Showing papers by "Bell Labs published in 1971"

Facilitation in recognizing pairs of words: Evidence of a dependence between retrieval operations.

Abstract: Two experiments are reported in which 5s were presented two strings of letters simultaneously, with one string displayed visually above the other. In Exp. I, 5s responded "yes" if both strings were words, otherwise responding "no." In Exp. II, 5s responded "same" if the two strings were either both words or both nonwords, otherwise responding "different." "Yes" responses and "same" responses were faster for pairs of commonly associated words than for pairs of unassociated words. "Same" responses were slowest for pairs of nonwords. "No" responses were faster when the top string in the display was a nonword, whereas "different" responses were faster when the top string was a word. The results of both experiments support a retrieval model involving a dependence between separate successive decisions about whether each of the two strings is a word. Possible mechanisms that underlie this dependence are discussed.

Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform

Abstract: The Fourier transform data communication system is a realization of frequency-division multiplexing (FDM) in which discrete Fourier transforms are computed as part of the modulation and demodulation processes. In addition to eliminating the bunks of subcarrier oscillators and coherent demodulators usually required in FDM systems, a completely digital implementation can be built around a special-purpose computer performing the fast Fourier transform. In this paper, the system is described and the effects of linear channel distortion are investigated. Signal design criteria and equalization algorithms are derived and explained. A differential phase modulation scheme is presented that obviates any equalization.

Behavior of the Electronic Dielectric Constant in Covalent and Ionic Materials

Abstract: Refractive-index dispersion data below the interband absorption edge in more than 100 widely different solids and liquids are analyzed using a single-effective-oscillator fit of the form ${n}^{2}\ensuremath{-}1=\frac{{E}_{d}{E}_{0}}{({E}_{0}^{2}\ensuremath{-}{\ensuremath{\hbar}}^{2}{\ensuremath{\omega}}^{2})}$, where $\ensuremath{\hbar}\ensuremath{\omega}$ is the photon energy, ${E}_{0}$ is the single oscillator energy, and ${E}_{d}$ is the dispersion energy. The parameter ${E}_{d}$, which is a measure of the strength of interband optical transitions, is found to obey the simple empirical relationship ${E}_{d}=\ensuremath{\beta}{N}_{c}{Z}_{a}{N}_{e}$, where ${N}_{c}$ is the coordination number of the cation nearest neighbor to the anion, ${Z}_{a}$ is the formal chemical valency of the anion, ${N}_{e}$ is the effective number of valence electrons per anion (usually ${N}_{e}=8$), and $\ensuremath{\beta}$ is essentially two-valued, taking on the "ionic" value ${\ensuremath{\beta}}_{i}=0.26\ifmmode\pm\else\textpm\fi{}0.04$ eV for halides and most oxides, and the "covalent" value ${\ensuremath{\beta}}_{c}=0.37\ifmmode\pm\else\textpm\fi{}0.05$ eV for the tetrahedrally bonded ${A}^{N}{B}^{8\ensuremath{-}N}$ zinc-blende- and diamond-type structures, as well as for scheelite-structure oxides and some iodates and carbonates. Wurtzite-structure crystals form a transitional group between ionic and covalent crystal classes. Experimentally, it is also found that ${E}_{d}$ does not depend significantly on either the bandgap or the volume density of valence electrons. The experimental results are related to the fundamental ${\ensuremath{\epsilon}}_{2}$ spectrum via appropriately defined moment integrals. It is found, using relationships between moment integrals, that for a particularly simple choice of a model ${\ensuremath{\epsilon}}_{2}$ spectrum, viz., constant optical-frequency conductivity with high- and low-frequency cutoffs, the bandgap parameter ${E}_{a}$ in the high-frequency sum rule introduced by Hopfield provides the connection between the single-oscillator parameters (${E}_{0},{E}_{d}$) and the Phillips static-dielectric-constant parameters (${E}_{g},\ensuremath{\hbar}{\ensuremath{\omega}}_{p}$), i.e., ${(\ensuremath{\hbar}{\ensuremath{\omega}}_{p})}^{2}={E}_{a}{E}_{d} \mathrm{and} {E}_{g}^{2}={E}_{a}{E}_{0}$. Finally, it is suggested that the observed dependence of ${E}_{d}$ on coordination number and valency implies that an understanding of refractive-index behavior may lie in a localized molecular theory of optical transitions.

Weakly guiding fibers.

Abstract: Thin glass fibers imbedded into a glass cladding of slightly lower refractive index represent a promising medium for optical communication. This article presents simple formulas and functions for the fiber parameters as a help for practical design work. It considers the propagation constant, mode delay, the cladding field depth, and the power distribution in the fiber cross section. Plots vs frequency of these parameters are given for 70 modes

Hopping Conductivity in Disordered Systems

Abstract: By considering a model in which charge is transported via phonon-induced tunneling of electrons between localized states which are randomly distributed in energy and position, Mott has obtained an electrical conductivity of the form $\ensuremath{\sigma}\ensuremath{\propto}\mathrm{exp}[\ensuremath{-}{(\frac{\ensuremath{\lambda}{\ensuremath{\alpha}}^{3}}{{\ensuremath{\rho}}_{0}\mathrm{kT}})}^{\frac{1}{4}}]$. Here $T$ is the temperature of the system, ${\ensuremath{\rho}}_{0}$ is the density of states at the Fermi level, $\ensuremath{\lambda}$ is a dimensionless constant, and ${\ensuremath{\alpha}}^{\ensuremath{-}1}$ is the distance for exponential decay of the wave functions. We rederive these results, relating $\ensuremath{\lambda}$ to the critical density of a certain dimensionless percolation problem, and we estimate $\ensuremath{\lambda}$ to be approximately 16. The applicability of the model to experimental observations on amorphous Ge, Si, and C is discussed.

Light waves in thin films and integrated optics.

Abstract: Integrated optics is a far-reaching attempt to apply thin-film technology to optical circuits and devices, and, by using methods of integrated circuitry, to achieve a better and more economical optical system. The specific topics discussed here are physics of light waves in thin films, materials and losses involved, methods of couplings light beam into and out of a thin film, and nonlinear interactions in waveguide structures. The purpose of this paper is to review in some detail the important development of this new and fascinating field, and to caution the reader that the technology involved is difficult because of the smallness and perfection demanded by thin-film optical devices.

Simple Molecular Model for the Smectic A Phase of Liquid Crystals

Abstract: The Maier-Saupe model of the nematic phase with an orientational order parameter is extended to the smectic $A$ phase by introducing a new order parameter, the amplitude of a density wave in the direction of the nematic preferred axis. Self-consistent equations for the two order parameters are derived from an anisotropic model interaction and are solved numerically. We calculate the order parameters, the entropy, and the specific heat as a function of temperature for several values of dimensionless interaction strength $\ensuremath{\alpha}$ for the smectic $A$ phase. The transition temperatures plotted versus $\ensuremath{\alpha}$ provide a theoretical phase diagram which resembles experimental plots of transition temperature versus alkyl chain length for homologous series of compounds. The model qualitatively reproduces chemical trends in transition entropies. Experiments are suggested to measure the order parameters in the smectic $A$ phase.

Valence-Band Parameters in Cubic Semiconductors

Abstract: A five-level $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ analysis is used to compute the principal effective-mass parameters at $k=0$ in diamond- and zinc-blende-type semiconductors. A semiempirical model is developed to describe the dependence of the momentum matrix elements on lattice constant, ionicity, and $d$-electron shells in the cores. Satisfactory agreement with available experimental data is achieved with six fitted parameters.

Separation of estimation and control for discrete time systems

Abstract: An attempt is made to coordinate the numerous results relating to separation of estimation and control in discrete time stochastic control theory. The results vary widely depending upon the assumptions about linearity, criteria, information pattern, constraints, and noise distributions. Some of the less well-known underlying concepts are discussed with the help of a fairly general model.

Stress singularities in a two-material wedge

Abstract: A method for the determination of stresses in a two-material wedge-shaped region is presented. The method is applicable for plane strain or plane stress problems and treats the general case where each region is a wedge of arbitrary angle. The results are obtained by the use of the Mellin transform and the theory of residues. The characteristic equation is investigated to determine the stress singularity resulting from certain combination of geometry and material properties. A formal solution is then presented for the case where the loading is in the form of a point dislocation along the interface. This solution is the Green's function for the more general mismatch problems and therefore has applications in solving other problems with compatible boundary conditions. The results obtained show that for small values of r the dominant effect is due to geometry and the secondary effect is caused by the choice of elastic constants of the materials.

Absorption, Reflectance, and Luminescence of GaN Epitaxial Layers

Abstract: Low-temperature absorption, reflectance, and emission spectra from a wide range of epitaxially grown GaN are reported. The best samples, which are far superior to any previously available, have spectral details that allow a definitive analysis. The reflectance results prove that GaN is a direct-gap semiconductor with its lowest exciton state at 3.474 \ifmmode\pm\else\textpm\fi{} 0.002 eV at 2 K. The absorption and luminescence spectra corroborate this observation. Crystal field and spin-orbit splittings have been resolved. The order of the valence-band states is ${\ensuremath{\Gamma}}_{9}g{\ensuremath{\Gamma}}_{7}g{\ensuremath{\Gamma}}_{7}$ with parameters ${\ensuremath{\Delta}}_{\mathrm{cr}}=22\ifmmode\pm\else\textpm\fi{}2$ meV and ${\ensuremath{\Delta}}_{\mathrm{so}}={11}_{\ensuremath{-}2}^{+5}$ meV. A correlation of these results with the emission data has led us to an interpretation in which we associate emission features with LO-phonon-assisted free-exciton decay, with bound-exciton decay at both donor and acceptor sites, and with distant donor-acceptor pair recombination. Binding energies derived from these associations are consistent with values obtained from independent experiments reported earlier. The internal consistency of the absorption, reflectance, and luminescence results provides strong support for our interpretation.

Current transport in metal-semiconductor-metal (MSM) structures

Abstract: The current-voltage characteristics of a metal-semiconductor-metal structure (essentially two metal-semiconductor contacts connected back to back) have been studied based on the thermionic emission theory. When a uniformly doped semiconductor is thin enough that it can be completely depleted before avalanche breakdown occurs, the structure can exhibit many novel transport behaviors. Two outstanding features of the structure are that (1) a wide range of high-level injection of minority carriers can be achieved by varying the barrier heights of the two contacts and (2) the critical voltage at which the minority carrier injection increases rapidly can be varied by varying the semiconductor doping and thickness. Experimental silicon MSM structures of PtSi-Si-PtSi have been made from n-type silicon with doping of 4×1014 cm−3 and thickness of 12 μm. The critical voltage at room temperature is about 30 V. The current increases over five orders of magnitude with only 10 per cent increase of the voltage. The above results and other measurements over wide temperature range do substantiate the theoretical predictions.

Linear and nonlinear optical properties of AgGaS 2 , CuGaS 2 , and CuInS 2 , and theory of the wedge technique for the measurement of nonlinear coefficients

Abstract: The refractive indices of the ternary A^{I}B^{III}C_{2}^{VI} semiconductors AgGaS 2 , CuGaS 2 , and CuInS 2 have been measured over the entire range of transparency of these crystals. The optical nonlinear coefficients for second-harmonic generation have also been determined. Three-frequency collinear phase matching is analyzed in detail for AgGaS 2 . The birefringences of CuGaS 2 and CuInS 2 are not large enough to permit three-frequency phase matching within the transparent regions. A parametric oscillator threshold calculation for a pump wavelength 0.89 μ, which is within the range of the GaAs injection laser, indicates that AgGaS 2 is promising for this application. The upconversion efficiency in AgGaS 2 for sum mixing of the CO 2 laser ( \lambda = 10.5 \mu ) with the xenon ion laser ( \lambda = 0.597 \mu ) is also calculated. The result indicates that, depending upon system requirements and the availability of high optical quality material, AgGaS 2 can be comparable to ZnGeP 2 for upconversion. In Appendix II, we present a theory of the wedge technique for the measurement of nonlinear coefficients. This theory takes into account losses and assumes a Gaussian beam geometry. Furthermore, a discussion of units in nonlinear optics is given.

Electrical Properties, Optical Properties, and Band Structure of CuGa S 2 and CuIn S 2

Abstract: Various optical and electrical properties of the I-III-V${\mathrm{I}}_{2}$ compounds CuGa${\mathrm{S}}_{2}$ and CuIn${\mathrm{S}}_{2}$ have been studied. From the results of low-temperature luminescence and reflectivity, both crystals are determined to have a direct band gap. The band gaps at 2 \ifmmode^\circ\else\textdegree\fi{}K are 2.53 eV for CuGa${\mathrm{S}}_{2}$ and 1.55 eV for CuIn${\mathrm{S}}_{2}$. CuIn${\mathrm{S}}_{2}$ has been made conducting both $n$ and $p$ type, while CuGa${\mathrm{S}}_{2}$ has been made $p$ type only. Electroreflectance measurements have been performed in an attempt to determine the band structure. The highest valence band appears to be a doublet with a large admixture of Cu $3d$ wave functions.

Specific contact resistance of metal-semiconductor barriers

Abstract: The specific contact resistance at zero bias, R c , serves as a measure of the ohmic or rectifying behavior of a metal-semiconductor barrier under operating conditions. It is thus an important design parameter for semiconductor devices. The values of R c have been calculated for Metal-Si and metal-GaAs barriers on p -type and n -type samples. The theoretical calculation is based on the generalized transport study of metal-semiconductor systems. The results, which are presented graphically, show the dependence of R c on temperature over the range 50°K–500°K, the barrier height from 0.2 to 1.0 eV, and the ionized impurity concentration from 10 14 to 10 21 cm −3 . Generally R c decreases exponentially with increasing temperature and with decreasing barrier height. For samples with lower dopings where the thermionic emission dominates, R c is essentially independent of doping; for higher dopings where the tunneling dominates, R c decreases rapidly with increasing doping. The experimental results of R c for various metals on silicon samples are in good agreement with the predictions.

Identification of human faces

Abstract: How well can human faces be identified by humans and by computers, using subjectively judged "feature" descriptions like long ears, wide-set eyes, etc.? Three classes of experiments are reported: 1) Gathering, analysis, and assessment of face-feature data for 255 faces. 2) Computer identification-studies. 3) Human identification-studies. A set of 22 features was evolved from an initially larger set to provide relevant, distinctive, relatively independent measures which can be judged reliably. Computer studies and a mathematical model established limits of performance of a person attempting to isolate a face from a population using feature descriptions. The model predicts that under certain conditions approximately 6 of an individual's features are required to isolate him from a population of 255. Human experiments under similar conditions showed unique identification occurred with an average of about 7 features. The model predicts that for a population of 4×106, only 14 feature-descriptions are required. These studies form a foundation for continuing research on real-time man-machine interaction for computer classification and identification of multidimensional vectors specified by noisy components.

Multiple scattering of EM waves by spheres part I--Multipole expansion and ray-optical solutions

Abstract: Solution to the multiple scattering of electromagnetic (EM) waves by two arbitrary spheres has been pursued first by the multipole expansion method. Previous attempts at numerical solution have been thwarted by the complexity of the translational addition theorem. A new recursion relation is derived which reduces the computation effort by several orders of magnitude so that a quantitative analysis for spheres as large as 10\lambda in radius at a spacing as small as two spheres in contact becomes feasible. Simplification and approximation for various cases are also given. With the availability of exact solution, the usefulness of various approximate solutions can be determined quantitatively. For high frequencies, the ray-optical solution is given for two conducting spheres. In addition to the geometric and creeping wave rays pertaining to each sphere alone, there are rays that undergo multiple reflections, multiple creeps, and combinations of both, called the hybrid rays. Numerical results show that the ray-optical solution can be accurate for spheres as small as \lambda/4 in radius is some cases. Despite some shortcomings, this approach provides much physical insight into the multiple scattering phenomena.

Principles of Optimal Page Replacement

Abstract: ABSTP~CT. A formal model is presented for paging algorithms under /-order nonstationary assumptions about program behavior. When processing a program under paging in a given memory, a given paging policy generates a certain (expected) number of page calls, i.e., its "cost." Under usual assumptions about memory system organization, minimum cost is always achieved by a demand paging algorithm. The minimum cost for /-order program behavior assumptions is expressed as a dynamic programming problem whose solution yields an optimal replacement algorithm. Solutions are exhibited in several 0-order cases of interest. Paging algorithms that implement and approximate the minimum cost are discussed.

On Euclid's Algorithm and the Theory of Subresultants

Abstract: : A key ingredient for systems which perform symbolic computer manipulation of multivariate rational functions are efficient algorithms for calculating polynomial greatest common divisors. Euclid's algorithm cannot be used directly because of problems with coefficient growth. The search for better methods leads naturally to the theory of subresultants. This paper presents an elementary treatment of the theory of subresultants, and examines the relationship of the subresultants of a given pair of polynomials to their polynomial remainder sequence as determined by Euclid's algorithm. This relation is expressed in the fundamental theorem of this paper. The results are essentially the same as those of Collins but the presentation is briefer, simpler, and somewhat more general. The fundamental theorem finds further applications in the proof that the modular algorithm for polynomial GCD terminates. (Author)

Conversion of printed text into synthetic speech

Abstract: A system is disclosed for converting printed text into speech sounds. Text is converted to alpha-numeric signal data, for example, by a scanner and dictionary lookup. Syntax of the input information is then analyzed to determine the proper phrase category, e.g., subject, verb, object, etc., of word intervals, and to assign pause, stress, duration, pitch and intensity values to the words. From these data a phonetic description of each word is found in a stored dictionary, modified by the accumulated data, and used to prepare synthesizer control signals.

Electron Transport in InSb, InAs, and InP

Abstract: The calculation of electron-transport properties of direct-gap semiconductors has been generalized to include arbitrary electron degeneracy as well as scattering by ionized impurities and heavy holes. Conduction-band nonparabolicity and electron wave-function admixture are retained throughout the calculation of drift mobility and thermoelectric power. Extensive comparison of the results with experiment confirms the present description over wide ranges of temperature and ionized-impurity concentration. Effects of multivalley conduction due to electron transfer into ${L}_{1c}$ satellite valleys appear in InSb above 700 \ifmmode^\circ\else\textdegree\fi{}K (just below the melting point at \ensuremath{\sim} 780 \ifmmode^\circ\else\textdegree\fi{}K) and in InP above 800 \ifmmode^\circ\else\textdegree\fi{}K. The lowest satellite valleys of InAs are sufficiently remote from the conduction-band edge at ${L}_{1c}$ that the results are expected to be accurate up to the melting point at \ensuremath{\sim} 1200 \ifmmode^\circ\else\textdegree\fi{}K.

On Euclid's Algorithm and the Computation of Polynomial Greatest Common Divisors

Abstract: This paper examines the computation of polynomial greatest common divisors by various generalizations of Euclid's algorithm. The phenomenon of coefficient growth is de- scribed, and the history of successful efforts first to control it and then to eliminate it is re- lated. The recently developed modular algorithm is presented in careful detail, with special atten- tion to the case of multivariate polynomials. The computing times for the classical algorithm and for the modular algorithm are analyzed, and it is shown that the modular algorithm is markedly superior. In fact, in the multivariate ease, the maximum computing time for the modular algorithm is strictly dominated by the maximum computing time for the first pseudo-division in the classical algorithm.

Laser Stimulation of Nerve Cells in Aplysia

Abstract: Laser radiation at 488 nanometers selectively stimulates neurons in the abdominal ganglion of the marine mollusk Aplysia californica. The laser radiation can be scanned over the surface of the ganglion and can be effectively utilized in mapping cellular interconnections. The laser appears to cause these changes through some mechanism other than damage.

Nitric oxide air pollution detection by optoacoustic spectroscopy

Abstract: Nitric oxide is detected by a new technique in which tunable infrared radiation from a spin-flip Raman laser is used to measure the absorption spectrumn of a gas sample by optoacoustic spectroscopy. This technique is sensitive enough to detect a concentration of 0.01 part per million of nitric oxide pollution in air samples.

Dispersion in Weakly Guiding Fibers

Abstract: Optical signals transmitted through cladded glass fibers are subject to delay distortion because of (1) dispersion in the material, (2) dispersion caused by the waveguide characteristic, and (3) delay differences between modes. We isolate these effects and evaluate their significance for cases of practical interest. These concern fibers in which the refractive index of the cladding is only slightly lower than that of the core.

Electron-Phonon Effects in the Infrared Properties of Metals

Abstract: Electron-phonon contributions to the infrared absorption in metals are examined from two points of view. First, "golden-rule" calculations are given for normal metals and superconductors which reproduce the phenomenological theory used by Joyce and Richards to analyze their data in lead. This theory is expected to be valid for weak electron-phonon coupling. Second, the Holstein transport theory for normal metals is used to examine the corrections arising from strong coupling. Solutions are found for the response function at general frequencies and wave vectors. The results are similar in form to the golden-rule theory, and provide a simple correction factor to this theory. It is suggested that optical measurements on both normal and superconducting materials may provide a valuable tool for analyzing the coupled electron-phonon system.