摘要:

Picosecond Raman spectroscopy has been employed to study electron–phonon interactions in the wide band‐gap semiconductor GaN. By using very intense picosecond laser pulses in the visible spectral range, electron‐hole pairs were generated through the two‐photon absorption in GaN. The relaxation of these high energy electrons and holes were used to interrogate electron–phonon interactions. We have found that electrons thermalize toward the bottom of the conduction band by emitting primarily longitudinal optical phonons. Our work demonstrates that the Fro¨hlich interaction is much stronger than the deformation potential interaction in wurtzite GaN. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115040

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report direct measurements of nonequilibrium electron distributions and electron drift velocities in a GaAs‐basedp‐i‐nnanostructure semiconductor by using transient subpicosecond Raman spectroscopy. Experimental conditions are such that the velocity overshoot phenomenon dominates the transport properties of the photoexcited carriers. These experimental results are compared with ensemble Monte Carlo calculations. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115041

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In the study of interdiffusion phenomena in layered structures of III‐V compounds by high resolution electron microscopy, contrast features in the micrographs can be correlated with the variation of the chemical composition of the crystals. For quantitative interpretation of the micrographs a fuzzy logic approach is adapted to extract chemical information. The linguistic variable ‘‘similarity of images’’ is derived from the standard deviation (SD) of their difference patterns, which proved to be an appropriate measure. The approach developed is used to analyze simulated contrast tableaus of GaAs/P (As/P variation) and experimental micrographs of Al/GaAs (Al/Ga variation). ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114375

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Nanocrystalline silicon aggregates imbedded in a predominantly amorphous silicon layer have been observed in anodically etchedp‐Si(100) by using valence band x‐ray photoelectron spectroscopy and lattice imaged high‐resolution transmission electron microscopy (XTEM). XTEM has identified the as‐prepared porous silicon to be a mixed phase of amorphous and nanocrystalline silicon, with the nanocrystalline aggregates being randomly dispersed throughout the full thickness of a 1 &mgr;m thick amorphous layer and exhibiting a size distribution from 2 to 5 nm in diameter. The abundance of the nanocrystalline aggregates seems to decrease as the anodic etching proceeds and as the sample is irradiated by x rays at room temperature in ultrahigh vacuum. Valence band photoelectron measurements show evidence for a crystalline‐to‐amorphous phase transition induced by x radiation which may, in part, be activated by photoelectron stimulated hydrogen desorption. The x‐ray irradiated samples also exhibit a significant reduction in photoluminescence yield, possibly caused by a reduction in the density of nanocrystallites. The observed mixed phase porous silicon and the metastability of the nanocrystallites help to explain apparent contradictory descriptions of the nature of porous silicon. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114376

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Superconducting electronic circuits surrounded by various configurations of holes in the superconducting ground plane have been imaged using a high resolution scanning superconducting quantum interference device (SQUID) microscope. These data demonstrate that in the weak field limit continuous moats trap flux more effectively to protect the circuits than small holes in the same configuration. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114377

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Measurements of transport alternating‐current (ac) losses in Bi2Sr2CaCu2O8+xpowder‐in‐tube Ag‐sheathed tapes at 4.2 and 65 K and at 20, 60, 200, and 400 Hz were carried out and comparisons to the theoretically predicted hysteretic loss plus ohmic loss were made. The measured loss, albeit larger than predicted, was found to agree with Bean’s critical state model. The loss, having a linear dependence on the frequency of the alternating transport current and increasing with decreasing critical current density, confirms hysteresis as the dominant source of power dissipation at both 4.2 and 65 K. However, at 4.2 K, the loss varies as the square of the amplitude of the transport current, which is not consistent with the model. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114378

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have investigated superconductor–normal metal–superconductor junctions with metallic perovskite SrRuO3barriers. Ramp‐type junctions with ion‐beam‐etched and wet‐chemical‐etched YBa2Cu3O7(YBCO) ramps have been compared with planar junctions where the interfaces were fabricated completelyinsitu. Regardless of the fabrication method, the junction properties were determined not by the metallic SrRuO3barrier, but by an insulating layer at the interface. The shape of theI–Vcurves, and the temperature dependence of the normal resistance of junctions, could be well explained by quasiparticle resonant tunneling via two localized states in the interface layer. Furthermore, a model for the Cooper pair transport will be discussed that assumes resonant tunneling via one localized state as transport mechanism. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114379

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

High quality epitaxial YBa2Cu3O7−x(YBCO) thin films have been deposited onto ferrimagnetic yttrium iron garnet (100) substrates using a double buffer layer lattice engineering scheme. The YBCO films arec‐axis oriented and show superconductive transition temperatures of 88–89 K with transition widths ≤0.5 K. The films also exhibitedJc’s of 2.7×106A/cm2in zero field and 1.1 ×106A/cm2in 2000 G at 77 K. Dielectric resonator measurements indicated unloadedQvalues of 29 000 at 77 K and 24 GHz for unpatterned films. These values correspond to a surface resistance (RS) of 500 &mgr;&OHgr; at 77 K, 10 GHz, which is the lowest obtained to date for a YBCO film on a ferrimagnetic substrate. Such films could be used for fabricating miniature‐high‐temperature superconducting microwave elements such as nonreciprocal and passive microwave devices, making possible, their integration on a single ferrimagnetic wafer. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114380

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Magnetic iron oxide films were successfully deposited on heated fused‐quartz substrates via the plume generated by the excimer laser ablation of poly(ferric methacrylate), an organometallic polymer, in an oxygen atmosphere. The investigation for the deposited films by means of x‐ray photoelectron spectroscopy, Raman spectroscopy, x‐ray diffraction, and scanning electron microscopy demonstrates that these films consist of iron‐rich Fe3O4and &agr;‐Fe2O3phases that are made up of ball‐like grains with nanometer‐size structure. Magnetic hysteresis loop of the deposited film was also measured. The values of saturation magnetization, remanence magnetization, and coercivity are 35.6 emu/cm3, 6.13 emu/cm3, and 200 Oe, respectively. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114381

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3system. At 125 K, the CMR of the La0.5Ca0.5MnO3reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114382

出版商:AIP    

年代:1995

数据来源: AIP