摘要:

Substrate selection presents particular challenges for the production of high‐quality high‐temperature superconducting (HTS) thin films suitable for applications. Because the substrate is generally a passive component, it is often ignored and assumed to have a negligible effect on the structure residing on top of it. There is also a technological motivation to use substrates that conventional wisdom would argue are unlikely to support high‐quality HTS films. These facts have led to rediscovery of many of the fundamental issues governing the role of the substrate in determining the properties of the thin film(s) it supports. For this reason, the study of issues in substrate selection for HTS materials presents a microcosm for substrate selection more generally. We consider the major issues governing the role of the substrate in HTS thin‐film technology and discuss many of the material classes and specific materials that have been studied for their suitability as substrates for HTS films. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362675

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Pr3+‐doped SrAl12O19has been investigated spectroscopically as a visible laser material. Its3P0fluorescence lifetime is rather long for an oxide, about 35 &mgr;s. This excited state exhibits only mild concentration and temperature quenching, so that concentrations high enough to give good ground state absorption into the3P0and3P1manifolds still give strong, long‐lived emission. The ground state absorption spectra indicate that excited state absorption into the 4f5dconfiguration is less likely to be significant at laser wavelengths in this host than in Y3Al5O12. The stimulated emission cross sections of major emission lines have been estimated, and are quite realistic for laser operation. Laser pumped laser operation has been demonstrated in the red (3P0→3F2) at room temperature and in the blue‐green (3P0→3H4) at cryogenic temperatures. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361085

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This article reports measurements of the group velocity surface of silicon in the region of the ⟨100⟩ axis. Pulsed ultrasonic beams generated by a piezoelectric longitudinal mode transducer are transmitted through water and focused onto a small spot on the surface of a (001) oriented silicon single crystal disk. This gives rise to transient elastic waves which propagate in a wide range of directions through the specimen and they are detected by a small piezoelectric longitudinal mode transducer fixed at the center of the opposite face. The imaging of the group velocity surface about the ⟨100⟩ axis is accomplished by scanning the focused beam in various directions along lines that intersect at epicenter. The scanned images reveal complex foldings of the slow transverse sheet of the ray surface about the ⟨100⟩ axis, in general agreement with the predictions of ray acoustics. However, the observed image manifests pronounced wave motions well beyond the cuspidal edges predicted by ray acoustics. This phantomlike feature or eidolon can be explained on the basis of diffraction resulting from the finite wavelength of the ultrasonic waves. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361086

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The phenomena occurring during the electrical explosion of segmented wires are described. It was observed that with a wire of varying thickness, the smaller diameter parts explode first, ejecting metal vapor radially. Breakdown occurs through the vapor, creating current carrying channels which bypass the larger diameter parts of the wire. This may result, in some cases, in the larger diameter parts not exploding at all. Only slight deformations occur at their ends. On the boundary between the exploding and nonexploding sections of the wire, such deformations may cause the evolution of an accelerated slug, similar to that of a ‘‘self‐forging fragment.’’ The phenomenon of the current bypassing metallic parts may offer an explanation to the way the ‘‘segmented diverters’’ work on a lightning stricken aircraft radome. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361087

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The GaAs/GaInAs/GaAs quantum‐well structures grown by metalorganic chemical vapor deposition were studied using high‐resolution x‐ray diffractometry and photoluminescence techniques. Diffraction profiles were fitted to experimental rocking curves by a simulation procedure, based on the direct summation of scattered waves. The analytical expressions obtained shed light on various relevant parameters and, together with a specific growth procedure, permitted determination of the thickness and composition of strained quantum wells. By following fine interference effects in the x‐ray diffraction spectra quantum wells as thin as 1.4 nm could be characterized. In order to check the validity of the procedure, the obtained quantum‐well parameters were used to calculate the peak positions in luminescence spectra and good agreement with experimental data was found. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361075

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Sequential ion implantation of As and Ga into SiO2and &agr;‐Al2O3followed by thermal annealing has been used to form zinc‐blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361088

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A magnetic resonance force microscope was used to demonstrate three‐dimensional nuclear magnetic resonance imaging with micrometer‐scale spatial resolution. The sample was mounted on a silicon nitride cantilever that served as a micromechanical force sensor. A nearby magnetic tip generated a field gradient of 22 G/&mgr;m. A three‐dimensional magnetic resonance force map of the1Hspins in the sample was produced by lateral scanning of the magnetic tip relative to the sample and by varying the rf frequency of the spin excitation. The real‐space spin density of the sample was reconstructed from the force map by means of a deconvolution technique. The spatial resolution achieved in the experiment was ∼3 &mgr;m in the axial direction. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361089

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The surfaces of virgin and chemically etched poly(tetrafluoroethylene) (PTFE) have been studied using scanning electron microscopy (SEM), and atomic force microscopy (AFM) in both contact and tapping modes. Contact mode AFM images of this relatively soft polymeric material are dominated by tip‐induced imaging artifacts. When subsequent, AFM imaging was performed in tapping mode these artifacts were eliminated, and comparable tapping mode AFM and SEM images were obtained for even the highly porous, unstable surface that results from sodium naphthalenide etching. Interfacial force microscopy force versus displacement, and creep experiments were performed to determine the nanomechanical nature of virgin PTFE. These experiments show that virgin PTFE is a viscoelastic material which is capable of supporting large forces on the millisecond time scale but creeps dramatically at longer times. Clearly, with scanning probe techniques which utilize constant probe force feedback, one should expect image distortions, as we observe, with soft materials such as virgin or etched PTFE. Conversely, with tapping mode AFM, rational images require contact times (&mgr;s) that are much shorter than creep times (ms). Thus, viscoelastic material characteristics determine the need for tapping mode AFM over contact mode AFM. By comparing tapping mode AFM images of virgin and etched PTFE surfaces, we can understand the three‐dimensional character of the etched surface necessary for mechanical interlocking and resultant strong metal adhesion.

ISSN:0021-8979    

DOI:10.1063/1.361090

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We studied electro‐optical properties of 5–13‐&mgr;m‐thick films of a short pitch (0.43 &mgr;m) chiral smectic C liquid crystal in bookshelf structure. The anchoring at the silane treated surfaces was relatively weak so that the relaxed bookshelf structure was helical also at the surfaces. Low fields cause only a deformation of the helix. Larger fields unwind the helix and give a quasibistable switching since the helix recovers at zero fields only slowly via a nucleation process. There is an intermediate field range where the sample is in a mixed state: In some areas the helical structure remains stable, while in other areas switching occurs between two unwound states. In the intermediate field range bistable switching with a gray scale is possible. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361091

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The extent of interdiffusion in Cd1−xMnxTe/CdTe quantum‐well structures grown by molecular‐beam epitaxy was monitored by photoluminescence and photoluminescence excitation spectra. Thermal annealing of as‐grown and ion‐implanted structures in over pressures of cadmium (or tellurium) provide clear evidence that diffusion is controlled by cation vacancies and are consistent with a strong dependence of the interdiffusion coefficient on the vacancy concentration. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361092

出版商:AIP    

年代:1996

数据来源: AIP