摘要:

The thermal‐induced escape out of the zero‐voltage state of a Josephson junction is discussed removing the main assumption of the resistively shunted junction model. This model assumes that the junction dissipation is dominated by a linear ohmic shunt resistor. However, the presence of quasiparticles excitations produces dissipation in the junction through nonlinear mechanisms. As a consequence, the dynamical resistance is strongly voltage dependent. In the present work, we study the effect of this voltage‐dependent dissipation on the supercurrent decay in connection with the general problem of a Brownian particle in a metastable state in the presence of a velocity‐dependent friction coefficient. An analytical expression for the lifetime of the metastable state is obtained, providing an answer to the open question of what resistance dominates the junction damping. A comparison of the theory with experimental results is also presented.

ISSN:0021-8979    

DOI:10.1063/1.346796

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper we outline an experimental method and a theoretical framework by which the strength of weak superconducting links in granular ceramic systems is compared and measured. This new method is based on dc measurement of screening properties of superconducting materials and renders a simultaneous determination of the critical magnetic field and the penetration length. In particular, the method is applied to a series of newly developed nonrandom silver‐YBa2Cu3O7−&dgr;composites. By measuring the field screening property in these materials, the effectiveness of various sintering processes can be judged.

ISSN:0021-8979    

DOI:10.1063/1.346797

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Measurements of the electrical resistanceRof polycrystalline samples of Bi1.6Pb0.4Sr2Ca2Cu3OYwith the 108‐K superconducting phase show that the Aslamazov–Larkin (AL) two‐dimensional paraconductivity corrections are observable in the vicinity of the mean‐field theory transition temperatureT0C. The effective thickness,d≊23 A˚, of the superconducting electronic sheet as found from the zero field data is comparable to thea,b‐plane separation. This result agrees well with the recent single‐crystal data. The data for magnetic fields up to 1.5 T also show paraconductivity effects.T0C(H) is found to decrease as a function of increasing magnetic fieldH; this result agrees with the prediction of the AL model.

ISSN:0021-8979    

DOI:10.1063/1.347184

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The amplitude of the current steps induced by a rf field in a Josephson junction is strictly related to the stability of the phase‐locked state. We show how the ac Josephson effect can be enhanced by means of a proper drive: when two phased ac fields of angular frequencies &ohgr; and 2&ohgr; are added together, the rf‐induced steps have maximum normalized amplitudes larger than the Bessel functions as predicted for a sinusoidal drive. Furthermore, in the limiting case of a driving signal that consists of an impulse train, the stability of the phase‐locked state can reach its maximum value, i.e., rf‐induced steps as large as the critical current can be obtained. Experimental evidence of the enhanced ac Josephson effect is reported for a highly hysteretic Nb/Al‐Al2O3/Nb Josephson tunnel junction. The enhanced ac Josephson effect may open new ways for the understanding of the nonlinear dynamics of Josephson junctions and for the design of more efficient Josephson voltage standard devices.

ISSN:0021-8979    

DOI:10.1063/1.346798

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Resistivity and transport critical current density have been measured as a function of temperature for patterned YBa2Cu3O7−&dgr;and Bi2Sr2CaCu2O8+&dgr;superconducting thin films on single‐crystal (100)‐oriented MgO substrates. These granular superconducting films were prepared by the metallo‐organic decomposition method. Transition temperaturesTc(&rgr;=0) are around 72 K for YBa2Cu3O7and 82 K for Bi2Sr2CaCu2O8films, both 1 &mgr;m in thickness. Critical current densities approaching 105A/cm2have been measured in the Bi2Sr2CaCu2O8films at low temperatures. The critical current densities in YBa2Cu3O7films are about 40 times lower than those in the Bi2Sr2CaCu2O8films. It was found that nearTcthe critical current density in both films is proportional to (Tc−T)2, in agreement with a superconductor–normal‐metal– superconductor model developed by DeGennes [P. G. De Gennes, Rev. Mod. Phys.36, 225 (1964)]. The reason for the much higher current density in the Bi2Sr2CaCu2O8film is believed to be due to the stronger coupling between the superconducting grains.

ISSN:0021-8979    

DOI:10.1063/1.346799

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Type‐II and multiply connected type‐I superconductors may experience magnetic forces that are significantly different from the well‐known repulsive Meissner effect. In particular flux trapped by a superconductor may result in a force which has a stable position. We have calculated the magnetic force acting on a superconductor, based on a trapped flux model; the results have been compared with experiments at 4.2 K on (1) a block of Nb3Sn, (2) a disk of YBa2Cu3Ox, and (3) a hollow cylinder of Pb. Most of the experimental results are in reasonably good agreement with the calculations. However, an interesting discrepancy exists in the frequency of small oscillations about the stable position.

ISSN:0021-8979    

DOI:10.1063/1.346800

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Magnetic Fe‐Bi multilayers have been, for the first time, synthesized by using electron‐beam evaporation at 140 K. The relationships between film structure and magnetic properties were investigated by means of measurements of magnetization, x‐ray diffraction, electron diffraction, and Mo¨ssbauer spectra. Films are ferromagnetic or paramagnetism, depending on Fe layer thickness,tFe. Films withtFeof about 1 nm exhibit perpendicular magnetic anisotropy while these with the thickertFehave in‐plane magnetism. Temperature dependence of magnetization in the Fe‐Bi multilayers was also studied in the temperature range of 77–600 K.

ISSN:0021-8979    

DOI:10.1063/1.346801

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A59Co nuclear magnetic resonance measurement and a transmission electron microscope observation are utilized to investigate the compositional distribution of Co‐Cr films with and without heat treatment. The spin‐echo spectra of evaporated films are found to be entirely different from those of Co‐Cr alloy powders. The spectra of films comprise a very complicated hyperfine field distribution and the resonant frequencies are much higher than those of powder samples, implying the existence of various kinds of segregation regions typical of a film. Chrysanthemumlike patterns, which disappear with heat treatment, are also observed for chemically etched films. This disappearance is accompanied by the disappearance of the highest‐frequency resonant line.

ISSN:0021-8979    

DOI:10.1063/1.346802

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A model is proposed to establish the relevant constitutional parameters of ferroelectric composite materials such as shape, composition, and connectivity which determine the piezoelectric, dielectric, and elastic properties of the composite. A proper Monte Carlo method is used to calculate the active volumetric fraction of ceramic. An equivalent circuit is proposed to calculate the effective dielectric constant. The effective medium expressions for the dielectric constant, elastic constant, and local field coefficients (LE,LD,LS,LT) are generalized to include shape effect. These results are used to calculate the piezoelectric coefficients. Recently, some authors have proposed a model and claimed to have established the degree of heterogeneity, determined by the scaled average grain diameter, to be the important constitutional parameter of a composite. We argue that the important constitutional parameter is the volumetric fraction of the ceramic. Our model is simpler, uses fewer parameters and its predictions compare better with experiment. The simplicity of the model can be exploited for predictions of the properties and design of multi‐phase ferroelectric composites.

ISSN:0021-8979    

DOI:10.1063/1.346803

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Yttrium oxide (Y2O3) thin films were deposited on indium‐tin‐oxide(ITO)‐coated glass substrates by the radio‐frequency‐sputtering method using an Y2O3‐sintered target. The relative dielectric constant &egr;rand the dielectric strengthEBDof the Y2O3films were studied. It was found that &egr;rand EBDhave a maximum value and a minimum value, respectively, at 1.3 Pa when the pressure of the sputtering gas, Ar+10% O2, is varied from 0.67 to 9.3 Pa. The x‐ray diffraction study showed that the Y2O3films deposited at 1.3 Pa are predominantly oriented along the ⟨332⟩ direction and their grain size is the smallest. Ion mass analysis showed impurity diffusion from ITO in the films deposited at 1.3 Pa. Furthermore, the dielectric properties of the Y2O3films deposited at 1.3 Pa are related to the structural properties, such as the ⟨332⟩ orientation, grain size, and impurity diffusion.

ISSN:0021-8979    

DOI:10.1063/1.346804

出版商:AIP    

年代:1990

数据来源: AIP