摘要:

An inhomogeneously textured ring prepared from a diffused Bi2Sr2CaCu2O thick film has been arranged for the measurement of longitudinal,VL, and transverse,VT, voltages with three diametrical pairs of contacts. The film was deliberately grown in such a way that one half of the ring has a high degree of texturing with theaaxis perpendicular to the sample plane. In the second, less textured half, a finer granular structure is present implying the presence of more grain boundaries. The application of an external magnetic field perpendicular and parallel to the sample plane clearly shows that an anisotropic weak link behavior from the inhomogeneous texturing is responsible for the minimum observed inVTbelowTc. The reduction of this minimum with increasing magnetic field is explained in terms of the loss of superconducting paths due to increased dissipation at grain boundaries in the more textured branch.

ISSN:0021-8979    

DOI:10.1063/1.350531

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Thin epitaxial films of monoclinic pure and cubic yttria‐stabilized (YSZ) ZrO2were deposited onto Si(100) by electron‐beam evaporation. The degree of crystalline perfection was controlled by the growth temperature. Epitaxial YBa2Cu3O7−xfilms were grown on these buffer layers by KrF excimer laser ablation. Optimum crystalline and electrical quality, characterized by a transition temperatureTc0of 86–89 K, a critical current densityjcof 106A/cm2at 77 K, and a channeling minimum yield of 12% was obtained on YSZ buffers showing a minimum yield of 7%. Even a 14‐nm‐thick YSZ buffer enabled the growth of an YBa2Cu3O7−xfilm withTc0of 86 K and a minimum yield of 12%. With decreasing quality of the YSZ buffer layers the crystalline quality of the superconductor also decreased. The disorder in the YBa2Cu3O7−xfilms, however, increased more slowly than in the buffer layers, so that even on an amorphous buffer the YBa2Cu3O7−xexhibited a pronounced texture with a minimum yield of 72%. The comparatively rough surface of the monoclinic pure ZrO2severely hampered thec‐axis alignment of the YBa2Cu3O7−x, resulting in superconductor films of inferior quality.

ISSN:0021-8979    

DOI:10.1063/1.350532

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have demonstrated that the normal‐state resistivity of sufficiently thick Y1Ba2Cu3O7−&dgr;thin films can be fitted very well by the Bloch–Gruneisen equation. As the film thickness decreases, the localization effect becomes predominant presuming the manifestation of surface defects and vacancies over the bulk region. The evaluated Debye temperatures span over a range which is consistent with experimental values. Below some critical thicknesses, the transport behavior in thin Y1Ba2Cu3O7−&dgr;films deviates from the Boltzmann conductivity and the temperature coefficient of the electric resistivity becomes negative, which can be explained by adopting quantum corrections for localization and interaction effects. The origin of nonmetallic behavior can be understood quantitatively in terms of the competition between the quantum corrections and the conventional Boltzmann conductivity. In these ultrathin superconducting films, the elastic mean‐free path arising from defect and heavy impurity scattering is very short and much smaller than the inelastic mean‐free path, even at room temperature.

ISSN:0021-8979    

DOI:10.1063/1.350533

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The temperature dependence of the superconducting critical current density (Jc) is investigated for the highTcBi1.6Pb0.4Sr2Ca2Cu3Ox. An abnormalJcchange is found from 50 to 80 K.

ISSN:0021-8979    

DOI:10.1063/1.350534

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Epitaxial thin films of YBa2Cu3O7have been prepared on SrTiO3and LaAlO3substrates by a high‐pressure planar dc‐sputtering technique. By covering the substrate heater with a frame of polycrystalline YBa2Cu3O7substantial improvements of the YBa2Cu3O7film properties were achieved. These are characterized by dc‐resistivity values &rgr;(T) of less than 50 &mgr;&OHgr; cm at 100 K and &rgr;(300 K)/&rgr;(100 K) values of up to 3.9. Significant deviations from the usual linear &rgr;(T) behavior were found. Critical temperatures above 90 K, resistive transition widths down to 0.3 K, and critical current densities of about 5 × 106A/cm2at 77 K confirm the high quality of the films. As indicated by Rutherford backscattering and high‐resolution transmission electron microscopy the films exhibit a microstructure characterized by a reduced density of lattice defects. However, lattice‐coherent precipitates with a diameter of about 5–10 nm were observed. As an outstanding feature the films exhibit, besides the initial steep falloff atTc, a further gradual decrease of the microwave surface resistance at 87 GHz below 50 K by at least one order of magnitude. These results are very promising for millimeter‐wave applications of epitaxial YBa2Cu3O7thin films.

ISSN:0021-8979    

DOI:10.1063/1.350535

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An analytic solution to the energy barriers of two interacting single domain particles is presented. Identical volumes and uniaxial anisotropies are assumed with easy axes parallel to an external magnetic field. The locations and heights of the system energy barriers are analytically determined when the line joining the two particles is either parallel or perpendicular to the easy axes and the external field. The lowest energy barriers are saddle points of the energy surface and correspond to reversal modes under thermal agitation. When dipole coupling is not strong, the mode of thermal switching isasymmetricfanningorasymmetriccoherentrotation, depending upon the bond angle, rather than symmetric fanning or coherent rotation that occurs at the nucleation field. An effective volume that describes the cooperative effect is calculated and good agreement with the numerical results using the Fokker–Planck equation is obtained. The energy barriers can be used to calculate the superparamagnetic relaxation time constants in the high barrier limit.

ISSN:0021-8979    

DOI:10.1063/1.351376

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The field dependence of the magnetization at 4.2 and 294 K and the temperature dependence of the spontaneous magnetization for two series of ternary Co‐based alloys, namely Co76−xB24Cxand Co88−yByC12have been measured. The metalloid content in these series ranges from 24 to 44 at.%, and extends almost to the critical concentration for the disappearance of ferromagnetism. The composition dependence of the average magnetic moment at 4.2 K compares well, for low metalloid content, with data for other Co‐based materials and also with the theoretical models that predict a linear dependence of the magnetic moment as a function of concentration. The thermal demagnetizing process at low temperatures is presented. The results have been analyzed in terms of Bloch’s law and the stiffness constant has been determined. The complete thermal demagnetization, i.e., down toTc, is presented only for the alloys with a Curie temperature below 300 K. The results show the well known flattening of theM(T) curve as compared with crystalline materials.

ISSN:0021-8979    

DOI:10.1063/1.350536

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report accurate temperature dependent measurements of optic indices of refraction, the birefringence, and the strain in the ferroelectric tungsten bronze crystals Ba0.25Sr0.75Nb2O6and two compositions of (Ba2−xSrx)2(K1−yNay)2(NbO3)10. These results are compared to our previous results in Ba0.4Sr0.6Nb2O6. From the experimental data, it appears that far above the ferroelectricTc, up to a temperatureTd, these crystals possess a local, randomly oriented polarization,Pd, with similarTdvalues, irrespective of their chemical composition andTc. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed.

ISSN:0021-8979    

DOI:10.1063/1.350537

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The properties of amorphous BaTiO3thin films deposited on Si substrates were studied as functions of annealing temperature. The film thickness decreased monotonically as the annealing temperature increased up to 600 °C. This reduction is believed to correlate with the densification process observed in the amorphous films during annealing. After the 500 °C annealing, a partial crystallization process was observed. It was also found that the dielectric constant and the index of refraction of the film showed significant changes as the annealing temperature increased from 400 to 500 °C. Correlations between the film density, index of refraction, and the dielectric constant are discussed. A severe reaction at the interface between the film and Si substrate was observed in the 750 °C annealed sample.

ISSN:0021-8979    

DOI:10.1063/1.350538

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have measured the optical properties of epitaxial CoSi2films on Si from 0.062 to 2.76 eV by ellipsometry and spectrophotometry. The energy dependencies of the dielectric constants show Drude behavior at energies lower than ∼0.2 eV with Drude parameters ℏ&ohgr;p=(5.8±0.2) eV and ℏ/&tgr;=(0.09±0.02) eV. Using the measured optical constants, the CoSi2film is shown to have maximum absorptance at a thickness of ∼20 nm for &lgr;≳1.4 &mgr;m. Finally, we have calculated the absorptance of a composite film of CoSi2particles embedded in Si and found that the absorptance peak due to a surface plasmon resonance in the CoSi2particles shifts to higher energy as the ellipsoidal particles become more elongated, in agreement with recent observations by Fathaueretal. [Phys. Rev. B44, 1345 (1991)].

ISSN:0021-8979    

DOI:10.1063/1.350539

出版商:AIP    

年代:1992

数据来源: AIP