Steep Threshold Research Articles

Characterisation of reaction layers on copper surfaces formed in aqueous chloride and sulphate ion containing electrolytes

AbstractCopper is in a passive state in a potential range of 50 mVH to about 300 mVH in contact with an aqueous electrolyte at neutral pH caused by the formation of a thin tarnishing layer consisting of copper(I)‐corrosion products. This layer inhibits more the anodic than the cathodic partial reaction of the corrosion process. The steep threshold in the transpassive range leads to the oxidation of copper and the formation of secondarily formed reaction layers.In sulphate containing electrolytes voluminous reaction layers of crystalline non‐protective copper(I)‐oxide are formed. The manifestation of corrosion is general attack. In chloride containing electrolytes a two‐fold reaction layer is formed, copper(I)‐chloride underneath copper(I)‐oxide. This amorphous copper(I)‐oxide is formed via hydrolysis of copper(I)‐chloride and inhibits anodic metal dissolution. The only observed manifestation of corrosion is repassivating pitting.

Room temperature photon avalanche up-conversion in Er-doped ZBLAN glass

Photon avalanche up-conversion emission at 550 nm with a steep threshold has been observed in Er-doped ZBLAN glass. This is, to the best of our knowledge, the first time that this effect was found for Er3+ ion, in a glass, and at room temperature. The excitation wavelength is 579.2 nm which is far away from any electronic absorption from the ground state. An extremely long rise time of the signal of up to about 4 s has been observed for the up-converted emission at 550 nm. An excitation mechanism of this photon avalanche process is proposed.

Implementation of new superconducting neural circuits using coupled SQUIDs

A novel superconducting neuron circuit and two types of variable synapses, which are based on superconducting quantum interferometer devices (SQUIDs), are presented. A neuron circuit with good input-output isolation and steep threshold characteristics is accomplished using a combination of a single-junction SQUID coupled to a double-junction SQUID. The quantum state of the single-junction SQUID represents the neuron state, and the output voltage of the double-junction SQUID, which is operated in a nonlatching mode with shunt resistors, is a sigmoid-shaped function of the input. Both variable synapse circuits are composed of multiple shunted double-junction SQUIDs. The first type changes its conductance value by using both superconducting and voltage states. The second variable synapse circuit changes its output current digitally by switching its bias currents. Besides numerical simulations of the circuit characteristics, we have fabricated superconducting neural chips in a Nb/AlO/sub x//Nb Josephson junction technology. The fundamental operation of each element and a 2-bit neural-based A/D converter have been successfully tested. A learning system with a variable synapse is also discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Otoacoustic emissions from the cochlea of the ‘constant frequency’ bats, Pteronotus parnellii and Rhinolophus rouxi

During stimulation with continuous pure tones, the cochlea of each individual of the mustached bat, Pteronotus parnellii, produces a strong evoked stimulus-frequency Otoacoustic emission (SFOAE) at about 62 kHz. The SFOAEs were on average 480 Hz above the dominant constant frequency component of the echolocation call (resting frequency, RF). In two out of nine individuals of Pteronotus the SFOAEs changed into spontaneous Otoacoustic emissions of 25–40 dB SPL. In the rufuous horseshoe bat, Rhinolophus rouxi spontaneous emissions were not detected and only in two out of seven animals were there weak SFOAEs about 300 Hz above the RF of 78 kHz. This difference may be due to a stronger damping of underlying resonant processes in Rhinolophus (Henson et al., 1985a). Acoustic distortion products behaved quite similar in both species. The first lower sideband distortion 2 f 1– f 2 was measurable over a wide frequency range between 10 and 100 kHz. The optimum frequency separation Δ of the two primary tones to evoke maximum 2 f 1– f 2 distortion was 0.8 to 5.8 kHz in Pteronotus and 1 to 7 kHz in Rhinolophus for frequencies outside the range of the constant frequency components of the call. This corresponds to ratios f 2 f 1 of about 1.03 to 1.2. At the frequency of the SFOAE in Pteronotus (480 Hz above the RF) and about 300 Hz above the RF in Rhinolophus the optimum δf decreased sharply to values of 31–63 Hz in Pteronotus (ratio f 2 f 1 of 1.0005–1.001), and to 39–590 Hz in Rhinolophus (ratio f 2 f 1 of 1.0005–1.007). In Pteronotus a second minimum of -If was found at about 90 kHz (values of 180–620 Hz. ratios f 2 f 1 of 1.002–1.007). In both bat species, the respective minima of Δf are located at or close to frequencies where neuronal tuning sharpness is exceptionally high. This indicates a mechanical origin of enhanced tuning. After adjusting the frequency of f 2 to match the optimum Δfs, 2 f 1– f 2 threshold curves were obtained. The distortion product threshold approximately parallels neuronal data and is in both species characterized by a pronounced insensitivity at the RF followed by a steep threshold minimum at frequencies 0.3–3 kHz above the RF. These features may be involved in reducing the cochlear response to the call such that the bats are able to focus on the Doppler-shifted echos which are slightly higher in frequency and thus within the range of the treshold minimum.

Spatial Light Modulators Using Super Twisted Nematic Liquid Crystals and Application to Projection Display

The super twisted nematic liquid crystal is applied to spatial light modulators. This liquid crystal has very steep threshold characteristics. These spatial light modulators work well even if the photoconductive layer is as thin as 1 µm. This is in contrast to conventional ones which require a thick photoconductive layer (>10 µm). Sensitivity is significantly increased (2 µW/cm2 at 50 Hz operation). This spatial light modulator is applied to a projection display. Images of objects illuminated by the fluorescent lamp can be passed through a lens, and large images are projected onto screens.

SEX RATIOS AND CONDITIONS REQUIRED FOR ENVIRONMENTAL SEX DETERMINATION IN ANIMALS

Conclusions and SummaryEnvironmental sex determination (ESD) is a system of sexual determination that is influenced by a variable environment. Once sex is determined it is then fixed for life. The model of Charnov &amp; Bull (1977) proposes that ESD is favoured by natural selection when an individual's fitness as a male or female is strongly influenced by environmental conditions and when the individual has little control over which environment it will experience. Adaptive sex ratio variation is considerably easier for organisms with ESD, and this feature is the ultimate cause for the evolution and maintenance of ESD. ESD is taxonomically widely expressed, and more cases are likely to be discovered. Both environmental and genotypic sex determination mechanisms are found in closely related species. Evidence of geographical variation in the degree and in the critical environmental values of ESD within the same species has also been discovered, e.g. in the fish Menidia menidia and in the crustacean Gammarus duebeni.The factors causing sex determination in invertebrates include temperature, daylength, nutrition, density, humidity, ionic composition of the environment, pH, carbon dioxide, UV light, metabolic products, parasites, exposure to the opposite sex of the same species, and in parasitoids also host size, age and type. In vertebrates temperature is the dominant factor causing sex determination, though in fish also pH, salinity, light, water quality and nutrition, and in turtles water potential of the substrate have some effect on the sex expression. Most of these factors influence growth through resource availability or developmental speed. In most cases of ESD in invertebrates and fish, the environmental factor has a gradual effect on the sex expression, in contrast to the typical steep threshold mode found in reptiles. These differences might be due to the fact that invertebrates exhibiting ESD are commonly parasitic or confined to aquatic environments, where less spatial microhabitat differentiation exists. Sex ratio data available from nature for animals with ESD are quite limited, except for reptiles. In the laboratory sex ratios can be varied more widely than what is observed in nature.There are a number of characteristic features some of which are found in each species exhibiting ESD: (1) Patchy environments, (2) variable sex ratios, (3) parthenogenesis in addition to bisexuality, (4) parasitism, (5) aquatic habitats, (6) sexual dimorphism, (7) females larger than males, and (8) local mate competition.

Potential optical gain with two crossed Fabry–Perot resonators: the influence of absorption

A crossed resonator Fabry-Perot device is analyzed for a collection of nonlinear-optical materials, including bulk semiconductors, the intraband transition in a multiple quantum well, organic polymers, and semiconductor-doped glasses, to determine its suitability for photonic switching and optical computing. We find that as an optical switch the crossed resonator offers a considerably steeper threshold and somewhat flatter wings than a single Fabry-Perot resonator and that to achieve optical gain the absorption per pass must be lower than 0.05.

Realization of the Logical Operation XOR by a Synergetic Computer

In this section I wish to show how the XOR operation (the exclusive “or”) can be performed by a dynamical system which has a close formal resemblance to a liquid forming hexagons in a nonequilibrium state. This will allow us to construct a synergetic computer for this operation. Historically, the XOR operation has played an important role in the discussion of parallel computers, or in other words, connectionist machines or neurocomputers. Following up the basic concepts of McCulloch and Pitts, Rosenblatt (1958) had developed his perceptron. The continuation of this work was “killed” (to use Sejmour Papert’s words) by the influential book by Minsky and Papert (1961), “Perceptrons”. In that book they showed that Rosenblatt’s perceptron cannot perform the XOR operation. Nowadays, the XOR operation can be performed by neurocomputers with several layers and a sigmoid activation curve (instead of the original steep threshold curve of McCulloch and Pitts).

Level of ribosomal RNA required for stimulation from quiescence increases during cellular aging in vitro of mammalian fibroblasts

We have investigated the relation between cell size in terms of cellular ribosomal RNA (rRNA) content and proliferation of diploid human and rat embryo fibroblasts during their aging in vitro. During phase III of the proliferative lifespan in vitro, cellular rRNA content increases by a factor of nearly 3. For very different regimes of stimulation of quiescent cells, a strict correlation was observed, between the proportion of cells stimulated and cellular rRNA content, resembling a steep threshold curve. During aging in vitro, these characteristic curves exhibit an essentially parallel shift to higher values of cellular rRNA content (to higher ‘thresholds’). Upon establishment as a permanent cell line, the relation between proliferation stimulation and cellular rRNA ceases to change with further subculturing. It is suggested that the essence of transformation of fibroblasts with a myc-type of oncogenes is a reduction and stabilizing of the critical rRNA content required for proliferation.

Control of oxidative phosphorylations in yeast mitochondria. Role of the phosphate carrier

This work describes the control of ATP synthesis and O 2 consumption as a function of external inorganic phosphate (P i) concentration at steady states which are characterised by a high external ( ADP ATP ) ratio. As a function of external P i concentration both fluxes vary closely with each other and have a biphasic behaviour characterised by a rapid increase below 2 mM and a slow increase above this concentration. Nevertheless, the control of the fluxes is not always the same. The adenine nucleotide carrier exhibits no control in either flux whatever the external P i concentration is. Cytochrome c oxidase is always a controlling step in both fluxes. At low P i concentration, the proton leak controls both fluxes, but at high P i concentration P i transport takes the place of proton reentry as the other main controlling step. The steep threshold in external P i concentration for which the exchange between the controlling steps occurs strongly depends on the proton permeability of the membrane. In ATP synthesis flux, proton permeability exerts a negative control (branched pathway) although the control is positive on the O 2 consumption flux. In accordance with the summation theorem, this property implies that the controls must be very different at some other steps of the two fluxes.

Thermal regeneration of the EL2 center unquenched configuration in semi-insulating GaAs

The EL2 center is a key defect in semi-insulating GaAs. Its nature is a matter of discussions. Recently, Levinson proposed a dipolar model that could explain the quenching properties previously related to a possible metastable state. According to this model, the EL2 defect could take two different quenched and unquenched geometrical configurations, respectively, denoted (0) and (0*); some levels in the band gap were shown to be typical of the (0) configuration; in this paper we propose to observe these levels by means of thermally stimulated conductivity or thermally stimulated optical absorption spectroscopy. Their behaviour after thermal cycles in the dark will express the conversion of the (0*) configuration into the (0) one. A steep thermal threshold is displayed atT=120 K, whereas other residual relaxation phenomena are also observed aboveT=220 K. The internal electronic mechanisms of the (0) configuration will also be discussed.