摘要:

Chlorophyllamicrocrystals can be electroplated from a suspension in an organic solvent. The optical‐absorption spectra of the resulting films indicate an interaction between the porphyrin rings of different molecules. It is shown that these films can store electronic charge. The charge storage was studied by plating the chlorophyll on silicon and using the well‐knownC(V) technique. It is also shown that monomeric films, made by heating the microcrystalline films at about 100 °C, have different charge‐storage properties. The charge‐injection and charge‐storage properties of the chlorophyll films are discussed in terms of the molecular arrangement of the films. The results are also compared with ac‐ and dc‐conductivity measurements of the films.

ISSN:0021-8979    

DOI:10.1063/1.324647

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Acoustic flux amplified from thermal background in semidegeneraten‐GaAs at liquid‐air temperature is accompanied by electric fields that initiate impact ionization. When the acoustic flux is concentrated in a narrow domain the interplay between the impact and the acoustoelectric effect brings about current oscillations of frequencies in the MHz range that increase monotonically with the net gain factor of the acoustic flux measured prior to the onset of impact. When wide stationary acoustic domains are generated in such samples, the impact causes an increase of the saturation current typical of the acoustoelectric effect. For initially applied fields higher than the threshold for impact, current modulation is observed when the acoustic flux is still in the embryonic stage and its incubation time is increased. The threshold field for impact in epitaxial layers is about 1500 V/cm, while in bulk samples it is much higher. At high power levels these instabilities may bring about the formation of current filaments that burn channels across the samples. The experimental results and a semiquantitative analysis of the instabilities are presented in detail in this work.

ISSN:0021-8979    

DOI:10.1063/1.324648

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A new pulsed nonlinearity measurement is described and applied to the characterization of thin‐film conductor stripes. The method is based on the phenomenon of ’’baseline shift’’, and is substantially simpler than the already familiar third‐harmonic method yet, at least potentially, just as sensitive. A model is proposed in which the observed nonlinearity is attributed to the temperature rise accompanying current in conductors and the associated resistance change. Good quantitative agreement has been obtained with predictions based on the model. A specific application of this method to reliability testing has been found in magnetic bubble memories, the lifetime of which correlated closely with pulse nonlinearity measurements.

ISSN:0021-8979    

DOI:10.1063/1.324649

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Schottky‐barrier diodes have been realized by cleavingn‐type Si in ultrahigh vacuum (UHV) in a stream of evaporating metal atoms. The properties of these Al, Mg, and Fe diodes are studied by means of standardI‐V,C−2‐V, and photoemission threshold measurements in the same UHV. With an error analysis it is shown that the barrier height is most accurately obtained fromC−2‐Vmeasurements. This is in contrast to the prevailing opinion that the barrier height from photoemission measurements is the most reliable measure of the barrier height. It will be shown that a correct value from the photoemission experiments is obscured by the difficulty in determining the exact number of absorbed photons in the metal (back illumination). Finally, reported large systematic discrepancies between barrier heights from photoemission measurements and those fromC−2‐Vmeasurements were not observed. All observed differences remain within the experimental error.

ISSN:0021-8979    

DOI:10.1063/1.324650

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of surface barriers to flux entry on the stability of superconductors at low fields is considered theoretically in the context of the usual critical‐state model. The presence of such a barrier is found to greatly reduce overall stability unless a thin normal‐metal overcoat is used to stabilize the superconductor. The relevance of these results to superconducting power transmission lines is discussed.

ISSN:0021-8979    

DOI:10.1063/1.324651

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

In an earlier work, we demonstrated that high self‐field and low‐field critical‐current densititesJcof the order of 106A cm−2can be attributed to flux pinning on a dispersed Nb5Ge3tetragonal phase present in Nb3Ge3layers grown by chemical‐vapor deposition (CVD). In this study, we examined the effect of impurity gas additions onJcand the critical temperatureTcof the A15 superconducting phase. The gas impurities were N2, C2H6, and CO2. The impurity concentration in the gas phase was varied over three to four orders of magnitude to establish tradeoffs betweenTcdeterioration andJcenhancement. The x‐ray phase analysis of samples containing the highest impurity concentrations indicated by the presence of niobium nitrides and carbides, respectively. TheTcwas affected least by N2and most by CO2additions. Doping by N2or C2H6resulted in A15 deposits free of the tetragonal phase and havingJc’s of the order of 106A cm−2, comparable to the best Nb5Ge3‐containing samples. The grain size of deposits was estimated from thinned specimens using transmission electron microscopy. Correlation ofJcwith the microstructure of the sample suggested that predominant flux pinning occurs on impurities incorporated in Nb‐Ge layers and on the accompanying defects.

ISSN:0021-8979    

DOI:10.1063/1.324652

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Flux pinning in irradiated superconductors having either dislocation‐loop or void microstrutures is reexamined. The elementary interaction forcefpcan be accurately computed for such defects if their size is known from transmission electron microscopy. Data exist for a range of defects for whichfpvaries by over a factor of 104. If the data for voids and dislocation loops are plotted asQ, the volume pinning force per defect, versusfpat a given reduced inductionb, the sets of data for each defect species overlap to form a single summation curve. This curve is not correctly described by either of the two most widely used summation models, the direct summation or the statistical summation of Labusch. The empirical summation curve runs parallel to, but a factor ?4 below, the direct summation curve at highfpbut bends to decrease approximately asf2pat lowfp. Increasingbproduces a shift in the summation curve, parallel to the direct summationQ=fpline, by a factor which is proportional to (1−b)2. This shift is the origin of the peak effect which appears at highbfor lowfpdefect microstructures. The failure of the statistical theory is thought to result from the presence of flux‐line dislocations in the flux‐line lattice which produce both a large decrease and a dispersion in the local threshold force necessary for pinning centers to be effective.

ISSN:0021-8979    

DOI:10.1063/1.324653

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We use qualitative mathematical methods for the analysis of the time dependence of the phase differences and, hence, the dynamicI‐Vcharacteristics for two coupled Josephson point junctions. All our qualitative results are supported by numerical calculations. The case of a single point junction is reviewed and extended; it is proved that periodic trajectories in the phase plane are attractors and that they are therefore unique for a given junction and dc driving currentI. Center of mass and relative coordinates are introduced for two inductively coupled junctions and the latter coordinate is shown to remain bounded. All static solutions are obtained and their stabilities analyzed. We find that the end of stability of a given solution is never a Hopf bifurcation and, hence, is marked by a finite jump to a dynamic or another static solution. The dynamic solutions are analyzed using perturbation theory for strongly coupled junctions. An approximation for the beating mode in weakly coupled junctions is presented and approximate expressions for the beat period derived. Interesting symmetries of the solution are found for a set of special values, a set which becomes dense for weak interjunction coupling.

ISSN:0021-8979    

DOI:10.1063/1.324654

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Dispersion characterisitcs of magnetoelastic surface waves in highly magnetostrictive cubic crystals are calculated for different bias fields applied along the direction of propagation. Effects of anisotropy are taken into account. The surface waves are found to be leaky or nonleaky depending on the direction of propagation. Velocity of surface waves is calculated both before and after saturation of the magnetic moment. Although most of the velocity change with bias field occurs while the magnetization vector rotates from the easy direction to the field direction, the velocity changes by a few percent even after the moment saturates. Theoretical results are in good agreement with experiment.

ISSN:0021-8979    

DOI:10.1063/1.324655

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The dynamical behavior of straight magnetic domain walls and propagating magnetic bubbles in single‐, double‐, and triple‐layer lanthanum gallium garnet films is reported. In the study of straight walls in the single‐layer films, a good agreement is obtained between the observed and the calculated values of the domain‐wall width parameter &Dgr;, the derivative of the peak velocity to in‐plane field at high in‐plane fields, and the derivative of the saturation velocity to in‐plane field at a moderate drive field and high in‐plane fieldsH1. The saturation velocity (atH1=0) decreases with decreasing film thickness, which is in disagreement with theoretical predictions. A propagating bubble in a single‐layer film exhibits an overshoot. In double‐ and triple‐layer films, the observed and calculated &Dgr; values disagree in most cases, and the straight wall and propagating bubble saturation velocities are significantly higher than in single‐layer films. Overshoots and low‐frequency oscillations are observed in the saturation regime of straight walls. The resulting high wall masses are attributed to stacking of horizontal Bloch lines on the film surface(s). From the measurements in in‐plane fields, we conclude that the saturation velocity and the peak velocity in a double‐ and a triple‐layer film depend on the polarity of the in‐plane field, which is always perpendicular to the straight wall. It is found that the saturation region in double‐ and triple‐layer films can be subdivided in three parts. This is explained qualitatively, and a calculation is made of the saturation velocity in double‐ and triple‐layer films. This calculation shows good agreement with experimental results.

ISSN:0021-8979    

DOI:10.1063/1.324656

出版商:AIP    

年代:1978

数据来源: AIP