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Volume 64, Issue 4, 15 August 1988

Diluted magnetic semiconductors
View Description Hide DescriptionWe review the physical properties of diluted magnetic semiconductors(DMS) of the type A^{II} _{1−x }Mn_{ x }B^{VI} (e.g., Cd_{1−x }Mn_{ x }Se, Hg_{1−x }Mn_{ x }Te). Crystallographic properties are discussed first, with emphasis on the c o m m o nstructural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the A^{II} _{1−x }Mn_{ x }B^{VI} alloys in the absence of an external magnetic field, stressing the close relationship of the s p electron bands in these materials to the band structure of the nonmagnetic A^{II}B^{VI} ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d ^{5} shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the m a g n e t i cproperties of the A^{II} _{1−x }Mn_{ x }B^{VI} alloys. In particular, we discuss the mechanism of the Mn^{+} ^{+}‐Mn^{+} ^{+} exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susceptibility of DMS in the paramagnetic range; we describe a somewhat empirical picture of the spin‐glasslike freezing in the A^{II} _{1−x }Mn_{ x }B^{VI} alloys, and its relationship to the short range antiferromagnetic order revealed by neutron scattering; and we point out some not yet fully understood questions concerning spin dynamics in
DMS revealed by electron paramagnetic resonance. We then discuss the s p‐dexchange interaction between the s p band electrons of the A^{II} _{1−x }Mn_{ x }B^{VI} alloy and the 3d ^{5} electrons associated with the Mn atoms. Here we present a general formulation of the exchange problem, followed by the most representative examples of its physical consequences, such as the giant Faraday rotation, the magnetic‐field‐induced metal‐to‐insulator transition in DMS, and the properties of the bound magnetic polaron. Next, we give considerable attention to the extremely exciting physics of quantum wells and superlattices involving DMS. We begin by describing the properties of the two‐dimensional gas existing at a DMS interface. We then briefly describe the current status of the A^{II} _{1−x }Mn_{ x }B^{VI} layers and superlattices (systems already successfully grown; methods of preparation; and basic nonmagnetic properties of the layered structures). We then describe new features observed in the magnetic behavior of the quasi‐two‐dimensional ultrathin DMS layers; and we discuss the exciting possibilities which the s p‐dexchange interaction offers in the quantum‐well situation. Finally, we list a number of topics which involve DMS but which have not been explicitly covered in this review such as elastic properties of DMS, DMS‐based devices, and the emerging work on diluted magnetic semiconductors other than the A^{II} _{1−x }Mn_{ x }B^{VI} alloys—and we provide relevant literature references to these omitted topics.

Generation of pure ion beams by field ionization of laser‐excited Rydberg atoms
View Description Hide DescriptionSodium atoms (10^{8}–10^{1} ^{1} cm^{−} ^{3}) effused from an oven were stepwise excited from their ground state to a Rydberg staten s or n d (20≤n≤26) by two pulsed dye lasers (∼5–10 ns, ∼100 μJ, ∼1.6‐mm‐diam spot). The laser energy density necessary to saturate the excitation processes from the ground state to the Rydberg state is ∼ several mJ/cm^{2}. About 50 ns after the laser irradiation, a pulsed electric field (0.5–5.5 kV/cm) was applied to the Rydberg atoms to ionize them and accelerate the resulting ions. Thus, a pulsed sodiumion beam of purity >99.99% was obtained with a maximum total electric charge of ∼5 pC and a pulse width of ∼120 ns full width at half maximum. The total electric charge of the ion beam is estimated including the effect of space‐charge field of the beam ions, and is in good agreement with the experimental values.

Attachment of low‐energy electrons to HCl
View Description Hide DescriptionThe electron‐attachment rate constants of HCl diluted in Ar and N_{2} were measured as a function of the reduced electric fieldE/N. These data were converted to the electron‐attachment cross section of HCl using the electron‐energy distribution functions of pure Ar and N_{2}. The dependence of the electron‐attachment rate constant and the mean electron energy on the fraction of HCl in each buffer gas was investigated. A comparison of the current result with both available experimental data and theoretical calculations is made.

Internal and near‐surface electromagnetic fields for a spherical particle irradiated by a focused laser beam
View Description Hide DescriptionTheoretical expressions for the internal and external electromagnetic fields for an arbitrary electromagnetic beam incident upon a homogeneous spherical particle are derived, and numerical calculations based upon this theoretical development are presented. In particular, spatial distributions of the internal and near‐surface electric field magnitude (source function) for a focused fundamental (TEM_{0} _{0} mode) Gaussian beam of 1.06 μm wavelength and 4 μm beam waist diameter incident upon a 5‐μm‐diam waterdroplet in air are presented as a function of the location of the beam focal point relative to the sphere center. The calculations indicate that the internal and near‐surface electric field magnitude distribution can be strongly dependent upon relative focal point positioning and may differ significantly from the corresponding electric field magnitude distribution expected from plane‐wave irradiation.

Comparison of the aperture and chromatic aberrations of magnetic round lenses with those of quadrupole doublets
View Description Hide DescriptionThe paraxial equation for rotationally symmetric magnetic lenses is integrated numerically by using the Numerov method. The normalized third‐order aperture and second‐order chromatic aberration coefficients (C _{3N } and C _{ c N }) are calculated. A Fourier representation of the lens field is used, and a search for minimum C _{3N } is carried out by varying the Fourier coefficients. Comparisons with comparable quadrupole doublets, and calculations of C _{3N } and C _{ c N } for actual coil systems are presented. The main conclusion is that magnetic round lenses are about a factor of 2 better than comparable magnetic quadrupole doublets as far as the third‐order aperture aberration is concerned, but the chromatic aberrations in both systems are comparable.

A study of octupoles as correctors
View Description Hide DescriptionIn an electron microscope equipped with sextupole correctors the ultimate resolution is limited by the fifth‐order spherical aberration of the optical system used to form the electron probe. We have examined the possibilities of using octupoles in order to correct this fifth‐order spherical aberration. We find that for an electrostatic octupole, the fifth‐order aberration is symmetric, while for a magnetic octupole the fifth‐order aberration is asymmetric but it is possible to reduce this asymmetry by properly arranging the initial conditions. Based on these results, we propose a system which could be used to correct fifth‐order aberration in a probe forming system equipped with a sextupole corrector.

Covariant transformations of wave equations for initial‐boundary‐value problems with moving boundary conditions
View Description Hide DescriptionIt is shown that the wave equation ψ_{ x x }−ψ_{ y y }=0 for the field ψ(x,y) in the domain R(x y) can be transformed into a wave equation Ψ_{ξξ}−Ψ_{ηη}=0 for the field Ψ(ξ,η) in the domain S(ξη). The transformation is accomplished through a complex functionF(x,y)=ξ(x,y) +iη(x,y), which is not analytic. For the transformation to exist, the real transformation functions ξ=ξ(x,y) and η=η(x,y) have to satisfy wave equations in the domain R(x y) and the first‐order partial equations ξ_{ x }=±η_{ y } and ξ_{ y }=±η_{ x } [‘‘±’’ distinguishes transformations of the first (+) and second (−) kinds]. Thus, the hyperbolic transformation theory is different from the conformal mapping theory, where the real transformation functions satisfy the Laplace equation and the Cauchy–Riemann conditions. As applications, the linear Lorentz transformation and nonlinear mappings of time‐varying regions into fixed domains are discussed as solutions of the indicated partial differential equations. Furthermore, an initial‐boundary‐value problem for the electromagnetic wave equation with moving boundary condition is solved analytically (compression of microwaves in an imploding resonator cavity).

Scattering of electromagnetic waves from a periodic surface with random roughness
View Description Hide DescriptionThe scattering of electromagnetic waves from a randomly perturbed periodic surface is formulated by the extended boundary condition (EBC) method and solved by the small perturbation method (SPM). The scattering from periodic surface is solved exactly and this solution is used in the SPM to solve for the surface currents and scattered fields up to the second order. The random perturbation is modeled as a Gaussian random process. The theoretical results are illustrated by calculating the bistatic and backscattering coefficients. It is shown that as the correlation length of the random roughness increases, the bistatic scattering pattern of the scattered fields show several beams associated with each Bragg diffraction direction of the periodic surface. When the correlation length becomes smaller, then the shape of the beams become broader. The results obtained using the EBC/SPM method is also compared with the results obtained using the Kirchhoff approximation. It is shown that the Kirchhoff approximation results show quite a good agreement with EBC/SPM method results for the h h and v v polarized backscattering coefficients for small angles of incidence. However, the Kirchhoff approximation does not give depolarized returns in the backscattering direction, whereas the results obtained using the EBC/SPM method give significant depolarized returns when the incident direction is not perpendicular to the row direction of the periodic surface.

Stability of Te–Cu amorphous alloy thin films for optical recording
View Description Hide DescriptionWe have studied the structure and optical stability of Te–Cu thin film alloy candidates for write‐once optical recording. Films prepared by rf diode sputtering with 20–50 at. % Cu are amorphous, as‐sputtered. One of these, Te_{65}Cu_{35} , has a relatively high crystallization temperature (150 °C), as determined by x‐ray diffraction. Near the eutectic composition (∼29 at .% Cu), alloy films have stable optical properties after accelerated aging at 60 °C and 85% relative humidity. The mechanism for film stability near the eutectic was studied by x‐ray photoelectron spectroscopy and depth profiling using ion scatteringspectroscopy. We found that a Cu‐enriched surface oxide, formed at ambient conditions, passivates the film and is responsible for its subsequent stability after accelerated aging. We also demonstrated that a 14 in. diam, multilayer optical disk with a Te_{65}Cu_{35} recording medium exhibits excellent linearity for 3 and 8 MHz pulses, good written pulse length stability, and high signal‐to‐noise ratio. Thus, a Te–Cu recording medium can effectively use run‐length‐limited codes, which allow very high data storage capacity and data transfer rates.

Characteristics of electron‐beam‐excited Xe^{*} _{2} at low pressures as a vacuum ultraviolet source
View Description Hide DescriptionThe performance of Xe^{*} _{2} as a 172‐nm fluorescence or laser source when pumped by a low‐current, long‐pulse electron beam was determined. The fluorescence efficiency of Xe^{*} _{2} is near the theoretical limit of ∼50% at modest pressures over a range of pump rates up to 10^{6} W/cm. The laser efficiency is limited to values <1% by a very strong medium absorption that is probably due to Xe^{*} _{2}photoionization. Laser performance is further degraded by early pulse termination that appears related to mirror degradation. An improved kinetics and extraction code was developed to model the performance of the Xe^{*} _{2} system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Rates for these processes were derived as described herein. With this model, good absolute agreement was obtained between experiments and calculated parameters at pressures as low as 0.5 atm.

Characteristics of electron‐beam‐excited Kr^{*} _{2} at low pressures as a vacuum ultraviolet source
View Description Hide DescriptionThe performance of Kr^{*} _{2} as a 146 nm fluorescence or laser source when pumped by a low‐current, long‐pulse electron beam has been determined. The fluorescence efficiency of Kr^{*} _{2} is near the theoretical limit of ∼46% at modest pressures over a range of pump rates up to 3×10^{5} W/cm^{3}. Lasing could not be achieved under the conditions studied here. An improved kinetics and extraction code has been developed to model the performance of the Kr^{*} _{2} system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Estimated rates for these processes are presented. With this model, good absolute agreement of model predictions with experimental results has been obtained at pressures as low as 0.5 atm.

Nonlinear optical waveguide directional coupler employing multiple quantum well structure
View Description Hide DescriptionThe nonlinear, lossy optical waveguide coupler employing GaAs/GaAlAs multiple quantum well structure has been analyzed. Using properly defined distributed circuit parameters, we model a system of coupled waveguides with a system of coupled transmission lines. The equivalent transmission line system parameters are derived in terms of the rectangular coupled waveguide system parameters. The coupled‐mode theory, developed for the study of the nonlinear lossy transmission line system, is then applied to the equivalent transmission line system. The coupled‐mode equations are solved numerically with the aid of the Runge–Kutta method, and the mode powers are calculated. It is shown that the variation of output power of the coupler and the division of powers between the two guides of the coupler depend strongly on the input power. The minimum input power required for the coupler to switch from the cross‐over state to the straight‐through state depends upon the losses of the guide as well as on the separation between the two guides of the coupler.

Characterization of planar Ti:LiNbO_{3} optical waveguides in the visible and near‐infrared spectral range
View Description Hide DescriptionSeveral Ti:LiNbO_{3}waveguides have been fabricated in dry and wet atmosphere, varying the diffusion time. The effect of water vapor on the diffusion process has been investigated. Effective indices and attenuation at λ=0.6328, 1.15, and 1.52 μm have been measured. In the dry case, the Ti in‐diffusion proceeds more rapidly than in the wet case, and the two diffusion coefficients have been calculated. Also, it has been observed that the presence of water vapor in the atmosphere does not completely prevent the phenomenon of Li_{2}O out‐diffusion, while the importance of the boat and the furnace materials in this process has been verified. Attenuation measurements showed that particular care must be taken, when working with waveguides diffused for short times, for two reasons. (1) The ratio diffusion length/wavelength, i.e., the normalized frequency v, decreases, and modes approach the cutoff, becoming more lossy. Of course, this is more evident working at long wavelength. (2) The surface roughness can strongly affect the attenuation value.

Stimulated Raman scattering in lead vapor pumped by a long‐pulse 1‐J XeCl excimer laser
View Description Hide DescriptionThe parametric dependence of the output energies and efficiencies for the Raman conversion of the radiation from a long‐pulse 1‐J XeCl excimer laser in Pb vapor was investigated. The effects of atom depletion and buffer gas on the stimulated Raman scattering output were discussed in detail. Multipeak structures of the waves of the Raman‐shifted pulses were observed which could be attributed to the coherent effects in Raman scattering. A computer model was introduced to explain the multipeak structures of the stimulated Raman scattering waveforms.

Reversible phase‐change optical data storage in InSbTe alloy films
View Description Hide DescriptionSome characteristics of reversible phase‐change optical data storage based on an amorphous‐crystalline transformation in InSbTe alloys are given. The reversible phase change was observed in a wide region of composition. The laser amorphized spot of a ternary compound In_{3}SbTe_{2} film could be crystallized using a diode laser pulse of less than 100 ns with an incident laser power of more than 10 mW. The crystallization temperature of the amorphized spot was 280 °C and the activation energy was about 1.8 eV which shows that long‐term data retention at room temperature is possible. The repetition number of static write and erase using the pulse of 50 ns reached above 10^{5}. These data show that the ternary compound film has potential for reversible optical data storage media with high‐speed erasing and long‐term data retention.

Comparative study of low‐pressure rare‐gas fluoride/chloride lasers excited by a short‐pulse electron beam
View Description Hide DescriptionOutput characteristics of the short‐pulse electron‐beam excited ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), and XeCl (308 nm) lasers were experimentally investigated at the same time on the same device for various mixtures all pumped at a high excitation rate of 2.3 MW/cm^{3}. The optimum pressure of rare gas forming its rare‐gas halide excimer increased for the sequence of ArF, KrCl, KrF, XeCl, and XeF, indicating that rare‐gas halides were more quenched by the heavier rare gases, such as Xe, even at near‐atmospheric pressures. At nearly atmospheric pressures the Ar‐buffered mixtures gave higher laser energy than Ne‐buffered mixtures except for the XeF laser. Through a series of measurements, the ArF laser showed the best result on the output energy of 96 J (5 J/l ) with an intrinsic efficiency of 3.4%.

Temperature variation of silicon’s nonlinearity constant and acoustic attenuation
View Description Hide DescriptionThe temperature variation of the nonlinearity constant in silicon has been studied for longitudinal waves along the [110] axis and for shear waves along the [110] axis with polarizations along the [11̄0] and [001] axes in the range 73–273 K. The temperature dependence of the nonlinearity constant estimated from the second‐ and third‐order elastic constants has been further used to evaluate the acoustic attenuation in the Akhieser region at 475 MHz. The estimated values of acoustic attenuation have been compared with the measured values available in the literature and are found to be in good agreement. The nonlinearity constant showed a positive temperature coefficient for the longitudinal waves, whereas it showed a negative temperature coefficient for the shear waves. These results resemble those obtained for germanium and copper.

Thermal‐wave imaging of epoxy/aluminum interfaces
View Description Hide DescriptionLaser‐excited thermal‐wave image magnitude and phase were acquired by photoacoustic detection in the 10–1000‐Hz modulation frequency range on epoxy/aluminum interface systems with surface (black) and bulk (white) laser‐absorbing epoxy, different planar interface distances, surface and subsurface flaws, and different surface angle boundaries. The following conclusions were reached from the thermal‐wave results: (1) thermally thin epoxy on aluminum layer thicknesses can be estimated from variable modulation frequency data; (2) planar and subsurface defect interface structures of epoxy/aluminum systems are best studied by examining both magnitude and phase images; (3) thermal‐wave images are more straightforward to interpret for the systems that absorb light at the irradiated surface rather than in the bulk. These conclusions are consistent with thermal diffusion length and thermal‐wave reflection effects predicted by theory for surface light‐absorption conditions.

Further studies of liquid‐metal flows and power losses in ducts with a moving conducting wall and a skewed magnetic field
View Description Hide DescriptionIn a previous paper the authors initiated studies of fully developed laminar liquid‐metalflows,currents, and power losses in a rectangular channel with a moving perfectly conducting wall and with a skewed homogeneous external magnetic field for high Hartmann numbers, high interaction parameters, low magnetic Reynolds numbers, and different aspect ratios. The channel had insulating side walls that were skewed to the external magnetic field, while the perfectly conducting moving top wall with an external potential and the stationary perfectly conducting bottom wall at zero potential acted as electrodes. These electrodes were also skewed to the external magnetic field. A mathematical solution was obtained for high Hartmann numbers by dividing the flow into three core regions, two free shear layers, and six Hartmann layers along the channel walls. The free shear layers were treated rigorously and in detail with fundamental magnetohydrodynamic theory. The previous work, however, left the solution for the velocity profiles in terms of a complex integral equation which was not solved. In the present work the integral equation is solved numerically
by the method of quadratures to give the velocity profiles, viscous dissipation and Joulean losses in the free shear layers. In addition, expressions for the viscous dissipation in the six Hartmann layers are presented. The best approximation to the viscous dissipation in the channel is the sum of the O(M ^{3} ^{/} ^{2}) contributions from the two free shear layers, the O(M ^{3} ^{/} ^{2}) contributions from the two Hartmann layers separating the free shear layers from the insulators, and the O(M) contributions from three of the Hartmann layers separating core regions from the walls. The best approximation to the Joulean power losses in the channel is the sum of the O(M ^{2}) contribution from the central core region which carries an O(1) current between the electrodes and the O(M ^{3} ^{/} ^{2}) contributions from the free shear layers. The expressions for the viscous dissipation and Joulean losses in each region involve the products of universal constants, electrical potentials and geometric factors. The theoretical magnetohydrodynamic model presented here was developed to provide data to help in the design of liquid‐metalcurrent collectors.

Heating of a dense plasma using a relativistic electron beam
View Description Hide DescriptionWe have carried out an experiment to measure the heating of a dense carbonplasma,n _{ e }∼10^{1} ^{8}–10^{1} ^{9} cm^{−} ^{3}, by a relativistic electron beam. Using spectroscopic methods, the temperature of the plasma was measured. By taking into consideration the heat loss mechanisms in the plasma, the amount of energy deposited into the plasma by the beam was determined. The results are consistent with theoretical predictions.