摘要:

(Bi,Pb)2Sr2Ca2Cu3OxAg‐sheathed tapes were fabricated by the standard ‘‘powder in tube’’ technique. Because of the platelike growth of the Bi HTc‐phase, it is possible to texture the wires by a rolling process, which improves the mechanical density. This implies an increase of the critical current density not only in zero magnetic field, but also in all external magnetic fields. By measuring &rgr;(T,B) curves, we have estimated the activation energy for thermically activated flux creep,U0, to vary between 200 and 20 meV in magnetic fields between 0.005 and 10 T. These lowU0values are approximately ten times smaller than in Y1Ba2Cu3O7−&dgr;. For the Bi 2223 tapes, a power lawU0∼B−&agr;with &agr;=0.277±0.030 was obtained.

ISSN:0021-8979    

DOI:10.1063/1.350785

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

(100)‐oriented FeCu films, containing 80 and 50 at. % Fe, have been deposited on (100) Si using (100) Pd/Cu seed layers. Heating of the samples helps identify the structures of the FeCu films, especially the one with 50 at. % Fe. The FeCu(50/50) film shows a structure with possibly two phases: one with a lattice spacing close to that of (100) Fe, and one to that of (100) Cu, henceforth assigned to the Fe‐ and Cu‐stabilized FeCu phases, respectively. The FeCu(80/20) film shows only one phase, with a lattice spacing close to that of (100) Fe. Ferromagnetic characteristics are detected up to an anneal of 30 min at 600 °C for the FeCu(80/20) films, with a reduced saturation magnetization above 400 °C. For the FeCu(50/50) films, their ferromagnetic characteristics remain little changed up to 300 °C, with reduced saturation magnetization at 400 °C, and disappear completely after an anneal at 500 °C. The results are compared with those containing elemental Fe layers deposited on different metal seeds, and the reaction mechanisms of the present structures are discussed.

ISSN:0021-8979    

DOI:10.1063/1.350786

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The positron annihilation technique was adopted to study the defect structure of the dielectric ceramics with the nominal composition (Sr1−xBi2/3x) TiO3, wherexwas 0, 0.06, 0.11, 0.16, 0.21, 0.25, 0.29, 0.33, 0.36, 0.40, and 0.43, respectively. A two phase field occurs above 0.36. The results show that the dielectric properties are closely related to the defect structure which varies as bismuth content changes. It is clear that the complex defect was formed in the materials, wherexranged from 0.11 to 0.36. The existence of the complex‐defect in the perovskite structure results in the typical ‘‘hopping ions’’ relaxation polarization.

ISSN:0021-8979    

DOI:10.1063/1.350787

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The effective dielectric constant, bulk modulus, and shear modulus of isotropic polycrystals with piezoelectric grains are studied using an effective medium approximation (EMA) and generalized Hashin–Shtrikman bounds. The EMA determines self‐consistently the electromechanical interaction of grains with the surrounding composite. Numerical values for the moduli are computed for barium titanate and compared with available experimental data, as well as with classical estimates for the moduli. Further assessment of the EMA is made by computing numerical values of the effective moduli for ideal polycrystals, based on numerical data for crystals with strong piezoelectric coupling and comparing the resulting values with classical estimates. Similar comparisons are made for the generalized Hashin–Shtrikman bounds. On ‘‘ideal’’ polycrystals the gap between the upper and lower bounds can be 30% narrower than the corresponding gap if piezoelectric coupling is neglected.

ISSN:0021-8979    

DOI:10.1063/1.350788

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Sol‐gel derived ferroelectric thin films of lead zirconate titanate have been annealed through the rapid thermal annealing (RTA) technique to investigate the effect of various annealing temperature‐time combinations. Crystallization of the film into the perovskite phase required 10 s at 600 °C and a mere 1 s at 700 °C. Rapid thermally annealed films recorded weak‐field permittivities greater than 1000, dissipation losses of 0.02–0.05, maximum remanent polarization of 29 &mgr;C/cm2, and coercive field around 40 kV/cm. RTA films are distinguished by superior breakdown strengths, and morphologically smoother surfaces. The frequency dependent dielectric constants have been discussed in terms of a lumped circuit model.

ISSN:0021-8979    

DOI:10.1063/1.350789

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have studied the time response of visible, red photoluminescence in anodically etchedp−silicon films. The principal features of our measurements are luminescence with two wavelength components, and a temperature dependent rise time of 24 &mgr;s and a decay time of 47 &mgr;s at room temperature. Results from our samples show some similarities to characteristics measured in amorphous Si, suggesting that some low‐dimensional or disordered Si phase may play a role in the observation of visible light from this new photonic material.

ISSN:0021-8979    

DOI:10.1063/1.350790

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Mg‐doped In0.32Ga0.68P epitaxial layers have been grown on (100) GaAs0.61P0.39substrates by liquid‐phase epitaxy using a supercooling method. The electrical properties of the Mg‐doped layers are investigated using capacitance‐voltage methods at 300 K. The full width at half maximum value of the 300‐K photoluminescence spectrum increases with hole concentration for Mg‐doped layers. The 77‐K photoluminescence spectra show two peaks and one broad band, and their relative intensities change with various hole concentrations. The Mg acceptor ionization energy obtained from 77‐K photoluminescence spectra is in the range from 20 to 38 meV. Finally, the photoluminescence spectra of Mg‐doped In0.32Ga0.68P layers with three hole concentrations measured at various temperatures between 20 and 150 K are presented for the gradual evolution of the peaks.

ISSN:0021-8979    

DOI:10.1063/1.350791

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Domain switching of 90° ferroelectric domains in tetragonal BaTiO3and PbTiO3is induced by the application of stress along specific crystallographic axes. For BaTiO3, single‐domain crystals are obtained from twinned specimens by the application of ∼1.1 MPa of stress parallel to theaaxis and twin boundaries are induced by application of ∼0.22 MPa of stress parallel to thecaxis. Similar piezoelectrically‐induced domain switching was observed in PbTiO3at elevated temperature. We observed the rotation of the crystallographic axes associated with domain switching via micro‐Raman spectroscopy. These results were consistent with optical microscope images of the domain switching which demonstrates the usefulness of micro‐Raman spectroscopy for the study of ferroelectric domain structures. A phenomenological treatment of domain switching in a piezoelectrically‐coupled system is described.

ISSN:0021-8979    

DOI:10.1063/1.350792

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Reflectance and photoluminescence spectroscopy have been used to study the optical properties of high quality InxGa1−xAs/GaAs (0.13≤x≤0.30) single quantum wells. The results show strong agreement with the theoretical model used taking into account the strain potential. The agreement of the theoretical model, though, deviates from the experimental results for large values of excess strain in the well. For the case of the large strain (x=0.30) the reflectance indicates the strain in the well is hydrostatic rather than biaxial. The relevance of this fact is discussed in relation to device performance.

ISSN:0021-8979    

DOI:10.1063/1.350793

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Heteroepitaxial layers of InP with thicknessDranging from 0.1 to 6.0 &mgr;m were grown by low‐pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function ofDwith transmission electron microscopy, x‐ray diffraction, and photoluminescence spectroscopy. ForD<2 &mgr;m, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasingD. ForD≳2 &mgr;m the portion of the layers away from the heterointerface by more than 1.5 &mgr;m shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasingD.Typical dislocation densities in the neighborhood of the top surface are in the mid 107cm−2range whenDsurpasses 4.0 &mgr;m. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x‐ray rocking curves diminishes from values larger than 500 arcsec forD<1.0 &mgr;m to about 200 arcsec forD≳4.0 &mgr;m. Secondly, the near‐band‐edge photoluminescence transitions, which forD<2.0 &mgr;m are predominantly determined by defect‐induced band tailing, display excitonic character. Thirdly, below‐band‐gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate.The strain removes the degeneracy between the light‐ and heavy‐hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for allD. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence‐band splitting decreases from 12.5 meV forD=0.3 &mgr;m to saturation values of 5.6 meV forD≳3.0 &mgr;m, indicating strain relaxation withDin qualitative agreement with x‐ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation againstDand the observed location of the dislocations throughout the layers.

ISSN:0021-8979    

DOI:10.1063/1.350794

出版商:AIP    

年代:1992

数据来源: AIP