Showing papers by "Alcatel-Lucent published in 1988"

Superconductivity near 30 K without copper: the Ba 0.6 K 0.4 BiO 3 perovskite

Abstract: It is well known that the breakthrough of Bednorz and Muller1 in discovering superconductivity in (La, Ba)2CuO4 was inspired in part by their knowledge of the superconducting properties of Ba(Pb, Bi)O3 (ref. 2). With a transition temperature, Tc, of ∼12 K, that compound was not generally considered anomalous despite the fact that its Tcis 3–5 times higher than that of traditional superconductors with comparable density of states3–5. The increases in Tc for copper-oxide-based materials continue to generate worldwide excitement, but from both a chemical and theoretical point of view it would also be exciting if high-Tcsuperconductivity were observed in another class of materials. Here we report the results of experiments leading us to the single-phase perovskite Ba0.6K0.4BiO3, which has a magnetically determined onset temperature of 29.8 K—a Tc considerably higher than that of conventional superconductors and surpassed only by copper-containing compounds. Superconductivity in this compound occurs within the framework of a three dimensionally connected bismuth-oxygen array. These results suggest that further research toward exploring the limiting Tcs for bismuth-oxide-based, high-temperature superconductors might be fruitful.

Infrared spectroscopy of Si(111) surfaces after HF treatment: Hydrogen termination and surface morphology

Abstract: Polarized internal reflection spectroscopy has been used to characterize HF‐treated Si(111) surfaces. The silicon‐hydrogen stretching vibrations indicate that the surface is well ordered, but is microscopically rough, with coupled monohydride, dihydride, and trihydride termination.

Rotation distance, triangulations, and hyperbolic geometry

Abstract: A rotation in a binary tree is a local restructuring that changes the tree into another tree. Rotations are useful in the design of tree-based data structures. The rotation distance between a pair of trees is the minimum number of rotations needed to convert one tree into the other. In this paper we establish a tight bound of In 6 on the maximum rotation distance between two A2-node trees for all large n, using volumetric arguments in hyperbolic 3-space. Our proof also gives a tight bound on the minimum number of tetrahedra needed to dissect a polyhedron in the worst case, and reveals connections 1 This is a revised and expanded version of a paper that appeared in the 18th Annual ACM Symposium on Theory of Computing, [9]. 2 Partial support provided by DARPA, ARPA order 4976, amendment 19, monitored by the Air Force Avionics Laboratory under contract F33615-87-C-1499, and by the National Science Foundation under grant CCR-8658139. 3 Partial support provided by the National Science Foundation under grant DCR-8605962. 4 Partial support provided by the National Science Foundation under grants DMR-8504984 and DCR8505517.

Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum

Abstract: The homogeneous (single‐cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Studies of oxygen-deficient Ba2YCu3O7−δ and superconductivity Bi(Pb)SrCaCuO

Abstract: Ambient temperature measurements of the crystallographic cell parameters for oxygen deficient Ba 2 YCu 3 O 7−δ prepared by gettered annealing indicate the presence of microscopic differences in the oxygen configuration at fixed δ dependent on annealing temperature. The loss of superconductivity with increasing oxygen deficiency is shown to be due to a step-like increase in length of the plane copper-apical oxygen bond, and not to the orthorhombic to tetragonal transition. The crystal structure of the 84K superconductor Bi 2 Sr 2 CaCu 2 O 8+δ is described, as is the stabilization of 110K superconductivity via partial Pb/Bi substitution with long time annealing.

Structural anomalies at the disappearance of superconductivity in Ba2YCu3O7−δ: Evidence for charge transfer from chains to planes

Abstract: In systematic studies of oxygen-deficient Ba 2 YCu 3 O 7−δ , we have found that a structural anomaly occurs at the composition where bulk superconductivity disappears. The observed step-like increase in the c lattice parameter, when associated with changes in the atomic positions within the unit cell, indicates that superconductivity is lost due to a charge redistribution from the copper-oxygen chains to the copper-oxygen planes. This is microscopic experimental evidence establishing the role of secondary structural elements in the layered copper-oxide high- T c superconductors.

Density of states and isotope effect in BiO superconductors: Evidence for nonphonon mechanism.

Abstract: Mesures des champs critiques et de la susceptibilite magnetique dans l'etat normal de Ba 0,6 K 0,4 BiO 3 et comparaison avec BaPb 0,75 Bi 0,25 O 3 . Mise en evidence du fait que ces supraconducteurs sont dans la limite du couplage faible qui exclut un couplage fort electron-phonon. L'appariement supraconducteur serait du aux excitations electroniques

Aluminum clusters: ionization thresholds and reactivity toward deuterium, water, oxygen, methanol, methane, and carbon monoxide

Abstract: Using the pulsed cluster beam flow reactor technique, the authors have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule they observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as O/sub 2/ > CH/sub 3/OH > CO > D/sub 2/O > D/sub 2/ > CH/sub 4/ with CH/sub 4/ showing no reaction under these experimental conditions. In addition, they are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.

Propagation around a bend in a multichannel electron waveguide.

Abstract: La magnetoconductivite dans des heterostructures de GaAs−AlGaAs, dans lesquelles un nombre restreint de sous-bandes unidimensionnelles transverses conduisent le courant, est mesuree. La resistance moyenne est en grande partie associee a la diffusion des porteurs de charge par le contact d'aluminium utilise pour la mesure

Cryptanalysis: a survey of recent results

Abstract: Cryptosystems are tested by subjecting them to cryptanalytic attacks by experts. Most of the cryptosystems that have been publicly proposed in the last decade have been broken. Some of the attacks that have been used are outlined, and some of the basic tools available to the cryptanalyst are explained. Attacks on the knapsack cryptosystems, congruential generators, and a variety of two key secrecy and signature schemes are discussed. There is also a brief discussion of the status of the security of cryptosystems for which there is no known feasible attack, such as the RSA, discrete exponentiation, and DES cryptosystems. >

In situ epitaxial growth of Y1Ba2Cu3O7−x films by molecular beam epitaxy with an activated oxygen source

Abstract: Highly oriented, epitaxial Y1Ba2Cu3O7−x thin films were prepared on MgO(100) by molecular beam epitaxy at a substrate temperature of 550–600 °C. The in situ growth was achieved by incorporating reactive oxygen species produced by a remote microwave plasma in a flow‐tube reactor. The epitaxial (001) orientation is demonstrated by x‐ray diffraction and ion channeling. In situ reflection high‐energy electron diffraction showed that a layer‐by‐layer growth has produced a well‐ordered, atomically smooth surface in the as‐grown tetragonal phase of an oxygen stoichiometry of 6.2–6.3. A 500 °C anneal in 1 atm of O2 converted the oxygen content to 6.7 to 6.8. Typical superconducting transport properties of an Y1Ba2Cu3O7−x film 1000 A thick are ρ(300 K)=325 μΩ cm, ρ(300 K)/ρ(100 K)=2.4, Tc(onset)=92 K, and Tc(R=0)=82 K. The transport Jc at 75 K is 1×105 A/cm2, and increases to 1×106 A/cm2 at 70 K.

Fast algorithms for multiple evaluations of the riemann zeta function

Abstract: The best previously know algorithm for evaluating the Riemann zeta function, ζ(σ+it), with σ bounded and t large to moderate accuracy was based on the Rienmann-Siegel formula and required on the order of t 1/2 operations for each value that was computed. New algorithms are presented in this paper which enable one to compute any single value of ζ(σ+it) with σ fixed and T≤t≤T+T 1/2 in O(te) operations on numbers of O(log t) bits for any e>0 for example. These algorithms lead to methods for numerically verifying the Riemann hypothesis for the first n zeros in what is expected to be O(n 1+ e) operations

Method and facilities for hybrid packet switching

Abstract: In fast packet-switching networks, packets of particular connections (CS1, CS2) (e.g., voice, moving-image) are to be treated with priority over packets of other connections (PS1, PS2, PS3) (e.g., data) (without delay jitter and loss of information). Prior art arrangements are divided into a circuit-switching portion (for CS1, CS2) and a packet-switching portion (for PS1, PS2, PS3). The circuit-switching portion operates on a time-division multiplex basis. On each incoming line, 70 packets each containing 40 octets, for example, are combined into a frame. If no special steps were taken, delays on the order of 40×70×8 bits=22,400 bits would result in each switching stage; for each input of a switching stage, 22,400 bits of storage would be necessary. According to the invention, all packets are divided within the exchange, into subpackets of equal length and distributed to subframes. Switching takes place on the basis of subframes. Delays and memory location requirements are reduced in the ratio of frames:subframes. Preferably, the connections (CS1, CS2) to be treated with priority are assigned fixed positions in each frame and each subframe throughout the call. Subpackets of the other packets are preferably inserted in sequence into free positions, and are recombined into complete packets within each switching stage.

Measurements of room‐temperature band‐gap‐resonant optical nonlinearities of GaAs/AlGaAs multiple quantum wells and bulk GaAs

Abstract: We present a systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs multiple quantum wells (MQW’s) at room temperature and compare them with bulk GaAs. The maximum change in the refractive index is greatest for the MQW’s with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 A MQW.

Growth and structural characterization of superconducting Ba1–xKxBiO3 single crystals

Abstract: Superconductivity near 30 K was discovered recently in a copper-free phase, Ba1–xKxBiO3 (refs 1, 2). This material has structural and electronic properties in common with both the cuprate high-transition-temperature (high-Tc) superconductors3 and with BaPb1–xBixO3, which has a Tc of ˜12 K (ref. 4). Here we report the growth and structural characterization of single crystals of Ba1–xKxBiO3. A maximum Tc of 30.5 K is found from d.c. susceptibility measurements for crystals of approximate composition Ba0.6K0.4BiO3. The structure for a crystal with x = 0.37 is determined by X-ray diffraction and is found to be a simple cubic perovskite with a = 4.2869 (6) A. A commensurate lattice distortion occurs for x = 0.04, which is identified as a distortion of the BiO6 octahedra. Effects of structural distortions and concomitant changes in electronic properties in this non-magnetic system may contain clues to the role of phonons and electronic interactions in the microscopic pairing mechanism of high-Tc superconductors.

Thermodynamics of isotopic polymer mixtures: Significance of local structural symmetry

Abstract: The dependence on composition (Φ) and degree of polymerization (N) of the effective segment–segment interaction parameter, χ=− 1/2 ∂2Γ/∂Φ2, has been determined for mixtures of perdeuterated and normal (protonated) poly(vinylethylene) and poly(ethylethylene) by small angle neutron scattering (SANS), where Γ represents the excess free energy of mixing. Owing to the small reduction in length and polarizability of carbon–hydrogen bonds with deuterium substitution, isotopic polymer mixtures are characterized by small positive χ parameters. However, deuterium substitution does not significantly influence the local structural symmetry (i.e., the liquid structure or reduced thermodynamic properties) of polymer isotopes, consistent with the assumptions of Flory–Huggins (FH) lattice theory. Nevertheless, we find that χ depends significantly on both Φ and N, contrary to the predictions of FH theory. A general theory, which corrects FH theory for monomer concentration fluctuations, is shown to closely account for these experimental results. In understanding the previous experimental investigations of χ for mixing chemically different species, the monomer concentration fluctuations discussed here must be addressed in addition to the effects due to local structural asymmetry.

Thermodynamics of isotopic polymer mixtures: poly(vinylethylene) and poly(ethylethylene)

Abstract: Melange etudie entre 26 et 100 o C, de melanges contenant 50% en volume de polymere perdeuterie

Direct inelastic scattering of N2 from Ag(111). II. Orientation

Abstract: The orientation of the angular momentum of N2 scattered from clean Ag(111) is determined by resonance‐enhanced multiphoton ionization. The orientation is the net helicity or handedness of the sense of rotation, i.e., clockwise vs counterclockwise. The orientation of the scattered N2 is measured along a direction perpendicular to the scattering plane. The degree and sign of the orientation is found to depend strongly on the final rotational quantum number J and on the final scattering angle. The results require that there are forces acting in the plane of the surface during the scattering. The observed behavior can be reproduced qualitatively by a conventional hard‐cube, hard‐ellipsoid model to which a tangential friction has been introduced to account for the in‐plane forces. This produces a splitting of the rotational rainbow peak which leads to changes of sign of the orientation as a function of rotational quantum number. Thus, orientation measurements provide a unique probe of in‐plane gas–surface forces.

Balancing sets of vectors

Abstract: For n>0, d>or=0, n identical to d (mod 2), let K(n, d) denote the minimal cardinality of a family V of +or-1 vectors of dimension n, such that for any +or-1 vector w of dimension n there is a v in V such that mod v-w mod >

Method of routing calls for a network of digital time-switched exchanges

Abstract: A method of routing calls for a network of digital time-switched exchanges (A, B, C, D, E, F, D) interconnected by groups (18 to 28) of PCM time multiplex trunks, with each group ingterconnecting two exchanges. Each exchange establishes a utilization cost for each of the groups connected thereto. This cost is based on the busy fraction of the least busy trunk in the group of trunks under consideration and it is transmitted to all of the exchanges in the network in order to enable them to determine least cost paths for the calls that they are requested to set up.

Ion-beam-induced destruction of superconducting phase coherence in YBa2Cu3O7- delta.

Abstract: We have explored the effects of ion beam irradiation on the electrical and structural properties of superconducting thin films of YBa/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub delta/. Damage created by nuclear-energy loss processes degrades the superconducting transitions. Data from resistance measurements as well as from Rutherford backscattering and channeling measurements suggest that ion bombardment is not changing the volume fraction of superconducting material substantially. Instead, it appears that islands of good superconductor are becoming progressively decoupled, destroying the phase coherence

Macromolecular stereochemistry: effect of pendant group structure on the conformational properties of polyisocyanides

Abstract: Preparation par amorcage avec NiII de polyisocyanures avec des substituants en α (un est optiquement actif)

High performance tunable 1.5 μm InGaAs/InGaAsP multiple quantum well distributed Bragg reflector lasers

Abstract: We describe the structure and performance of tunable four‐quantum‐well InGaAs/InGaAsP distributed Bragg reflector lasers. We observe total tuning range as large as 94 A, differential efficiency of 32%/front facet, thresholds of 17 mA, low‐chirp high‐speed digital operation, and linewidths as low as 5.75 MHz at only 2 mW output.

Cellular array processor with individual cell-level data-dependent cell control and multiport input memory

Abstract: In an array of processing cells arranged as a single instruction multiple data processor, each processing cell contains logic which enables the cell to determine individually whether it will perform an arithmetic or logic operation or be in an idle condition. This cell-level logic includes a control register whose contents determine the operating or idle condition of the cell, and also includes PUSH/POP/COMPLEMENT stack mechanisms to represent multiple complex levels of conditions, and mechanisms to load the results of a cell-level test or arithmetic instruction into the control register, providing data-dependent control at the cell level.

Deformation-dependent hydrodynamic interaction in flows of dilute polymer solutions

Abstract: We have extended the bead–spring model of Zimm to include hydrodynamic interaction that depends upon the conformation of the polymer and hence the deformation rate. Our computational scheme is based on the model of Fixman, and it allows us to investigate numerically chains as large as 500 beads. The major approximation is the replacement of the hydrodynamic interaction tensor with its configurational average. We find that the incorporation of configuration‐dependent hydrodynamic interaction qualitatively changes the Zimm predictions for both the shear and the elongational viscosity. In particular, the elongational viscosity becomes a multivalued function of the rate of elongation. This result is consistent with the coil‐to‐stretch transition predicted by de Gennes and Hinch.

Method of demodulating digitally modulated signals, and apparatus implementing such a method

Abstract: A signal may be demodulated without using any analog components by using a digital processor. A digitizer delivers the amplitude of the signal to be demodulated at a sampling rate. The signal is acquired from the digitizer and stored in a common memory. The amplitude samples are converted to phase samples. Symbol rate phase hypotheses are generated for splitting the time signal into slices. The carrier phase for one symbol is then calculated for each symbol rate phase hypothesis so that the carrier phase can be estimated over a plurality of (S) symbols. The symbol phase is compared to the carrier phase so that the demodulated signal may be extracted and so that the quality of the demodulation may be estimated.

Balancing sets of vectors

Abstract: For n>0, d>or=0, n identical to d (mod 2), let K(n, d) denote the minimal cardinality of a family V of +or-1 vectors of dimension n, such that for any +or-1 vector w of dimension n there is a v in V such that mod v-w mod >

Control of pinholes in epitaxial CoSi2 layers on Si(111)

Abstract: The growth of ultrathin (<50 A thick) uniform CoSi2 layers at low temperatures (<450 °C) has been reported recently. Pinholes are formed in these silicide layers when the temperature is raised to above ∼550 °C. An important driving force for the generation of pinholes has been identified as a change of the surface structure from CoSi2‐C, stable at low temperature, to the high‐temperature stable CoSi2‐S. Treatment of the surface of CoSi2 facilitates this transition and prevents the formation of pinholes. A few important parameters in the silicide reaction are shown to govern the morphology of the reacted CoSi2 layers.