摘要:

Experiments were performed on the modulation of CO2laser radiation (10.6 &mgr;) at the piezoelectric resonance frequencies of a GaAs sample. The GaAs crystal length was 5.09 cm in the[11¯0]direction with a cross section of 0.291 cm [110] × 0.278 cm [001]. Experimentally, one shear and two thickness vibration modes were observed. The voltages to yield half‐wave retardation by means of the piezoelectro‐optic effect were found to be 4.15 V at the shear resonance frequency (736.900 kHz) and 15.4 and 51.0 V at the first (675.800 kHz) and second (1019.130 kHz) thickness resonance frequencies, respectively. The bandwidths were in the range 30–100 Hz. When the electro‐optic modulation scheme is used instead of the resonant piezoelectro‐optic modulation scheme, 5.2 kV are required to obtain half‐wave retardation with the same sample. The shape of the sample precluded analytically solvable models for the observed phenomena. However, approximate models yield reasonable order‐of‐magnitude agreement with the observed values.

ISSN:0021-8979    

DOI:10.1063/1.1663573

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Investigations on luminescent properties of good‐quality GaN single‐crystalline epitaxial layers at temperatures in the range 1.6–300 K are reported. The high‐energy region (3.27–3.48 eV) at 1.6 K is dominated by different bound exciton transitions. Peaks are observed at 3.469, 3.454, 3.447, 3.400, 3.377, 3.355, and 3.287 eV at 1.6 K in this region; their possible origin is discussed. At higher temperatures (T> 30 K) the freeA‐exciton emission is also clearly observed, and thus its positionEAx=3.475±0.0005 eVat 1.6 K could be established (neglecting polariton effects) together with a value of 6.4±0.4 meV for the binding energy of an exciton to a neutral donor. The energy region 3.0–3.3 eV is dominated by the previously studied donor‐acceptor pair spectra, which peak at 3.263±0.003 in all our good‐quality samples (undoped and doped). From the assignment of a subsidiary high‐energy peak as free‐to‐bound transitions, a value of 29±6 meV for the donor binding energy is experimentally determined, in good agreement with the theoretical effective mass value. The region below 3.0 eV is in Cd‐doped material dominated by a broad band centered at 2.72 eV, unlike previous observations. This band is an envelope composed of several discrete series of emissions observable at low temperatures. The solubility of Cd in GaN appears to be rather low, and high‐Ohmic single‐crystalline material was not obtained, in contrast to our results from Zn‐doping.

ISSN:0021-8979    

DOI:10.1063/1.1663574

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The wavelength‐modulated reflectivity spectra in the visible and near uv of GdIG, LuFeO3, FeBO3, Fe2O3, Fe[NiFe]O4, and BaFe12O19are reported. Features of both the exciton sideband and the charge‐transfer spectra are observed. Comparison of these spectra allows some site assignments to be made. In particular the octahedrally coordinated Fe3+ion is found to contribute a characteristic charge‐transfer spectrum that occurs at lower energies than the tetrahedral Fe3+charge‐transfer spectrum. A further charge‐transfer process between octahedral and tetrahedral Fe3+sites is suggested to explain the garnet spectra.

ISSN:0021-8979    

DOI:10.1063/1.1663575

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Fluorescence owing to organic impurities in a Raman spectroscopic sample is quenched by the photochemical reaction, which improves the Raman spectroscopy. The quenching technique is demonstrated in the Raman spectroscopy of liquid methanol.

ISSN:0021-8979    

DOI:10.1063/1.1663576

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Point‐contact weak‐link junctions often possess hysteresis in their current‐voltage characteristics. We describe experimental evidence which suggests that this hysteresis is not due to the effects described by Stewart and McCumber but rather that this hysteresis is a self‐heating phenomenon.

ISSN:0021-8979    

DOI:10.1063/1.1663577

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A method to measure magnetic quantities of ferrimagnetic and ferromagnetic samples under the effect of time‐dependent fields [Ha(t)] is presented. The method is based on the alternating field method together with the powerful technique of synchronous detection. Interesting quantities such as the total Bloch wall area present during the magnetization reversal, the reversible permeability, and its first derivative with respect to the magnetic field at any point of the hysteresis loop are plotted rapidly, even in the case of very small samples (for instance, iron whiskers). Reversible permeability is obtained without need of extapolation. The switching time and the dynamic coercive field are easily measured as well. The sensitivity of the method is very high, and the noise is widely rejected. The technique is applied to iron whiskers and polycrystalline YIG.

ISSN:0021-8979    

DOI:10.1063/1.1663578

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The effect of diamagnetic shielding currents on the magnetic field in a tape‐wound superconducting solenoid is calculated on the basis of the isothermal critical‐state model and compared with measured values. The calculation is found to be in reasonable agreement with the data. Additional calculations showing the effect of the tape width on the diamagnetic correction to the central field are presented.

ISSN:0021-8979    

DOI:10.1063/1.1663579

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The influence of several surface preparation techniques, such as electropolishing, chemical polishing, anodizing, and firing in an UHV furnace with different cool‐down cycles, onQ0values, peak rf fields, and surface mean free pathlshave been studied in a weldedS‐band cavity having large beam tubes. With special treatments, peak fields up to 810 G atQ0≈1011have been obtained, whereas the surface mean free paths stayed fairly short(≲300 Å )not changing systematically with (surface) treatment. Hence, to obtain low rf losses and high rf fields, the purity is not important, but, as we will explain, the spatial homogeneity is very important. Measurements with Nb cylinders, processed like the cavities, showed that the variation of flux penetration depth &lgr;(Hdc) with a dc magnetic fieldHdcparallel to the cylinder surface changed markedly with surface treatment. This variation of &lgr;(Hdc) shows several features, which are related to surface properties, namely, the reversibility nearHc1and the signal atHc2as indications for roughness or oxygen in the surface sheath, the irreversibility betweenHc1andHc2as an indication for inhomogeneities, especially NbO precipitates, in a Nb surface layer. For example, by this method we observed that the usual cool down from 400 to 50°C in the UHV furnace deteriorates a Nb surface layer of several &mgr; thickness. These surface properties obtained with &lgr;(Hdc) are not only important for achieving better rf cavities but also for ac power transmission lines.

ISSN:0021-8979    

DOI:10.1063/1.1663580

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The influence of several surface preparation techniques on bothQ0values and peak rf fields has been studied in a welded beam hole cavity between 2 and 4 GHz. In the TE011mode peak rf fields up to 810 G atQ0≈1011have been obtained, whereas, in modes subject to electron loading, the limitations are below 400 G, corresponding toE≲30 MV/m. The significant difference in the breakdown field of modes with and without electron loading show that the observed frequency dependence ofHcritcan easily be explained by electron loading and radiation damage.

ISSN:0021-8979    

DOI:10.1063/1.1663581

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A technique has been developed for determining unambiguously the effective barrier height &phgr; and the average effective thicknessSof the insulating layer in a metal‐insulator‐metal tunnel junction. The quantityS&phgr;3/2is measured from the slope of the Fowler‐Nordheim plot. The experimental data are then compared with a family of calculated current‐voltage curves by using various pairs ofSand &phgr; withS&phgr;3/2fixed at the measured value, in order to determine the pair which gives the best fit. The method has been tested on the best understood tunnel barrier, those grown thermally on an Al film, and is found to give good agreement with independent measurements. From a macroscopic viewpoint, the tunnel barrier seems adequately described by a trapezoidal barrier model of nearly uniform height but with thickness variations on a microscopic scale over the junction area. The use of the method is illustrated on barriers grown thermally on Cr, thin‐film V, and bulk V. We present a series of graphs at varyingSand &phgr; for six different values ofS&phgr;3/2to facilitate the determination of approximate barrier parameters with lengthy computer calculations.

ISSN:0021-8979    

DOI:10.1063/1.1663582

出版商:AIP    

年代:1974

数据来源: AIP