摘要:

Optical spectra of rare‐earth impurity ions in crystalline solids are inhomogeneously broadened in mixed crystal systems. This principle has been used in Y3(Al1−xGax)O12: Nd to lower the cross section of the 1.06‐&mgr;m laser transition by 3.3 relative to Y3Al5O12: Nd, while not substantially altering the thermal conductivity, thereby providing a host for Nd with improvedQ‐switched laser performance. Spectroscopic and paramagnetic resonance studies are reported to elucidate the detailed nature of the broadening.

ISSN:0021-8979    

DOI:10.1063/1.1663331

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A circuit‐theoretic treatment is presented for thin‐film optical waveguides using a class of real anisotropic and gyrotropic materials. The analysis is based on the two‐mode approximation in normal‐mode theory. The terminal behavior of those guides is described by the 2 × 2 transmission matrix and expressions for matrix elements of six systems that are introduced as canonical elements in circuit synthesis are derived. Properties of canonical elements are discussed with particular attention on their reciprocity. Using the transmission matrix, we can treat the design of thin‐film optical devices by a simple matrix operation familiar in conventional transmission‐line circuit synthesis. As a typical application the design of various nonreciprocal integrated‐optical devices is treated in detail. The desired response is synthesized by cascading selected canonical elements in an appropriate order. Examples include the gyrator, unidirectional mode converter, differential phase shifter, isolator, and circulator.

ISSN:0021-8979    

DOI:10.1063/1.1663332

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A theory was worked out to analyze the effect of intense magnetic fields on the forward current gainhFE. The quantum splitting of energy levels (Landau levels) was utilized. The theory is in very good agreement with available experimental data.

ISSN:0021-8979    

DOI:10.1063/1.1663333

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Epitaxial layers of PbSe and Pb1−xSnxSe have been grown on cleaved BaF2and SrF2single‐crystal substrates by vacuum deposition of the evaporated binary compounds. The influence of the effusion conditions on the stoichiometry of the molecular beam is considered and it is shown that evaporation of PbSe results in two‐phase layers that contain precipitated lead. Coevaporation of selenium compensated the excess lead and permitted the control ofn‐ orp‐type carrier concentrations in epitaxial layers in the range 3×1016−6×1017cm−3. Structural studies indicate that the layers are single crystals with about the same concentration of low‐angle grain boundaries as observed on the substrates. Low‐temperature Hall mobilities were measured that are as large as those obtained with annealed bulk crystals. The value 4.8×105cm2/V sec for a PbSe layer with 9.2×1016hole cm−3is the largest mobility measured for this material. Below 77°K the mobilities depend markedly on carrier concentration and thermal history of the specimens.

ISSN:0021-8979    

DOI:10.1063/1.1663334

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The absolute signs and magnitudes of all the optical second‐harmonic generation coefficientsdijfor ferroelectric guanidinium aluminum sulfate hexahydrate (GASH) have been obtained by using single‐domain crystals and a 1.06‐&mgr;m Nd laser. It is found that in esu unitsd22= − 0.25 × 10−9,d31=d15= + 0.020 × 10−9, andd33= + 0.047 × 10−9, where the absolute directions are defined in the usual manner. The magnitudes ofd22andd31are consistent with values estimated from data previously reported by others. The vector part of the &dgr; form of thedtensor is &dgr;&ngr;= 2&dgr;31+ &dgr;33= + 1.2 × 10−7esu. The value &dgr;&ngr;/Ps= + 3.4 × 10−7and is about −5 times the value characteristic of a number of oxide‐type ferroelectrics. Upon polarization reversal all thedijcoefficients change sign with respect to a fixed coordinate system, which is consistent with structure data and symmetry considerations.

ISSN:0021-8979    

DOI:10.1063/1.1663335

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A study has been made of the effect of dislocation density on the properties ofp‐type GaAs (Ge‐doped,p= 1 × 1019cm−3) grown homoepitaxially on GaAs and heteroepitaxially on GaP. It has been found that dislocation densities > 5 × 106cm−2reduce the minority‐carrier diffusion length in thep‐type material and, consistent with this reduction, degrade the junction electroluminescence andp‐layer photoluminescence. At high dislocation densities (> 5 × 106cm−2), it is also found that subgrains form in the GaAs epitaxial layers grown either heteroepitaxially on GaP or homoepitaxially on GaAs. It is surmised that the dislocations polygonize during growth to form subgrain boundaries rather than form due to nucleation at the substrate. Finally, the results indicate that the electron diffusion length is limited to the average dislocation spacing with the dislocations acting as nonradiative recombination centers for electrons.

ISSN:0021-8979    

DOI:10.1063/1.1663336

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Levitation of high‐speed transportation vehicles carrying superconducting magnets may be achieved by repulsion from the currents induced in a conducting guideway. The vertical accelerations of such vehicles moving over a guideway of finite roughness will lead to alternating currents in the superconducting magnets and thus to power dissipation in the cryogenic system. Previous investigators have concluded that the resulting contribution to the cryogenic heat load may be as great as that from all other sources and thus impose serious weight and space penalties on the over‐all system design. These investigators have quite generally assumed that the alternating fields will completely penetrate the superconducting wire at all field levels and that the use of twisted multifilament composite wire is thus essential if ac losses are to be minimized. We show that this assumption is incorrect and that it results in a significant overestimate of the loss in multicore wire at the low alternating field levels appropriate for the full‐scale vehicle‐guideway system. Our analysis further predicts, and our experiments confirm, that an order‐of‐magnitude improvement inlow‐levelac loss performance can be achieved merely by the substitution of single‐core superconducting wire for the multicore wire ususally proposed. This should reduce the ac loss to about 1.3 W/vehicle from the 14 W for a typical multicore wire. We find a cubic dependence of the loss rate on the alternating field level, and this implies that an additional loss reduction would accrue if the coil bundle could be expanded to reduce the average current density while preserving the total current and lift force. By these means the ac loss contribution to the cryogenic heat load of a levitated vehicle could be made quite negligible.

ISSN:0021-8979    

DOI:10.1063/1.1663337

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Measurements of the x radiation and the electron loading in a niobium cavity resonant in the TM010mode at 1208 MHz have been made in order to study enhanced electron field emission in superconducting cavities. Measurements have also been made of the low‐temperature dc electron field emission from bismuth, niobium, copper, and tungsten utilizing a configuration which approximates the point‐to‐plane geometry. In the rf cavity measurements, it is found that the x‐radiation count rate data follow a modified Fowler‐Nordheim equation, which takes into account the time‐dependent rf fields, the electron dynamics in the rf fields, the emission of secondary electrons, and the production and absorption of x radiation. Further, it is found that the x radiation from the cavity can be reduced by a factor of 104−106by helium‐ion sputter processing at low temperature. From x‐radiation data and electron loading data interpreted with the modified Fowler‐Nordheim equation, it has been shown that, after helium‐ion sputter processing at low temperature, the electron field emission process can be consistently described by ordinary Fowler‐Nordheim electron field emission from geometrical projections at which the electric field is enhanced. Before processing, the electron field emission process cannot be consistently described by ordinary Fowler‐Nordheim electron field emission. A point of view which is consistent with the data is that before helium‐ion sputter processing the electron field emission is enhanced by resonant tunneling through surface states due to adsorbed atoms, and that as a result of processing, the adsorbed atoms are removed. This point of view is supported by calculated sputtering rates during the processing. In the dc measurements, it is found that the low‐temperature electron field emission is consistent with the Fowler‐Nordheim theory for an emission tip that has been cleaned by reverse field desorption. However, it is found that the dc electron field emission increases slowly with time, presumably as a result of adsorption of material ejected by electron bombardment of the anode. The resulting changes in the dc electron field emission characteristics are of just the type expected for resonant tunneling through adsorbate surface states, and are remarkably similar to the differences in x‐radiation count rate data observed for a superconducting cavity before and after helium‐ion sputter processing.

ISSN:0021-8979    

DOI:10.1063/1.1663338

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A simple theory for interaction energy of dislocations with magnetization through magnetostriction is developed for Permalloy films with varying composition. It is found that the inclusion of the dislocation energy in the total anisotropic energy in the pair‐strain model gives a better fit with experimental results.

ISSN:0021-8979    

DOI:10.1063/1.1663339

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Epitaxial films of Sm0.4Y2.6Ga1.2Fe3.8O12have been grown under carefully controlled and essentially identical conditions on Gd3Ga5O12substrates cut in four different orientations: {112}, {111}, {110}, and {100}. It is found that the two‐parameter model for growth‐induced anisotropy is inadequate to describe the anisotropies experimentally determined from this group of films. In the context of the preferential site occupation mechanism for growth‐induced anisotropy, these results indicate that the site‐preference factors for the yttrium and samarium ions depend on the orientations of the growth faces in a more complicated way than has heretofore been considered.

ISSN:0021-8979    

DOI:10.1063/1.1663340

出版商:AIP    

年代:1974

数据来源: AIP