摘要:

High‐quality epitaxial Nb thin films (thickness, 2000 A˚), characterized by flat and clean surfaces, high superconducting critical temperature (max.Tc, 9.44 K), large resistivity ratio (RR, resistivity at room temperature divided by resistivity at 10 K, max.RR, 44), and very small quantities of grains of different orientation, have been formed on sapphire‐A and sapphire‐C substrates at a relatively low substrate temperature (350–530 °C) using an ultrahigh vacuum arc method (4×10−7– 4×10−6Pa). Structures and orientations of the films deposited on MgO(100), sapphire‐A, and sapphire‐C substrates are investigated by means of several techniques and they depend on symmetry properties of the substrates. 200‐A˚‐thick Nb films deposited on the MgO and the sapphire‐C substrates showed good crystallinity and planarity. High‐quality single‐crystal films of Ta (thickness, 90 A˚) were formed on single‐crystal Nb films (7000 A˚) obtained with the sapphire‐A plane.

ISSN:0021-8979    

DOI:10.1063/1.334684

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In measurements of superconducting thin‐film penetration depths, the need for a reference penetration depth has been eliminated by a modification of the direct‐coupled SQUID technique. Furthermore, the technique is convenient for accurate measurements of insulator and thin‐film resistor thicknesses in superconducting integrated circuits. Measurements of the penetration depths and insulator thicknesses have been obtained on different chips from a few wafers of a single fabrication run. The results indicate for the first time, on a small scale, the degree of chip‐to‐chip and wafer‐to‐wafer control presently obtainable for these parameters.

ISSN:0021-8979    

DOI:10.1063/1.334685

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A two‐dimensional model of Josephson junction of overlap type is presented. The energy input is provided through induced magnetic fields modeled by a set of boundary conditions. In the limit of a very narrow junction, this model reduces to the one‐dimensional model. Further, an equation derived for the critical current leads in this limit to the critical current obtained from the one‐dimensional model. Comparisons between stationary fluxon velocities obtained from the two models by means of numerical computations show that the difference is negligible. This supports the experimental observation that measurements are insensitive to the width of the junction.

ISSN:0021-8979    

DOI:10.1063/1.335478

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A system of three Josephson junctions driven by a direct current is modelled and studied both numerically and with a perturbative approach. The results are compared and the relation with experiment is discussed. Hysteretic behavior is observed and predicted with the perturbation. Chaotic behavior is also observed and analyzed. Extensions of the approach to other models is discussed.

ISSN:0021-8979    

DOI:10.1063/1.334686

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Chaotic behavior in the rf‐biased Josephson junction is studied through digital simulations of the Steward–McCumber model. Chaotic states are characterized by Poincare sections, Liapunov exponents, and power spectra. Models are presented which explain some features of the chaotic spectra. The parameter range over which chaotic behavior occurs is determined empirically for a broad range of dc bias, rf bias, and hysteresis parameters for a fixed rf frequency. It is shown that chaos does not occur if either the dc bias or the rf bias is very large. An attempt is made to explain the boundaries of the chaotic region in terms of simple models for chaotic behavior.

ISSN:0021-8979    

DOI:10.1063/1.334687

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Superconducting NbN was produced on commercial carbon fibers and sapphire substrates by dc reactive magnetron sputtering. Uniform coating was achieved by spreading 9000 filaments into a strip located between two planar magnetrons facing each other. The conditions forB1 phase formation and homogeneous coating have been studied. Over a rather wide range of total pressures,pAr+pN2, the highest superconducting transition temperature was found for a nitrogen partial pressurepN2of about 0.05 Pa. In addition, tensile or compressive strains were produced in the NbN film on the carbon fiber as a function of bias. The NbN grain size was controlled in the range between 10 and 25 nm. Without a bias, the usual (111) preferential orientation of the grains was observed in the NbN film on the carbon fiber. The intensity ratioI200/I111could be controlled between 0.74 and 0.4. Under an increased bias, the lattice parameter grew from about 0.4385 to about 0.4395 nm. This behavior may be due to a variety of reasons. The superconducting critical currentsjcin a self‐magnetic field are about 105A/cm2for NbN on carbon fibers (C‐NbN) and about 2×106for NbN on sapphire (S‐NbN). Various microstructures and, to some extent, also mechanical defects are thought to be responsible for this difference. At 13 T, the correspondingjcvalues are 2×104and 105A/cm2for NbN films of 1‐&mgr;m thickness. The extrapolatedBc2(4K) values for C‐NbN are about 30–35 T, for S‐NbN about 25 T. The nucleation conditions for NbN depended strongly on the substrate; this was found to be true for the existence of theB1 phase with respect to gas composition, microstructure, and deposition rate. The analysis of the composition of the films, which was performed by Rutherford backscattering and scanning Auger depth profiling, revealed uniform compositions with approximately 1‐at. % carbon and approximately 0.1‐at. % oxygen contaminations.

ISSN:0021-8979    

DOI:10.1063/1.334688

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A previously described method for calculating the Lorentz factorLfor orthorhombic polar crystals is modified to allow discussion of dipoles of finite size. In this approach, the field due to a lattice of point charges ±qseparated by a distancedis calculated. The Lorentz factor, which relates the field at a lattice site to the macroscopic polarization, is found as a function of the dipole lengthd, and is seen to vary appreciably with changes in the separation between the charges. Assumption of a value of 1/3 forLcan thus lead to significant errors in determining the self‐consistent polarization of crystals consisting of polarizable molecules.

ISSN:0021-8979    

DOI:10.1063/1.334689

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A calculation of the equilibrium polarization and of the piezoelectric and pyroelectric coefficients of &bgr;‐phase poly(vinylidene fluoride) (PVF2) is presented. The effects of both the orthorhombic crystal structure and of the finite separation between the monopoles of the molecular dipole are taken into account. Good agreement is obtained with the experimentally measured polarization and piezoelectric and pyroelectric coefficients of &bgr;‐PVF2films.

ISSN:0021-8979    

DOI:10.1063/1.334690

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Piezoelectricity and ferroelectricity in 80 mol % vinylidene fluoride/20 mol % tetrafluoroethylene copolymers (VDF‐TFE) were measured at various crystallinities. In order to control the crystallinity, the crystallization pressure was varied from 1 bar to 5 kbar. All the samples prepared by this method contain only form I crystals but with various crystallinity and crystal thickness. Especially, a band structure characterizing an extended chain crystal was observed in the sample crystallized under more than 4 kbar. Remanent polarization (Pr) obtained by theD‐Ehysteresis curve increases with the crystallinity, and the extrapolatedProf 126 mC/m2nearly coincides with the value of 140 mC/m2calculated from the unit cell dimension and dipole moment. The coercive field (Ec) decreases with the crystallinity. This would be explained by the expansion of intermolecular distance and a decrease in the number of folds, which affect the rotation of dipoles under an electric field.

ISSN:0021-8979    

DOI:10.1063/1.334691

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The oscillator strength of small‐polaron absorption is studied for various thin films of tungsten oxide, WOx(x≤3), prepared by rf reactive magnetron sputtering, reactive ion plating, and vacuum evaporation. The oscillator strength greatly varies depending on the procedure of film preparation and treatment. For WOxfilms prepared by vacuum evaporation, the effective oscillator strength decreases down to 0.04 by annealing in air at 200 °C and increases up to 0.24 by annealing in air at 400 °C. The film deposited by reactive ion plating at an rf power of 300 W exhibits a large effective oscillator strength of 0.31. Concerning the film prepared by rf reactive magnetron sputtering, the oscillator strength drastically increases with an increase in rf power. For the film deposited at an rf power of 1000 W, an effective oscillator strength of 0.51 is obtained, which is five times greater than that of the film prepared by vacuum evaporation. X‐ray diffraction and XPS measurements reveal that the oscillator strength is affected by the two factors, that is, crystallinity and oxygen content of the film. It is concluded that the enhancement of oscillator strength occurs through crystallization of the film from its amorphous state and a lack of oxygen in the film. Using a configuration coordinate diagram, the mechanism of the oscillator strength enhancement is discussed in terms of a delocalization of polaron wave function.

ISSN:0021-8979    

DOI:10.1063/1.334692

出版商:AIP    

年代:1985

数据来源: AIP