Showing papers by "Bell Labs published in 1979"

Robust Locally Weighted Regression and Smoothing Scatterplots

Abstract: The visual information on a scatterplot can be greatly enhanced, with little additional cost, by computing and plotting smoothed points. Robust locally weighted regression is a method for smoothing a scatterplot, (x i , y i ), i = 1, …, n, in which the fitted value at z k is the value of a polynomial fit to the data using weighted least squares, where the weight for (x i , y i ) is large if x i is close to x k and small if it is not. A robust fitting procedure is used that guards against deviant points distorting the smoothed points. Visual, computational, and statistical issues of robust locally weighted regression are discussed. Several examples, including data on lead intoxication, are used to illustrate the methodology.

Optimization and Approximation in Deterministic Sequencing and Scheduling: a Survey

Abstract: The theory of deterministic sequencing and scheduling has expanded rapidly during the past years. In this paper we survey the state of the art with respect to optimization and approximation algorithms and interpret these in terms of computational complexity theory. Special cases considered are single machine scheduling, identical, uniform and unrelated parallel machine scheduling, and open shop, flow shop and job shop scheduling. We indicate some problems for future research and include a selective bibliography.

Norm-Conserving Pseudopotentials

Abstract: A very simple procedure to extract pseudopotentials from ab initio atomic calculations is presented. The pseudopotentials yield exact eigenvalues and nodeless eigenfunctions which agree with atomic wave functions beyond a chosen radius ${\mathcal{r}}_{c}$. Moreover, logarithmic derivatives of real and pseudo wave functions and their first energy derivatives agree for $\mathcal{r}g{\mathcal{r}}_{c}$ guaranteeing excellent transferability of the pseudopotentials.

Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys

Abstract: The pronounced glass-forming tendencies of alloys of S and Se with Ge and/or As are discussed topologically. An atomic model is introduced which for predominantly covalent forces constitutes the first microscopic realization of Kauzmann's description of the glass transition as an entropy (not enthalpy or volume) crisis. The model contains no adjustable parameters and predicts the glass-forming tendency as a function of composition in excellent agreement with experiment. Several related properties, including phase diagrams, radial distribution functions and crystal structures are discussed in the context of chemical bonding and short-range order in the non-crystalline covalent networks of these materials.

Password security: a case history

Abstract: This paper describes the history of the design of the password security scheme on a remotely accessed time-sharing system. The present design was the result of countering observed attempts to penetrate the system. The result is a compromise between extreme security and ease of use.

Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry

Abstract: Using measured dielectric function data from 2.1 to 5.5 eV for chemical-vapor-deposition---grown smooth amorphous ($a$-Si) and microscopically rough fine-grained polycrystalline ($p$-Si) films, we show that the dielectric properties of microscopically rough layers of thicknesses 100-500 \AA{}A are accurately modeled in the effective-medium approximation. These microscopically rough layers show essentially no macroscopic light scattering, and thus are inaccessible to measurement by usual scattering techniques. The unambiguous identification of microscopic roughness, as opposed to, e.g., an overlying oxide, is shown to require a spectroscopic capability. Statistical-analysis techniques are introduced to determine model parameters systematically and objectively, and also to establish correlations and confidence limits that show which parameters are defined by the data and which are statistically indeterminate. A best-fit five-parameter model for the sample with the thickest surface region shows that the density profile is characteristic of hemispherical, not pyramidal, irregularities. This indicates that surface roughness arises from a three-dimensional nucleation and growth process in these samples. In a comparison of the three one-parameter effective-medium models, Bruggeman and Maxwell Garnett(2) theories are found to adequately represent the data, while the Lorentz-Lorenz model, previously used exclusively to model roughness in single-wavelength applications, predicts only qualitatively the spectral dependence and gives poor results.

Asymptotically optimal block quantization

Abstract: In 1948 W. R. Bennett used a companding model for nonuniform quantization and proposed the formula D \: = \: \frac{1}{12N^{2}} \: \int \: p(x) [ E(x) ]^{-2} \dx for the mean-square quantizing error where N is the number of levels, p (x) is the probability density of the input, and E \prime (x) is the slope of the compressor curve. The formula, an approximation based on the assumption that the number of levels is large and overload distortion is negligible, is a useful tool for analytical studies of quantization. This paper gives a heuristic argument generalizing Bennett's formula to block quantization where a vector of random variables is quantized. The approach is again based on the asymptotic situation where N , the number of quantized output vectors, is very large. Using the resulting heuristic formula, an optimization is performed leading to an expression for the minimum quantizing noise attainable for any block quantizer of a given block size k . The results are consistent with Zador's results and specialize to known results for the one- and two-dimensional cases and for the case of infinite block length (k \rightarrow \infty) . The same heuristic approach also gives an alternate derivation of a bound of Elias for multidimensional quantization. Our approach leads to a rigorous method for obtaining upper bounds on the minimum distortion for block quantizers. In particular, for k = 3 we give a tight upper bound that may in fact be exact. The idea of representing a block quantizer by a block "compressor" mapping followed with an optimal quantizer for uniformly distributed random vectors is also explored. It is not always possible to represent an optimal quantizer with this block companding model.

Make — a program for maintaining computer programs

Abstract: In a programming project, it is easy to lose track of which files need to be reprocessed or recompiled after a change is made in some part of the source. Make provides a simple mechanism for maintaining up-to-date versions of programs that result from many operations on a number of files. It is possible to tell Make the sequence of commands that create certain files, and the list of files that require other files to be current before the operations can be done. Whenever a change is made in any part of the program, the Make command will create the proper files simply, correctly, and with a minimum amount of effort. The basic operation of Make is to find the name of a needed target in the description, ensure that all of the files on which it depends exist and are up to date, and then create the target if it has not been modified since its generators were. The description file really defines the graph of dependencies;Make does a depth-first search of this graph to determine what work is really necessary. Make also provides a simple macro substitution facility and the ability to encapsulate commands in a single file for convenient administration.

Evidence for a Liquid-to-Crystal Phase Transition in a Classical, Two-Dimensional Sheet of Electrons

Abstract: Experimental evidence is presented for an electron-liquid to electron-crystal phase transition in a sheet of electrons on a liquid-He surface. The phase transition has been studied for electron areal densities from 3\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ to 9\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ and has yielded melting temperatures between 0.35 and 0.65 K. The phase transition occurs at $\ensuremath{\Gamma}=137\ifmmode\pm\else\textpm\fi{}15$, where $\ensuremath{\Gamma}$ is a measure of the ratio of potential energy to kinetic energy per electron.

Trapping characteristics and a donor-complex ( DX ) model for the persistent-photoconductivity trapping center in Te-doped Al x Ga 1 − x As

Abstract: Photocapacitance measurements have been used to determine the electron photoionization cross section of the centers responsible for persistent photoconductivity in Te-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$. The cross-section data, which have been obtained at various temperatures and for crystals of various alloy compositions, are fitted by a theoretical line shape that is valid for large lattice relaxation. The line shape and thermal broadening can best be fit by a binding energy of 0.10\ifmmode\pm\else\textpm\fi{}0.05 eV and a Franck-Condon energy of 0.75\ifmmode\pm\else\textpm\fi{}0.1 eV. These values are in good qualitative agreement with the large-lattice-relaxation model of persistent photoconductivity which we recently proposed. We show that the 0.10-eV binding energy is also consistent with experiments that locate this energy relative to the Fermi level. The dependence of the properties of the persistent-photoconductivity center on the donor doping of the samples leaves little doubt that this center involves a donor atom, but because the center is not effective-mass-like, we believe that it is a complex also involving another constituent. Accordingly, we designate it as a "$\mathrm{DX}$" center. The anomalously-large Franck-Condon energy (Stokes shift) and apparent fact that the unoccupied state of the $\mathrm{DX}$ center is resonant with the conduction band, yet sufficiently localized to produce a large relaxation, are thus well established. These considerations lead us to the propose that the most likely model for $\mathrm{DX}$ centers in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$, and perhaps in other compound semiconductors as well, is a complex involving a donor and an anion vacancy. We show that such a model is qualitatively consistent with the overall trends in persistent-photoconductivity behavior observed in a variety of III-V and II-VI semiconductors.

Universality of data retrieval languages

Abstract: We consider the question of how powerful a relational query language should be and state two principles that we feel any query language should satisfy. We show that although relational algebra and relational calculus satisfy these principles, there are certain queries involving least fixed points that cannot be expressed by these languages, yet that also satisfy the principles. We then consider various extensions of relational algebra to enable it to answer such queries. Finally, we discuss our extensions to relational algebra in terms of a new programming language oriented model for queries.

Electric field depinning of charge density waves

Abstract: The pinning of charge-density waves by impurities is considered in systems that exhibit at least short-range order in three dimensions. Impurities are classified into strong and weak with quite different pinning properties. The pinning of spin-density waves is weak and the phase values at impurity sites are almost random, in agreement with a recent experiment. The electric field required to depin the charge-density wave is estimated. The coupling between a drifting charge-density wave and carriers either from a remnant Fermi surface or thermal excitation is considered. Attention is focused on umklapp scattering of carriers by phasons as a coupling mechanism at finite temperature. The conductivity in the high-electric-field depinned limit can be large. Dislocations in the charge-density-wave lattice are examined with particular emphasis on the piecewise motion of the charge-density wave through the motion of dislocations. We also discuss the generation of dislocations by the analog of Frank-Read sources. The unusual nonlinear conductivity observed in Nb${\mathrm{Se}}_{3}$ is interpreted in terms of depinning of charge-density waves. The possibility of observing similar effects in other systems is briefly examined.

Chunking in recall of symbolic drawings

Abstract: Three experiments explored memory for symbolic circuit drawings using skilled electronics technicians and novice subjects. In the first experiment a skilled technician reconstructed circuit diagrams from memory. Recall showed marked “chunking”, or grouping, by functional units similar to Chess Masters’ recall of chess positions. In the second experiment skilled technicians were able to recall more than were novice subjects following a brief exposure of the drawings. This advantage did not hold for randomly arranged symbols. In the third experiment the size of chunks retrieved systematically increased with additional study time. Supplementary analyses suggested that the chunking by skilled subjects was not an artifact of spatial proximity and chunk statistics, and that severe constraints are placed on any explanation of the data based on guessing. It is proposed that skilled subjects identify the conceptual category for an entire drawing, and retrieve elements using a generate-and-test process.

A new theory of the spin-Peierls transition with special relevance to the experiments on TTFCuBDT

Abstract: We develop a new theory of the spin-Peierls transition in spin-\textonehalf{} Heisenberg chains, treating the phonons in a mean-field random-phase approximation (RPA) as in previous work, but calculating the relevant response functions of the spin system using the procedure of Luther and Peschel. We show that the RPA on the phonons (and therefore our whole calculation) should be good for the experimentally important system tetrathiafulvalenium bis-cis- (1,2-perfluoromethylethylene-1,2-dithiolato)-copper (TTFCuBDT). It is also exact for a model system in which planes of atoms perpendicular to the chains are constrained by lattice forces to move together: we have derived some exact results for the spin-Peierls transition in this model system. We find a new linear dependence of the transition temperature ${T}_{c}$ on the spin-phonon coupling, and an enhancement of ${T}_{c}$ and also of the rate of phonon softening above ${T}_{c}$. Predictions of some other signatures of the transition, such as the specific-heat jump ratio, are however not much changed from previous work. Exact results are found for the leading dependence of the ground-state energy $E$ and gap $\ensuremath{\Delta}$ in the excitation spectrum on the lattice distortion $\ensuremath{\delta}: E\ensuremath{\propto}{\ensuremath{\delta}}^{\frac{4}{3}}$, $\ensuremath{\Delta}\ensuremath{\propto}{\ensuremath{\delta}}^{\frac{2}{3}}$.

Predictive coding of speech signals and subjective error criteria

Abstract: Predictive coding methods attempt to minimize the rms error in the coded signal. However, the human ear does not perceive signal distortion on the basis of rms error, regardless of its spectral shape relative to the signal spectrum. In designing a coder for speech signals, it is necessary to consider the spectrum of the quantization noise and its relation to the speech spectrum. The theory of auditory masking suggests that noise in the formant regions would be partially or totally masked by the speech signal. Thus, a large part of the perceived noise in a coder comes from frequency regions where the signal level is low. In this paper, methods for reducing the subjective distortion in predictive coders for speech signals are described and evaluated. Improved speech quality is obtained: 1) by efficient removal of formant and pitch-related redundant structure of speech before quantizing, and 2) by effective masking of the quantizer noise by the speech signal.

Solid State Electrodes for High Energy Batteries

Abstract: A new class of electrode materials for high energy density, rechargeable batteries based on topochemical reactions of lithium and transition metal compounds is evolving. The physical and structural properties relevant to the ability of transition metal oxides with framework structures to topochemically incorporate lithium are discussed. Perovskite-related structures are particularly attractive hosts for lithium.

Possible explanation of nonlinear conductivity in thin-film metal wires

Abstract: We introduce the idea of an incoherence length ${L}_{2}$ at which inelastic collision broadening equals electronic energy-level separation. The two-dimensional conductance crosses over at ${L}_{2}$ from a $\mathrm{ln}L$ dependence suggested recently by Abrahams et al. to Ohmic behavior. These ideas and plausible relaxation and heating models are used to explain the nonlinear conductivity observations of Dolan and Osheroff.

Processing determinants of reading speed.

Abstract: Two groups of university undergraduates differing in reading ability were tested on a number of reaction-time tasks designed to determine the speed of encoding visual information at several different levels. In addition, the subjects were given tests of sensory functions, verbal and quantitative reasoning ability, short-term auditory memory span, and ability to comprehend spoken text. The groups did not differ on the sensory tests. However, the faster reader group had faster reaction times on all of the reaction-time tasks, and the size of the fast-reader advantage increased with the mean reaction time. Faster readers also performed more accurately in verbal and quantitative reasoning, short-term auditory memory, and speech comprehension. Regression analyses suggested that the ability to comprehend spoken material and speed of accessing overlearned memory codes for visually presented letters represented two important independent correlates of reading ability in our sample of subjects. Two variables reflecting these abilities--the percentage of correct answers to a listening comprehension test and the reaction time for correct responses in a letter-matching task--accounted for nearly all of the variance in reading ability tapped by both of our reading tests. In a second experiment, no reaction-time difference was found between fast and average readers in a matching task requiring no long-term memory code access but considerable visual information processing as indexed by overall mean reaction time. The results supported the conclusion that one skill allowing fast readers to capture more information from each reading fixation is faster access to letter codes from print.

Characterization of the electron mobility in the inverted l100g Si surface

Abstract: The effective mobility of electrons in the inverted 〈100〉 Si surface was measured over a wide range of temperatures, gate voltages, and back-bias voltages. At a first glance the mobility appears to be strongly dependent on the channel surface doping. When the data were reanalyzed under a new approach where the mobility is plotted as a function of an effective perpendicular electric field experienced by the inversion layer, a universal curve has developed from the experimental data which indicates that the inversion layer mobility is not a function of doping density in the range (N A 2 system and not a parameter sensitive to nominal process variations encountered in the present n-channel Si-Gate Technology.

Cellular applications of 31P and 13C nuclear magnetic resonance

Abstract: High-resolution nuclear magnetic resonance (NMR) studies of cells and purified mitochondria are discussed to show the kind of information that can be obtained in vivo. In suspensions of Escherichia coli both phosphorus-31 and carbon-13 NMR studies of glycolysis and bioenergetics are presented. In rat liver cells the pathways of gluconeogenesis from carbon-13-labeled glycerol are followed by carbon-13 NMR. In the intact liver cells cytosolic and mitochondrial pH's were separately measured by phosphorus-31 NMR. In purified mitochondria the internal and external concentrations of inorganic phosphate, adenosine diphosphate, and adenosine triphosphate were determined by phosphorus-31 NMR while the pH difference across the membrane was measured simultaneously.

Optimal noise figure of microwave GaAs MESFET's

Abstract: The optimal value of the minimum noise figure F o of GaAs MESFET's is expressed in terms of either representative equivalent circuit elements or geometrical and material parameters in simple analytical forms. These expressions are derived on a semiempirical basis. The predicted values of F o for sample GaAs MESFET's using these expressions are in good agreement with the measured values at microwave frequencies. The expressions are then applied to show design optimization for low-noise devices. This exercise indicates that shortening the gate length and minimizing the parasitic gate and source resistances are essential to lower F o . Moreover, a simple shortening of the gate length may not bring an improved F o unless the unit gate width is accordingly narrowed. The maximum value of the unit gate width is defined as the width above which the gate metallization resistance becomes greater than the source series resistance. Short-gate GaAs MESFET's with optimized designs promise a superior noise performance at microwave frequencies through K band. The predicted values of F o at 20 GHz, for example, for a half-micrometer gate device and a quarter-micrometer gate device are 3 and 2 dB, respectively. These devices could be fabricated with the current technology.

Infrared personnel locator system

Abstract: An infrared personnel locator system using a periodic unique infrared identification code emitted from a battery-powered transmitter unit to identify the person carrying the transmitter unit to an overhead infrared receiver as the person enters the receiver's monitoring zone. The transmitter unit identification code together with the receiver identification code is communicated to a common control unit which displays the location of all of the transmitter units. For paging, the system provides alerting units which are selectively operated in the zone closest to the person being paged. Additionally, the system controls electronic locks for restricting access of users to certain locations or equipment. A telephone interface provides system access from the telephone network.

Polarization effects in fiber Raman and Brillouin lasers

Abstract: Raman and Brillouin gains in fibers are a factor of 2 higher if linear polarization is maintained. Birefringent single-mode fibers are used to demonstrate this gain difference. Threshold powers of 200 mW have been achieved in Raman oscillators.

Sliding-Mode Conductivity in Nb Se 3 : Observation of a Threshold Electric Field and Conduction Noise

Abstract: Two new effects associated with Fr\"ohlich sliding-mode conductivity have been observed in Nb${\mathrm{Se}}_{3}$ First, nonlinear resistance is observed only after a well-defined threshold field is reached Second, electric fields above the threshold result in noise in the potential difference across a current biased specimen We interpret the threshold field as direct evidence of charge-density wave depinning and the noise as a result of charge-density wave motion

Path integrals and stationary-phase approximations

Abstract: The general formalism for path integrals expressed in terms of an arbitrary continuous representation (generalized coherent states) is applied to give $c$-number formulations for the canonical algebra and for the spin algebra, and is used to derive meaningful stationary-phase approximations for these two cases. Some clarification of a recent discussion by Jevicki and Papanicolaou for the spin case is given.

Dynamics of Photoexcited GaAs Band-Edge Absorption with Subpicosecond Resolution

Abstract: Time-resolved measurements of optically induced changes in the near-band-gap transmission spectrum of GaAs at 80\ifmmode^\circ\else\textdegree\fi{}K, following excitation with an ultrashort laser pulse, provide a means of directly monitoring the hot-carrier distribution as it cools to the lattice temperature with a time constant of 4 psec. Exciton screening and band-gap renormalization are observed to occur in less than 0.5 psec.

Electronic surface resonances of crystals

Abstract: Electrons incident on a crystal surface can be temporarily trapped in surface states at energies above the vacuum level. These temporary or nonstationary surface states are observed as narrow fluctuations of elastic scattering intensity with respect to variation of electron energy and incidence direction. The scattering process is called electronic surface resonance scattering. The temporary surface states that are intermediate states in resonance scattering are called electronic surface resonances. The article surveys both experimental and theoretical research on electronic surface resonances as observed by scattering of low-energy (<1 keV) electrons. A critical account of experiments on Al(001), W(001), Ni(001), and oxygenated Ni(001) surfaces is offered together with theoretical commentary. Plots of the electronic surface resonance band structure E (k/sub parallel/) (E=resonance energy, k/sub parallel/ =reduced surface-parallel momentum) are compiled and the signficance of E (k/sub parallel/) plots for surface characterization is indicated.