摘要:

The Larkin and Ovchinnikov (LO) theory for the summation of local pinning forces on flux vortices in type II superconductors describes the collective interaction between a perfectly elastic vortex lattice and a subthreshold pinning potential. Using a similar minimization procedure, it is shown that the elastic solution derived by LO does not always represent the lowest energy state of the vortex lattice. It is argued that as the magnetic field approachesBc2, the vortex lattice inevitably becomes defective and that the defect structure determines the total pinning force, the static and dynamic response of the vortex lattice, and history effects when they occur. Calculations are presented for two and three dimensions, in two dimensions, the equilibrium defect structure predicts a peak effect in good agreement with observed critical currents. It is suggested that the situation in three dimensions is always nonequilibrium.

ISSN:0021-8979    

DOI:10.1063/1.335447

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A simple model for the Josephson sampling gate is presented. The sampler response was calculated analytically for a step‐function signal. Two tradeoffs were found: one between time resolution and overshoot and another between time resolution and sensitivity. The model also explains a prepulse in the sampling gate response.

ISSN:0021-8979    

DOI:10.1063/1.335448

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A Pb‐alloy film preparation technique is proposed and applied to high‐reliability Josephson junctions with small threshold current spreads. Fine‐grained films with the average grain sizes as small as 30 nm are obtained. This is accomplished by condensing Pb, In, and Au molecular beams onto liquid‐nitrogen‐cooled substrates. Fine grains result from AuIn2particles being dispersed finely and uniformly in alloy films. Josephson junctions are fabricated using these fine‐grained films as base electrodes. The threshold current standard deviation is reduced to 2.4%. This is due to the fine grain size and surface passivation treatment for the base electrodes. The cumulative failure is found to be less than 0.13% after 8400 thermal cycles between 4.2 K and room temperature. This is the best value reported thus far. This technique achieves fine‐grained metal alloy films providing new properties. These properties promise to aid development of high‐performance Josephson IC’s.

ISSN:0021-8979    

DOI:10.1063/1.335449

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Obliquely evaporated films of silicon oxide, SiOx, widely used to align the nematic liquid crystals in electro‐optics devices, were investigated optically by attenuated total reflection on a pyramid with a rectangular base covered with a thin layer of gold. This configuration allowed us to make measurements in two perpendicular directions and to determine any in‐plane anisotropy. Reflectivity measurements versus angles of incidence exhibited minima associated with both resonant guided modes and surface plasmon oscillations (SPO). Analyses revealed that the obliquely deposited SiOxlayers were slightly birefringent and porous, the porosity being determined by the SPO shift when air was replaced by a liquid, hexane. Here the voids surrounding the columnar deposits of SiOxbecame filled with either air or hexane and, in spite of the heterogeneous structure of the films, effective indices of refraction for the SiOxlayer could be determined. Applying the Maxwell–Garnett theory for spherical particles embedded in a host medium (air and hexane), we found that the dielectric constant of our SiOxlayers were close to that of stoichiometric SiO2with a slight optical anisotropy.

ISSN:0021-8979    

DOI:10.1063/1.335450

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The temperature dependence of the refractive indexnof cubic crystals undergoing phase transitions (PT) is measured with an accuracy of the order &dgr;n=10−6at temperatures between 3.5 and 1000 K with a computer‐controlled Jamin–Lebedeff interferometer. Microscopic monitoring forinsitusample orientation and single domain work is provided. Results are presented for KMnF3(antiferrodistortive PT) and NiO (antiferromagnetic PT) and discussed within the framework of a generalized phenomenological indicatrix perturbation theory. It predicts the proportionality between the order parameter autocorrelation function ⟨&dgr;&eegr;2⟩ and the refractive index anomaly preceding the PT. Comparison with recent theoretical predictions is performed.

ISSN:0021-8979    

DOI:10.1063/1.335451

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The microwave eddy current loss in a powder sample has been examined both theoretically and experimentally. It is shown that under certain conditions, the absorption power of the conductive powder inserted in the region of the maximum microwave magnetic field in the cavity is proportional to the bulk conductivity, and therefore the temperature dependence of the bulk conductivity can be easily deduced from experiments on powders. The method is applicable to resistivities ranging from 10−1to 10−6&OHgr; m. Examples of measurements at 9.4 GHz are given for Si, Fe3O4, and VSe2.

ISSN:0021-8979    

DOI:10.1063/1.335452

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Measurements of the temperature rise of a laser‐illuminated field emitter show that a tip temperature in excess of 1000 °K can be obtained with a focused laser beam of modest power, and that the temperature rise per unit irradiance varies approximately in inverse proportion to the radius of the focal spot. By measuring the irradiance of the central fringe in the Fraunhofer diffraction pattern of a field emitter in a focused laser beam, the profile of the field emitter is mapped without removing it from the vacuum tube. A calculation based on the observed shank profile, and taking into account diffraction in a simple approximation, predicts a peak temperature rise in good agreement with experiment. These findings have important practical applications in photofield emission.

ISSN:0021-8979    

DOI:10.1063/1.335453

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We present a simple molecular model for the propagation of shock‐induced detonation waves in energetic solids. The explosive propagation of molecular dissociation is explained by energy transmission from one molecule to the next one. A numerical study shows that the model can explain the main experimental features of detonations in energetic compounds and particularly the fact that the detonation speed is characteristic of the material and not of the initial excitation, the existence of two unequal dissociation and detonation thresholds, and the possibility of ‘‘superdetonations’’ for large initial excitations. This model allows us to point out microscopic conditions that a solid must fulfill to sustain detonation waves. It demonstrates that the intramolecular potential plays a significant role in determining the characteristics of a detonation. The model predicts that a materials sensitivity to shocks and the velocity of the detonation wave are largely independent. It is then applied to solid nitromethane: The parameters that it involves are determined for this compound and we show that nitromethane meets the requirements to sustain detonation waves with a propagation speed which is in agreement with the experimental value estimated for this compound in solid phase.

ISSN:0021-8979    

DOI:10.1063/1.335454

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An experimental investigation of the atomic carbon laser’s energy storage was conducted with a rotating back mirror. The high rotational speed of the mirror allowed the laser cavity to be switched to a highQconfiguration for various time delays after the discharge. This study, which combines results from experiments and computer simulations, revealed a short upper laser level lifetime of about 2 &mgr;sec. Thus, the lifetime of the upper laser state is too short to account for the delay observed (∼5 msec) in previous experiments on the atomic carbon nuclear‐pumped laser.

ISSN:0021-8979    

DOI:10.1063/1.335504

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A comparison is made here between the magnetic field and electric potential produced by a thin strip of frog heart muscle. An experimental test is made of the theory which states that the wave front of a single fiber (or parallel bundle of fibers as in this strip) can be represented, for both the magnetic field and electric potential, by the same single‐current dipole. First, an experimental measurement is made of the ratio of magnetic field/electric potential produced by an actual current dipole in an electrolytic tank. Then the dipole is replaced by the muscle strip and a measurement is again made of the ratio; this is done for three muscle strips at eight different source‐to‐detector distances ranging from 1 to 5 cm. It is found, in all cases, that the muscle ratios are equal to those of the actual dipole to within the experimental uncertainty of ±10%. Therefore, to this extent the theory is verified for this case of a thin strip of frog heart tissue.

ISSN:0021-8979    

DOI:10.1063/1.335402

出版商:AIP    

年代:1985

数据来源: AIP