摘要:

An x‐ray photoelectron‐spectroscopy study of flux‐grown YBa2Cu3O7−&dgr;crystals left in moist atmospheric conditions has shown some aspects of the degradation process. Besides peaks which correspond to YO and BaO bonding in the superconducting phase, higher binding‐energy components can be attributed to BaCO3, YOH, and/or YCO3bonding. An x‐ray photoreduction of the copper species is observed in the ultrahigh vacuum. Fluorine‐gas treatments carried out at low temperature drastically modify the surface properties of the crystals. The peaks attributed to MO bonding (M=Y, Ba, or Cu) disappear and are substituted by MF bonding with a subsequent increase inEb. In the F 1sregion, the peak corresponding to those bondings is observed aroundEb&bartil;684.5 eV, a value which is in agreement with those claimed for the corresponding inorganic fluorides. The signature of those surface fluorides remains, even when the samples are left for long periods in moist air. Within the F 1senvelope, chemisorbed fluoro species withEb&bartil;686.5 eV are observed, especially when the fluorination process is performed at room temperature. These species are eliminated by vacuum annealing at 350 °C. In specific conditions, a high‐binding‐energy contribution arises (Eb&bartil;689 eV) which can be ascribed to the formation of surface ‘‘carbon fluoride.’’

ISSN:0021-8979    

DOI:10.1063/1.347124

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The magnetostatic self energy is calculated for the magnetization curling mode in a finite cylinder, for the model proposed by Ishii and Sato [J. Appl. Phys.65, 3146 (1989)]. It is shown that slight modifications of that model can eliminate all the cumbersome computations and allow all the integrations to be done analytically.

ISSN:0021-8979    

DOI:10.1063/1.347125

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

X‐ray diffraction studies carried out on a Ho0.85Tb0.15Fe2−xCox(x=0–2.0) system revealed that all the compositions investigated retained theC15‐type cubic Laves phase structure. Magnetization and electrical resistivity measurements were carried out in the temperature range 77–750 K. The Curie temperature was found to increase initially from 630 K forx=0 to a value of 690 K forx=0.5 and then decrease to 130 K forx=2.0. Several anomalies observed in the resistivity measurements have been identified corresponding to spin reorientations occurring in this system. Some of these spin reorientations have also been observed in the magnetization measurements.

ISSN:0021-8979    

DOI:10.1063/1.347160

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Domain structures in an applied field are simulated by minimizing the total energy in two dimensions. Wall positions and magnetization directions in the applied field are determined mainly by the balance of magnetostatic energyEsand Zeeman energyEm. On the other hand, the number of domains is determined by the balance of anisotropy energy and wall energy. Using this calculation, a soft magnetic film of low aspect ratio, such as a magnetic head core, is analyzed in the applied field. It was found that the energiesEsandEmare seemed to be determined by the film shape independently of the number of domains, and the ratio ofEstoEmis about 1 to −2. Moreover, the change in domain structure induced by applied field agrees with observations using a Kerr microscope. By analyzing a trapezoidal film which is similar to the thin‐film head core, we can obtain the angle of the film side surface whose domain structure is stable.

ISSN:0021-8979    

DOI:10.1063/1.347126

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The structure of pseudobinary Co‐Zr and Cu‐Zr metallic glasses, where Co and Cu were substituted by Fe, was studied by means of  57Fe‐Mo¨ssbauer spectroscopy. The resulting spectra can be divided into two different classes: (1) For Zr concentrations larger than 67 at. %, the structure of the glass, as derived from Mo¨ssbauer spectroscopy, is nearly independent from a substitution, leading to only small changes of the average isomer shifts and quadrupole splittings. These small variations of hyperfine parameters can be ascribed to charge transfer effects. (2) For lower Zr contents, pronounced systematic changes of hyperfine parameters occur upon Co or Cu substitution. Beyond changes of the average hyperfine values, the corresponding quadrupole‐splitting distributions become dependent on the degree of substitution, i.e., there is a concentration dependent splitting into two peaks. For the amorphous binary Zr‐Fe system, these two peaks have been connected to two amorphous phases with concentrations near Zr2Fe and ZrFe2. At least for the Fe‐doped Cu‐Zr metallic glasses it could be demonstrated that Fe does not substitute the Cu sites but rather creates preferred environments.

ISSN:0021-8979    

DOI:10.1063/1.347127

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

To understand the ac voltage aging of low‐mobility materials, the amount of energy involved in the degradation process must be known. Electroluminescence experiments are a choice environment for solving this problem since luminescence spectra give direct information on the energy released during aging. Carrier injection is needed for electroluminescence to take place, but it is not clear whether luminescence occurs at the injection threshold or later on. We investigate this problem using two very sensitive arrangements: a charge detector and a photon‐counting system. The experimental threshold fields for charge injection and electroluminescence are compared, as are their ac time variations. In polyethylene, charge injection is detected prior to electroluminescence. But it is found that the two phenomena begin at the same electric field if the space‐charge effect is taken into account in the estimation of the luminescence threshold. A discussion of the experimental results yields the picture of a recombination‐controlled luminescence. The energy released during the radiative as well as the nonradiative recombination processes induce chemical damage of the dielectric.

ISSN:0021-8979    

DOI:10.1063/1.347197

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The temperature dependence of the energy gaps for sulfur‐annealed copper indium disulfide has been studied by photoreflectance in the temperature range of 10–300 K. The sulfur‐annealed sample has been found to have larger transition energies, smaller positive temperature coefficients of energy gaps, and larger spin‐orbit splitting energy than the as‐grown sample. This can be explained by the reduction ofd‐level contributions in the upper valence band probably caused by the variation of lattice distance due to native defects.

ISSN:0021-8979    

DOI:10.1063/1.347128

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Various approximations in the calculations of electron energy‐loss cross section and x‐ray absorption coefficient have been evaluated using the SiL2,3edge as an example. It was found that the random phase approximation, for which the cross terms are neglected in the expressions of electron energy‐loss cross section and x‐ray absorption coefficient, is a very good approximation. The dipole approximation in the calculation of the electron energy‐loss cross section was found to begin to break down even at small acceptance half angles, such as 12.5 mrad. For energy losses larger than that of the SiL2,3edge the dipole approximation will break down faster. This approximation also fails for a large scattering angle even at a small energy loss. The electron energy‐loss cross section and x‐ray absorption coefficient of the SiL2,3edge has been calculated and compared. The large difference in relative intensity reported experimentally between the two spectra is not confirmed in this study. More experimental comparisons between electron energy loss and x‐ray absorption spectra are recommended for Si and other compounds.

ISSN:0021-8979    

DOI:10.1063/1.347129

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The low‐temperature photoluminescence (PL) spectra of CuGaS2are investigated in this work. At 4.2 K, high‐quality single crystals grown by both the iodine‐transport and the melt‐growth methods showed many sharp PL lines. Free‐exciton (2.504 eV) and bound‐exciton (2.501, 2.495, and 2.493 eV) lines are studied in detail with respect to transition energy, temperature, and excitation intensity. Phonon‐assisted transitions have been observed in the low‐energy regions of zero‐phonon lines at 2.493, 2.404, and 2.397 eV. Most of them have been well interpreted on the basis of the phonon energies previously obtained by Raman measurements.

ISSN:0021-8979    

DOI:10.1063/1.347131

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The results of our studies on the reduction of the emission current from an osmium‐alloy‐coated dispenser cathode (derivative of theMcathode) due to exposure to various gases or vapors are reported. The primary interest of these studies is poisoning by chemical agents that might be encountered in the vacuum system of an induction linear accelerator. Of the constituents normally found in a tight unbaked vacuum chamber, the cathode is most sensitive to partial pressures of water vapor. Of the vapors studied which might be encountered in an induction accelerator, Freon was the most virulent poison.

ISSN:0021-8979    

DOI:10.1063/1.347132

出版商:AIP    

年代:1990

数据来源: AIP