Search Results - (Author, Cooperation:J. Beyer)

2018-06-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Geochemistry, Geophysics, Planetary Science

2018-07-26

American Physical Society (APS)

0031-9007

1079-7114

Physics

Condensed Matter: Electronic Properties, etc.

2013-04-16

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1573-7357

Springer Online Journal Archives 1860-2000

Physics

Abstract The frequency-dependent conductivity of a dirty superconductor is calculated as a function of temperature by solving a kinetic equation that describes the effect of electron-phonon scattering at frequencies well below the gap frequency. The numerical solutions of the kinetic equation are compared with results based on a relaxation time approximation.

Electronic Resource

1573-7357

Springer Online Journal Archives 1860-2000

Physics

Abstract We present a general formalism based on the quasiclassical Green's function for calculating charge imbalance in nonequilibrium superconductors. Our discussion is sufficiently general that it applies at arbitrary temperatures, and under conditions when the width of quasiparticle states are appreciable due to pair breaking processes, and when strong coupling effects are significant. As a first application we demonstrate in detail how in the limit of small pair breaking and for a weak coupling superconductor the collision term in the formalism reduces to the one in the quasiparticle Boltzmann equation. We next treat the case of charge imbalance generated by tunnel injection, with pair breaking by phonons and magnetic impurities. Over the range of temperatures investigated experimentally to date, the calculated charge imbalance is rather close to that evaluated using the Boltzmann equation, even if pair breaking is so strong as almost to destroy superconductivity. Finally we consider charge imbalance generated by the combined influence of a supercurrent and a temperature gradient. We give calculations for a dirty superconductor with scattering by phonons as the pair breaking mechanism, and the results give a reasonable account of the experimental data of Clarke, Fjordbøge, and Lindelof. We carry out calculations for the case of impurity scattering alone which are valid not only in the clean and dirty limits, but also for intermediate situations. These enable us to see how the large contribution to the charge imbalance found for energies close to the gap edge in the clean case is reduced with increasing impurity scattering.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The microwave surface resistance of an YBa2Cu3O6.9 superconducting thick film was measured over the range 7.0–16.7 GHz at 77 K. This was done by placing a sample in a TE01n wavemeter cavity and observing the change in selectivity of the cavity. The material's surface resistance is of the same order of magnitude as that of silver at 77 K from 8 to 12 GHz and improves about another order at 4.2 K. The power-law behavior of surface resistance with frequency is probably close to quadratic. This is similar to the behavior of low critical temperature superconductors.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The linearity of sensitive superconducting quantum interference device (SQUID) magnetometers fabricated from high critical temperature (high-Tc) superconductors and operated in a direct-coupled flux-locked loop with bias reversal was investigated. The system nonlinearity was determined by applying a sinusoidal test signal and measuring the output signal harmonics using a setup which ensures the elimination of nonlinearities arising from the test signal source and the spectrum analyzer. The experimental setup enables the simultaneous measurement of the harmonic distortion of a single magnetometer and of a first-order electronic gradiometer formed by two individual magnetometers. Test signal amplitudes and frequencies were chosen to simulate typical laboratory interferences. An analysis of the results was performed using analytical equations in which the SQUID's inherent nonlinear character is taken into account. At signal frequencies above about 0.1% of the system bandwidth, the system nonlinearity was found to be mainly caused by the nonlinear behavior of the SQUID magnetometers, whereas at low frequencies nonlinearities arising from read-out electronics components predominate. For signal peak-to-peak amplitudes up to 1 μT and frequencies up to 0.5 kHz, total harmonic distortions below about −105 dB for a single sensor channel and −100 dB for an electronic gradiometer were obtained. © 1999 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have investigated three-terminal single-layer thin-film superconducting devices made of YBaCuO and Nb. The devices incorporate regions of weak superconductivity in multiple parallel links that are influenced by current in a separate control line. These experiments were designed to study the possible application of this device as an rf amplifier. With the device biased, rf power was applied to the control line and the transmission coefficient, S21, was measured. The reverse transmission coefficient, S12, was also measured for comparison. Upon biasing into a flux flow state, the S21 of the device at rf frequencies was found to increase 10–15 dB over the zero bias value and over the reverse feed (S12 ) value. The device behaved linearly up to power inputs of +5 dBm (1-dB compression point). The bandwidth was limited only by the impedance transformer.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A wide-band traveling wave amplifier using vortex flow transistors is proposed. A vortex flow transistor is a long Josephson junction used as a current controlled voltage source. The dual nature of this device to the field effect transistor is exploited. A circuit model of this device is proposed and a distributed amplifier utilizing 50 vortex flow transistors is predicted to have useful gain to 100 GHz.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

This paper presents measurements of transresistance of the vortex flow transistor (VFT). These measurements were obtained at frequencies up to 500 MHz which was one-tenth of the transit time cutoff frequency. A signal tagging scheme was developed to detect these high-frequency signals that would otherwise be obscured by high crosstalk levels. The measured frequency response of the VFT indicates that the transresistance signal is present at frequencies greater than one-tenth of the transit time cutoff frequency. Thus, the VFT is a viable active device candidate for rf superconducting electronic systems.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A completely noninvasive method is presented for the investigation of semiconductor wafers with high spatial resolution utilizing a superconducting quantum interference device (SQUID) magnetometer system. The method is based on the detection of the magnetic field caused by photocurrents generated in the semiconductor sample using a sensitive SQUID magnetometer. The photocurrents arise when laser light with a photon energy exceeding the band gap of the semiconductor is focused onto the sample surface in a region of a doping gradient. The spatial resolution of this detection method is mainly determined by the size of the excitation focus of about 20 μm. We report on measurements of silicon wafers with small growth-related doping fluctuations. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have fabricated and characterized a low-noise direct-coupled magnetometer based on a 100 pH YBa2Cu3O7−x dc superconducting quantum interference device (SQUID) on a 10 mm×10 mm SrTiO3 bicrystal substrate with 30° misorientation angle. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The SQUID magnetometer was operated using direct-coupled flux-locked-loop electronics with bias reversal. The sensor had a usable voltage swing of 39 μV and a white magnetic field noise of 32 fTHz−1/2 with a 1/f corner at 2 Hz, including electronics and environmental noise. The voltage versus flux (V−Φ) characteristic showed a pronounced distortion on the negative slope. Numerical simulations were performed to explain the distorted V−Φ characteristic. Measurements of magnetocardiograms demonstrate the suitability of this sensor for biomagnetic applications. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

An approach for the localization and characterization of electrically active defects in multicrystalline silicon and of artefacts in photovoltaic devices is presented. The essential idea of the method is the noninvasive magnetic detection of photocurrents generated in the sample by local illumination. Magnetic imaging with high spatial and magnetic resolution is performed by scanning the sample under investigation and synchronously measuring the magnetic field of the photocurrents using highly sensitive superconducting quantum interference device magnetometers. The technique enables the nondestructive and nonpolluting detection and investigation of electrically active grain boundaries in multicrystalline silicon wafers. Results obtained from photovoltaic devices show that typical, performance-limiting defects can be localized, as they cause distinct features in the magnetic field topography. © 2000 American Institute of Physics.

Electronic Resource

1398-9995

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

The NK cell activity of mononuclear cells as well as monocyte-depleted, Percoll-fractionated, NK cell-enriched effector cells against K 562 target cells was inhibited by methylprednisolone (MP) and hydrocortisone (HC) in a dose-dependent manner. The effector/target cell conjugate formation was studied in a single cell agarose assay, and it was shown that MP and HC partly inhibited the NK cell activity by inhibition of the adhesion of effector cells to target.

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1095-8649

Blackwell Publishing Journal Backfiles 1879-2005

Biology

A new model approach to gastric evacuation in predatory fishes was shown to give accurate and reliable estimates of evacuation. It is believed that such a model would prove particularly useful in understanding predator-prey interactions in natural systems. The model is a simple, geometric abstraction of the square root model predicting that evacuation time is proportional to the square root of meal size in accordance with extensive empirical evidence. Digestive processes are assumed to be restricted to an outer surface of total stomach content that is represented by the curved side of a cylinder. This way, total stomach content is considered a cylinder of constant length the mass of which is gradually reduced by successive peeling off its side leaving the ends unaffected. The fundamental rate δ describes mean thickness of the layer that is peeled off the cylinder per time unit. In the model, δ is constant independent of cylinder thickness but inversely proportional to the square root of cylinder length. Anatomical and dynamic characteristics of the stomach render this formulation of δ plausible. Using mean evacuation rate over time during evacuation of a meal, the model disregards prey heterogeneities that influence instantaneous evacuation rate. Relationships between prey heterogeneities and evacuation patterns were indicated by application of a general power model to evacuation data, and it was suggested how the effects of prey characteristics might be incorporated into the square root model to produce a generic model of gastric evacuation.

Electronic Resource

1095-8649

Blackwell Publishing Journal Backfiles 1879-2005

Biology

In the distributional overlap volume of Baltic cod Gadus morhua and its prey, studied in the Bornholm Basin in the southern Baltic Sea, only a fraction of the sprat Sprattus sprattus population vertically overlapped with the Baltic cod population. Sprat occurred in the intermediate water, in the halocline and in the bottom water, while herring Clupea harengus and Baltic cod occurred exclusively in the halocline and in the bottom water. Only parts of the sprat population were hence accessible for Baltic cod, and only a fraction of the sprat had access to the Baltic cod eggs below the halocline. Baltic cod–clupeid overlap volumes appeared to be determined by salinity stratification and oxygenation of the bottom water. Hydrography time series were used to estimate average habitat volumes and overlap from July to September in 1958–1999. In the 1999 survey spawning Baltic cod had greater ratios of empty stomachs and lower average rations than non-spawning Baltic cod. The average ration for Baltic cod caught within 11· 4 m from the bottom (demersal) did not differ from the average ration of Baltic cod caught in shallower waters (pelagic), because spawning and non-spawning Baltic cod in both strata were caught at equal rates. The diet of the Baltic cod caught demersally contained more benthic invertebrates, especially Saduria entomon, but Baltic cod caught pelagically also had fresh benthic food in their stomachs, indicating vertical migration of individual fish.

Electronic Resource

1095-8649

Blackwell Publishing Journal Backfiles 1879-2005

Biology

A mechanistic gastric evacuation model of mixed stomach contents was established. Its simple generic nature should prove useful for studying predator-prey interactions in fish communities. Only prey lengths were required as additional input. Surface dependent digestive processes were considered to act on equal fractions of individual prey-cylinder surfaces. Primary and interactive effects of size, energy density and resistance to digestion of individual prey in a stomach were described by the model. Model predictions of results from experiments on gastric evacuation of meals composed of different prey types demonstrated the capability of this model, as opposed to previously applied model principles, to predict evacuation of mixed stomach contents involving the three above-mentioned prey characteristics. The study furthermore illustrated that estimates of food ration might be severely biased by using improperly formulated effects of prey characters on gastric evacuation, and demonstrated that the new model holds the potential to predict food ration and diet composition for wild populations of predatory fishes.

Electronic Resource

0022-1325

Psychology