecular dynamics simulation to investigate the role of Ar ions in the ion physical vapor deposition (IPVD) process for different Ar+-to-Cu+ ratios, and to analyze the influence of different Ar+-to-Cu+ ratios on the trench filling morphology. Also compared are the trench filling morphology observed for the IPVD process with that found in the conventional collimated magnetron deposition process. The molecular dynamics simulation includes a trench model and a deposition model, and uses the many-body, tight-binding potential method to represent the interatomic force acting among neutral atoms. The interatomic force acting between the ions and the neutral atoms is modeled by the pairwise Moliere potential method. The simulation indicates that the incident Ar ions influence the trench filling mechanisms in two significant ways; peeling of the cluster atoms, which promotes migration of the cluster atoms along the sidewall, and breaking of the bridge which forms when two clusters of atoms join. Both phenomena are beneficial since they promote a more complete filling of the trench. © 2002 American Institute of Physics.
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  • 73
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3750-3758 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: FeCoV/TiNx and FeCoV/Ti multilayers having tFeCoV=30–700 Å prepared by dc magnetron sputtering are investigated by x-ray diffraction, stress, and magnetization measurements. The x-ray diffraction data of the FeCoV/TiNx system show the presence of interstitial N atoms in the FeCoV layers due to reactive sputtering of Ti with nitrogen. The interstitial N causes an expansion of the FeCoV lattice in FeCoV/TiNx for small tFeCoV. However, for the samples with large tFeCoV, no lattice expansion is observed. In addition to the lattice expansion caused by the intake of N atoms, a change in the crystalline texture of FeCoV layers is also observed as indicated by the enhancement of the FeCoV(200) peaks. The magnetic hysteresis measurements on the samples show that the easy direction of magnetization lies in the plane of the layers. They further show that there are easy and hard axes of magnetization within the plane of the FeCoV layers. The stress anisotropy present in the plane of the samples induces a magnetic anisotropy through magnetostrictive effects leading to the formation of the in- plane easy axis. The hysteresis and stress measurements carried out on these samples clearly show the influence of N on the in-plane magnetic anisotropy. The magnetoelastic energy in the case of the FeCoV/TiNx system, calculated from the stress data and from the magnetization measurements as a function of tFeCoV is found to agree over a large range of thickness, whereas the curves deviate significantly for small layer thickness. This deviation may be due to the role of the FeCoVNx phase. Hysteresis measurements also show that the remanence is about 95% for all the samples of the FeCoV/TiNx system. In contrast, the coercivity increases linearly with increasing tFeCoV in this system. The coercivity of the FeCoV/Ti system is larger and increases more rapidly with tFeCoV, as compared with the FeCoV/TiNx system. This behavior is attributed to a smaller grain size in the FeCoV/TiNx system due to the reactive sputtering of the Ti layers. However, there is no significant influence of N on the saturation magnetization of both systems. © 2002 American Institute of Physics.
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  • 74
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3764-3768 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The amorphous alloys Nd40Fe40Co5Al8B7, Nd57Fe20Co5Al10B8, and Nd57Fe20Cu5Al10B8 were prepared by copper mold casting, melt spinning, and mechanical alloying. Despite their similar x-ray diffraction patterns, samples display different magnetic and thermal behavior correlated with the method of preparation. The fully amorphous melt-spun ribbons exhibit relatively soft magnetic properties with coercivities (approximate)40 kA/m at room temperature and a Curie temperature (TC)(approximate)474 K. Apparently only the mold-cast cylinders of 3 mm diameter show hard magnetic behavior with a coercivity in the range of 258–270 kA/m (depending on composition) and have approximately the same TC as that of the melt-spun ribbons. An additional magnetic transition at 585 K due to the presence of Nd2Fe14B phase in the case of Nd40Fe40Co5Al8B7 cast rod has been observed. Heat treatment above crystallization temperature in as-cast Nd57Fe20Co5Al10B8 and Nd57Fe20Cu5Al10B8 samples destroys the hard magnetic properties. In contrast, mechanically alloyed amorphous samples are soft magnetic with maximum coercivity up to 11 kA/m but show an entirely different TC(approximate)680–740 K, which is rather characteristic of an Fe solid solution. The magnetic properties are discussed in terms of different local atomic environment and cluster sizes in amorphous samples prepared by different methods. © 2002 American Institute of Physics.
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  • 75
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3769-3774 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The phase evolution, microstructure, and magnetic properties of Nd8Fe86B6−xCx (x=0, 2, 4, 5, 6) melt-spun ribbons were systematically studied as a function of C content. It was found that the addition of C decreases the glass-forming tendency of the as-spun ribbons significantly. A uniform nanoscale exchange coupled Nd2Fe14(BC)/α-Fe microstructure with an average grain size of 20–25 nm can be developed in the directly quenched ribbons with C contents up to 4 at. %. Further increase of C content to x=5 leads to, in the optimally quenched ribbons, the presence of an undesirable Nd2Fe17Cx phase in addition to the 2:14:1 and α-Fe phases, whereas the alloy ribbon containing 6 at. % C consists almost entirely of the soft magnetic Nd2Fe17Cx and α-Fe phases. Subsequent annealing induces a transformation of the 2:17:Cx phase to the 2:14:1 phase +α-Fe in the ribbons with x=5 and 6, resulting in the formation of a composite 2:14:1/α-Fe structure having relatively large crystallite sizes. Magnetic measurements revealed that, for the optimally processed samples, replacement of up to 4 at. % of B by C significantly increases the coercivity iHc, with only slight reduction in remanence Jr; an optimum coercivity of 542 kA/m was obtained in the Nd8Fe86B2C4 ribbon compared with 430 kA/m for the Nd8Fe86B6 ribbon. Excessive substitution of C (x〉4) causes a drastic deterioration of both iHc and Jr due to the microstructural coarsening. Moreover, the Curie temperature of the 2:14:1 phase in the samples decreases progressively with increasing C content from 312 °C for x=0 to 270 °C for x=6. © 2002 American Institute of Physics.
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  • 76
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3797-3805 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Domain structures of unpoled as well as poled (along [001]- and [110]-direction) Pb(Zn1/3Nb2/3)O3 (PZN)-8% PbTiO3 (PT) and Pb(Mg1/3Nb2/3)O3 (PMN)-29% PT single crystals have been investigated by scanning force microscopy (SFM) in the piezoresponse mode, at room temperature. Antiparallel domain structures have been detected mostly in unpoled crystals of both materials, with a fingerprint pattern in (001)-oriented PZN-8% PT crystal. The ferroelastic domain wall has been identified in poled (110)-oriented PZN-8% PT crystal. "Writing" of ferroelectric domains has been performed by applying a dc voltage to the SFM tip. Local re-poling has been observed for all unpoled as well as for poled (001)-oriented crystals at the voltage ±60 V. Local electrical switching was successful in poled (110)-oriented PMN-29% PT at higher voltage (±120 V) but was not successful in poled (110)-oriented PZN-8% PT crystal. Domain-engineered crystals poled in [110]-direction seem to exhibit more stable (in the sense of local re-poling properties) domain arrangement. Hysteretic d(E) dependencies were observed by local application of an ac voltage. © 2002 American Institute of Physics.
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  • 77
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3785-3796 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In the study of elastic and piezoelectric fields in semiconductors due to buried quantum dots (QDs), the semicoupled piezoelectric model is commonly adopted. However, its accuracy and suitability have never been studied. In this article, by developing a fully coupled piezoelectric model and deriving the analytical elastic and piezoelectric fields based on this and the semicoupled models, we are able to verify that when the piezoelectric coupling is weak, like GaAs with the electromechanical coupling factor g=0.04, the semicoupled model predicts very accurate results as compared to those based on the fully coupled model. However, if the piezoelectric coupling is relatively strong, like AlN with g=0.32, we have shown that the semicoupled model gives very serious errors or even totally wrong results. Applying these two models to a uniformly strained AlN layer grown along the polar axis has also confirmed our observation. Therefore, for semiconductors like AlN, the fully coupled model presented in this article must be employed in order to give a reliable and accurate prediction for the elastic and piezoelectric fields. Also presented in this article is the distribution of the piezoelectric field on the surface of a half-space GaAs due to a buried QD located at 2 nm below the surface with a volume 4π/3 (nm)3. It is observed that the horizontal electric field on the traction-free and insulating surface shows some special features and its maximum magnitude can be as high as 3.5×107 V/m when the uniform mismatch eigenstrain is 0.07. Furthermore, the piezoelectric field on the traction-free and conducting surface exhibits different characters as compared to the traction-free and insulating case. © 2002 American Institute of Physics.
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  • 78
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3829-3840 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Optically thin palladium metal films evaporated on different silicon based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide and silicon nitride substrates is used to recover information regarding the reflectivity inversion and α/β-phases of the palladium complex after both first and multiple gas cycling. Atomic force microscopy confirms the formation of metal nanostructures following exposure to hydrogen of the optically thin palladium films. © 2002 American Institute of Physics.
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  • 79
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3847-3854 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A detailed systematic study on the growth morphology of carbon nanotubes (CNTs) on Si in atmospheric pressure thermal chemical vapor deposition was undertaken. The role of NH3 for vertical alignment of CNTs was investigated. The direct cause for the alignment was a dense distribution of the catalytic metal particles, but that the particles are maintained catalytically active under amorphous carbon deposits was established by NH3. It allows a dense nucleation of the CNTs, and consequently, assists vertical alignment through entanglement and mechanical leaning among the tubes. The CNTs grew in a base growth mode. Since Ni is consumed both by silicide reaction and by capture into the growing tube, the growth stops when Ni is totally depleted. It occurs earlier for smaller particles, and thus a long time of growth results in a thin bottom with poor adhesion. © 2002 American Institute of Physics.
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  • 80
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3864-3868 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Pt/Ti/WSi/Ni ohmic contacts to n-SiC, initially annealed at 950 and 1000 °C for 30 s, were evaluated for thermal stability via pulsed/cyclic thermal fatigue and aging experiments at 650 °C. Modifications of material properties in response to cyclic thermal fatigue and aging tests were quantitatively assessed via current–voltage measurements, field emission scanning microscopy, atomic force microscopy, and Rutherford backscattering spectrometry. Negligible changes in the electrical properties, microstructure, and surface morphology/roughness were observed for both annealed ohmic contacts in response to 100 cycles of acute cyclic thermal fatigue. Aging of the 950 °C annealed contact for 75 h at 650 °C resulted in electrical failure and chemical interdiffusion/reaction between the contact and SiC substrate. The 1000 °C annealed contact retained ohmicity after 100 h of aging and was found to be chemically and microstructurally stable. These findings indicate that the 1000 °C annealed Pt/Ti/WSi/Ni ohmic contact to n-SiC is thermally stable and merits strong potential for utilization in high temperature and pulsed power devices. © 2002 American Institute of Physics.
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  • 81
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 4983-4987 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Transmission electron microscopy and x-ray diffraction measurements reveal the presence of stacking faults (SFs) in undoped cubic GaN thin layers. We demonstrate the importance of the defects in the interfacial region of the films by showing that the SFs act as nucleation sites for precipitates of residual impurities such as C and Si present in the GaN layers grown on SiC(001) substrates. We used the imaging secondary ion mass spectroscopy technique to locate these impurities. The systematic decrease of the SF density as a function of the layer thickness is explained by an annihilation mechanism. Finally, the effects of usual dopants on the structural properties of GaN layers are discussed. It is shown that Mg has a tendency to incorporate out of the Ga site by forming Mg precipitates for a concentration higher than 1019 cm−3 in contrast with the results found for heavily Si doped layers. © 2002 American Institute of Physics.
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  • 82
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5016-5023 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Results of thermally stimulated photoluminescence (TSL) measurements in poly(2,5-dioctoxy p-phenylene vinylene) (DOO–PPV) are reported. The obtained results are analyzed in terms of the hopping model of TSL in disordered organic materials. It is shown that the experimentally obtained TSL curve can be fitted on the basis of a double-peak Gaussian density-of-states distribution. The upper peak is associated with intrinsic localized states while the lower one can be ascribed to aggregates. The latter assignment is also supported by measurements of steady-state and time-resolved photoluminescence in DOO–PPV films and solutions. Possible mechanisms of charge carrier photogeneration in DOO–PPV are discussed. © 2002 American Institute of Physics.
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  • 83
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5029-5034 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A theoretical analysis based on a perturbation method is used to elucidate the results of attenuated total reflection (ATR) measurements performed on silicon oxide layers of different thicknesses on silicon substrates. This analysis shows that the absorbance ATR spectrum in p polarization is the image of the layer energy loss function, under specific conditions. It is pointed out that the enhanced sensitivity of ATR is controlled by the air gap thickness, the optical properties of the media involved, and the probing light polarization. An exact ATR spectrum simulation using a matrix formalism showed that straightforward interpretation in terms of the layer dielectric function is limited to a very narrow layer thickness range. The ATR spectrum fitting process is considered for layers out of this range and evaluated for the interpretation of experimental silicon oxide layer spectra. © 2002 American Institute of Physics.
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  • 84
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5041-5044 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The effect of Nb codoping on the optical properties of the PbWO4:Mo scintillator is investigated by radio- and thermoluminescence, scintillation decay, and light yield measurements. Steady-state radioluminescence efficiency of PbWO4:Mo,Nb with optimized doping concentrations (2750 and 350 molar ppm, respectively) becomes up to 20 times higher with respect to that of undoped PbWO4 and is comparable to that of Bi3Ge4O12. However, slow components down to several tens of microseconds appear in the time decay. Their existence may be related to the presence of traps monitored by thermoluminescence. © 2002 American Institute of Physics.
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  • 85
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5051-5054 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The thermal expansion coefficients of a single crystal and ceramic of Nb2O5 are measured in the temperature range of −200–500 °C by the dilatometer technique. Both single crystals and ceramics of Nb2O5 and Nb2O5(1−x):xTiO2 show negative thermal expansion in this temperature range. Some contribution to the result could be due to the presence of the Magneli phases. The main phase transition temperature, which also matches with the dielectric anomaly, occurs at (approximate)150 °C. © 2002 American Institute of Physics.
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  • 86
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5066-5071 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Germanium-doped ZnSe epilayers have been grown on (001)-oriented semi-insulating GaAs substrates by molecular beam epitaxy. The photoluminescence (PL) spectra exhibit strong near-band edge emission similar to those from undoped, chlorine-doped and gallium-doped samples, though some differences exist. The prominent PL peak at 2.795 eV (10 K) is attributed to the germanium-bound exciton recombination and is accompanied by free exciton (2.802 eV), Ia-type exciton (2.785 eV) and Iv-type exciton (2.775 eV) emission peaks. Following an increase in temperature, the intensity of all the IGe, Ia, and Iv emission peaks decreases gradually, indicating the presence of nonradiative recombination mechanisms with thermal activation energies of 40, 70, and 50 meV respectively. However, for the Iv peak, there is one additional nonradiative recombination mechanism in accordance with the thermally activated transfer of excitons from the Iv-type centers to Ia-type centers. This nonradiative recombination mechanism with activation energy of 9 meV is responsible for the decrease of the Iv peak intensity when the sample temperature is changed from 15 to 100 K. Following an increase in temperature, the Iv peak, Ia peak, and germanium-related peak disappear gradually and successively. Finally, the PL spectrum is dominated by free exciton emission at temperatures exceeding 210 K. © 2002 American Institute of Physics.
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  • 87
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5089-5092 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Electronic property variations of a two-dimensional electron gas (2DEG) in modulation-doped step quantum wells due to an embedded potential barrier were studied by performing Shubnikov–de Haas (SdH), Van der Pauw–Hall-effect, and cyclotron resonance measurements on two kinds of InxGa1−xAs/InyAl1−yAs step quantum wells which were one without and the other with an embedded barrier. The fast Fourier transformation results for the SdH data at 1.5 K indicated the electron occupation of two subbands in both step quantum wells. The total electron carrier density and the mobility of the 2DEG in the step quantum well with an embedded barrier were smaller than those in the quantum well without an embedded barrier. The electron effective masses were determined from the slopes of the main peak absorption energies as functions of the magnetic field, and satisfied qualitatively the nonparabolicity effects in both quantum wells. The electronic subband energies, the wave functions, and the Fermi energies were calculated by using a self-consistent method taking into account exchange-correlation effects together with strain and nonparabolicity effects. These present results indicate that the electronic parameters in modulation-doped InxGa1−xAs/InyAl1−yAs step quantum wells are significantly affected by an embedded barrier. © 2002 American Institute of Physics.
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  • 88
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5116-5124 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The direct and assisted-by-trap elastic tunnel current in metal–oxide–semiconductor capacitors with ultrathin gate oxide (1.5–3.6 nm) has been studied. Bardeen's method has been adapted to obtain the assisted tunnel current, in addition to the direct tunnel current. The dependence of the assisted current on the trap distribution in energy has also been analyzed. This allows us to obtain the trap distribution in energy from experimental current curves. Finally, we have analyzed the role of the image force, the inclusion of which can avoid a barrier height dependence on the oxide thickness. © 2002 American Institute of Physics.
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  • 89
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5135-5140 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: X-ray photoemission and x-ray photoabsorption were used to study the composition and the electronic structure of ytterbium-doped strontium fluoroapatite (Yb:S-FAP). High resolution photoemission measurements on the valence band electronic structure and Sr 3d, P 2p and 2s, Yb 4d and 4p, F 1s, and O 1s core lines were used to evaluate the surface and near surface chemistry of this fluoroapatite. Element specific density of unoccupied electronic states in Yb:S-FAP were probed by x-ray absorption spectroscopy at the Yb 4d (N4,5 edge), Sr 3d (M4,5 edge), P 2p (L2,3 edge), F 1s, and O 1s (K edges) absorption edges. These results provide measurements of the electronic structure and surface chemistry of this material. © 2002 American Institute of Physics.
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  • 90
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5149-5154 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Extremely thin titanium nitride (TiN) barrier layers for Cu based interconnects were deposited using metal organic chemical vapor deposition. The effect of the subsequently performed nitrogen/hydrogen plasma treatment on the microstructure, composition, and electrical properties of these films is studied using conventional and high resolution transmission electron microscopy, Auger electron spectroscopy, and four point probe resistivity measurements. In the studied system the crystallization of the TiN film starts from an amorphous matrix and a polycrystalline morphology is developed upon the H2/N2 plasma treatment. After a short plasma treatment, most of the film is already crystalline and consists of grains of a few nanometers in diameter. Continued plasma treatment leads to grain growth and a significant reduction of contaminants such as oxygen and carbon. The resistivity of the films drops with plasma treatment time, and a correlation between resistivity and oxygen content is found, which suggests that oxygen in the grain boundaries plays a decisive role for the resistivity of the films. It is shown that the oxygen in the grain boundaries leads to an electron reflectance of 0.9. Thus, the oxygen accumulation in the grain boundaries is the limiting factor for the reduction of film resistivity by plasma treatment. © 2002 American Institute of Physics.
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  • 91
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5155-5157 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The effect of periods on the accumulation and release of stress in GaAs/AlInAs superlattices structure is reported here. It is observed that in GaAs/AlInAs superlattices, when the Indium (In) content is greater than 10%, stress accumulates monotonically as the number of period increases. In GaAs/AlInAs superlattices with an In content of 5%, the accumulated stress is larger when the number of periods is less than 10. However when the number of periods exceeds 10, it was observed that suddenly there is a significant increase in defects and stress release. However, with any further increase in period number, there is once again an accumulation of stress. © 2002 American Institute of Physics.
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  • 92
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5158-5162 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Five electron traps were detected successfully in heavily Si-doped GaAs and AlxGa1−xAs of low Al content with a Si concentration of above 1×1019 cm−3 using deep level transient spectroscopy. The junctions were grown by liquid phase epitaxy and were strongly compensated. The traps were investigated for functions of the Si concentration and the AlAs mole fraction. The traps are discussed in terms using their spectra and concentration as opposed to the previous results which used point defects in the GaAs and AlGaAs. The traps show distinctive features, which can be attributed to strongly Si-compensated crystals. Three traps among them were confirmed to be DX centers. © 2002 American Institute of Physics.
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  • 93
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5170-5175 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Using a theoretical model of a bilayer organic light emitting diode, we calculate numerically the evolution of carrier densities and electric fields inside the device. The obtained results allow us to detail injection and accumulation of carriers during transient excitation. Charge densities as a function of applied voltage present two distinct thresholds which determine three operating ranges: no injection, unipolar injection, and bipolar injection. Dynamically these thresholds depend on the rise time of the applied voltage and have a clear signature in the current densities. We show that the electroluminescence threshold has static and dynamic values which may be different. Calculated external current in response to a voltage ramp presents two steps related to the evolution of the capacity of the device. This capacitive behavior is observed experimentally. © 2002 American Institute of Physics.
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  • 94
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5176-5181 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: This paper analyzes a simplified rate equation model of localized exciton emission in GaInN. Expressions for temperature dependent photoluminescence (PL) efficiency and decay time are derived and compared with time integrated (TIPL) and time resolved photoluminescence (TRPL) data for a series of multiquantum well light emitting diodes with varying In composition in the active region. Time resolved photoluminescence is measured up to relatively high temperature (540 K) and a decreasing efficiency coupled with a peak energy decay time that is weakly dependent on temperature is observed. The decay time at peak emission energy begins to decrease at a temperature that depends on the In content in the quantum wells. The analysis developed here demonstrates that application of the expressions τr=τpl/η and τnr=τpl/(1−η) is not sufficient to determine radiative and nonradiative lifetimes from TRPL and TIPL data in the GaInN system. (Here τr is the radiative decay time, τnr is the nonradiative decay time, τpl is the measured PL decay time, and η is the measured TIPL intensity normalized to the low temperature intensity.) GaInN with even small amounts of In exhibits highly efficient luminescence due to recombination through localizing centers. As relaxation occurs into both defects and localizing states after initial generation with the above GaInN band gap excitation, the number of carriers arriving at localization centers can change with temperature. This temperature dependent change should be considered when calculating relevant decay times from TRPL and TIPL data. This mechanism is distinct from an increase in the intrinsic radiative decay time obtained by applying conventional analysis to extract radiative and nonradiative lifetimes from TRPL and TIPL data. © 2002 American Institute of Physics.
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  • 95
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5200-5202 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Avalanche breakdown voltages were determined for a series of In0.53Ga0.47As p-i-n diodes with i-region thicknesses ranging from 0.4 to 4.79 μm using measurements of reverse dark current and phase-sensitive photomultiplication. Despite its narrower bandgap In0.53Ga0.47As is found to have a very similar breakdown voltage to GaAs. © 2002 American Institute of Physics.
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  • 96
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5203-5207 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Multi-quantum-well GaN/InGaN heterojunction diodes prepared by metalorganic chemical vapor deposition on sapphire showed effects of strong tunneling in their I–V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice (SL). The injection of moderately high forward currents through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL. These results may have relevance to the aging characteristics of light-emitting diodes under bias. © 2002 American Institute of Physics.
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  • 97
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5221-5226 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Current and voltage noise measurements have been carried out on nanoparticle WO3 films. The fluctuation dissipation theorem holds, which indicates that the observed noise is an equilibrium phenomenon. Results on the thinnest films show that noise measurements can be used for quality assessment of nanocrystalline insulating films. © 2002 American Institute of Physics.
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  • 98
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    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5227-5229 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Fe–Co–Nd–Dy–B glassy alloys are a current type of ferromagnetic material. To increase their glass-forming ability against the growth of a crystallization phase, the effects of the addition of transition metals TM (TM=V, Nb, Ta, Cr, Mo, and W) on the glass-formation ability and magnetic properties of Fe62Co9.5Nd3Dy0.5B25 glassy alloy have been investigated. The substitution of 2 at. % elements TM (=Nb, Ta, Mo, and W) for Fe and Co increases crystallization temperature Tx and decreases the onset temperature of solidification Tm, leading to a significant increase in the thermal stability against crystallization for Fe60.3Co9.2TM2Nd3Dy0.5B25. The difference (ΔTx=Tx−Tg) between Tx and the glass transition temperature Tg increases from 55 K at 0 at. % TM to 87 K at 2 at. % TM. The bulk glassy alloy with a diameter up to 1.2 mm was produced by copper mold casting. Also no distinct changes in Tg, Tx and ΔTx are seen for the addition of Cr and V. The results can be explained by the difference of atomic size in the additional elements. The saturation magnetization decreases slightly by the addition of 2 at. % TM elements. The saturation magnetization and coercive force of glassy Fe60.3Co9.2TM2Nd3Dy0.5B25 alloys are 1.13 to 1.19 T and 3.85–4.98 A/m, respectively. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 99
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5234-5239 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Ti/Co/NiFe/Al-oxide/NiFe (F1) and Co/NiFe/Al-oxide/NiFe (F2) junction films were characterized using high-resolution electron microscopy (HREM), Lorentz transmission electron microscopy (LTEM), and alternating gradient force magnetometry (AGFM). HREM images showed that the Ti seed layer induced a strong 〈111〉 texture in the bottom Co/NiFe bilayer. The ferromagnet/Al-oxide interfaces in F1 showed correlated waviness, while the interface waviness in F2 appeared uncorrelated. Thus, "orange-peel" coupling effect was more significant in F1 than in F2, which was confirmed by the steep slope of the magnetization curve in the "antiparallel" magnetization configuration for F1. The LTEM in situ magnetizing experiment results and the AGFM measurement of magnetization curves showed that both junction films possessed a two-stage magnetization reversal characteristic—magnetization of the top NiFe layer reversed first followed by the reversal of the bottom Co/NiFe bilayer. LTEM observation revealed that the magnetization reversal of the top NiFe layer was via domain wall motion, while the reversal of the bottom Co/NiFe bilayers was mainly by wall motion together with a small degree of moment rotation. Domain wall mobility in the Co/NiFe bilayer of F1 was higher due to the strong crystallographic texture and large grain size appeared in the bilayer. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 100
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% with a squareness ratio of 1 and no degradation of MR ratio at 103 Ω μm2 ordered junction resistance. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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