Showing papers by "NEC published in 1992"

New one-dimensional conductors: Graphitic microtubules.

Abstract: On the basis of realistic tight-binding band-structure calculations, we predict that carbon microtubules exhibit striking variations in electronic transport, from metallic to semiconducting with narrow and moderate band gaps, depending on the diameter of the tubule and on the degree of helical arrangement of the carbon hexagons. The origin of this drastic variation in the band structure is explained in terms of the two-dimensional band structure of graphite.

Large-scale synthesis of carbon nanotubes

Abstract: INTEREST in carbon fibres1,2 has been stimulated greatly by the recent discovery of hollow graphitic tubules of nanometre dimensions3. There has been much speculation about the properties and potential application of these nanotubes4–8. Theoretical studies predict that their electronic properties will depend on their diameter and degree of helicity4,5. Experimental tests of these ideas has been hampered, however, by the lack of macroscopic quantities of the material. Here we report the synthesis of graphitic nanotubes in gram quantities. We use a variant of the standard arc-discharge technique for fullerene synthesis under a helium atmosphere. Under certain conditions, a carbonaceous deposit forms on one of the graphite rods, consisting of a macroscopic (diameter of about 5 mm) cylinder in which the core comprises pure nanotubes and nanoscale particles in high yield. The purity and yield depend sensitively on the gas pressure in the reaction vessel. Preliminary measurements of the conductivity of the bulk nanotube material indicate a conductivity of about 100 S cm–11.

Pentagons, heptagons and negative curvature in graphite microtubule growth

Abstract: THE standard carbon-arc synthesis for fullerenes also produces graphitic microtubules with helical structures1. In most cases the cylindrical tubes are closed by polyhedral caps, some being first transformed into a conical shape before closure2. Here we present images from transmission electron microscopy of a further kind of growth morphology, in which cone-like growth is transformed into cylindrical growth by the incorporation of a defect that induces negative curvature. We suggest that the defect in the hexagonal network responsible for negative curvature may be a single heptagonal ring. Three-dimensional, open negatively curved graphitic structures have been proposed recently by Terrones and Mackay3. We discuss more generally the effect of pentagons and heptagons on the growth morphologies of these tubules, and the constraints on the number of pentagonal defects imposed by tube closure.

Growth model for carbon nanotubes.

Abstract: Evidence is presented, through high-resolution electron microscopy images, for the open-end growth of carbon nanotubes Terminations of incomplete layers of carbon, seen on the tube surfaces and cone tips, suggest that the extension and thickening of the tubes occur by the island growth of graphite basal planes on existing tube surfaces The nucleation of positive (pentagons) and negative (heptagons) disclinations on open tube ends results in changes of growth directions, producing different morphologies A novel structure that shows complete ``turn around growth,'' involving pentagon-heptagon pairs, of the tubular assemblies is presented

Physics and Device Applications of Optical Microcavities

Abstract: Optical microcavities are resonators that have at least one dimension on the order of a single optical wavelength. These structures enable one to control the optical emission properties of materials placed inside them. They can, for example, modify the spatial distribution of radiation power, change the spectral width of the emitted light, and enhance or suppress the spontaneous emission rate. In addition to being attractive for studying the fundamental physics of the interaction between materials and vacuum field fluctuations, optical microcavities hold technological promise for constructing novel kinds of light-emitting devices. One of their most dramatic potential features is thresholdless lasing. In this way and others, controlled spontaneous emission is expected to play a key role in a new generation of optical devices.

Method for producing multilayer printed wiring boards

Abstract: A multilayered board is formed by applying a photosensitive insulating resin layer on a laminated plate on which via holes and a circuit pattern are formed, followed by the formation of photoviaholes through the photoprinting method, plating and etching. Then, the multilayered board is adhered to another multilayered board prepared in the same manner through a prepreg layer and a conductive paste while applying heat and pressure to give a multilayer printed wiring board. According to this method, electrical connections between the conductive layer of the upper-most layer and the inner conductive layers, between the inner conductive layers, and between the lower-most conductive layer and the inner conductive layers can be achieved through the photoviaholes and the conductive paste. Therefore, it is not necessary to form through-holes for the electrical connection therebetween. The multilayer printed wiring boards can be substantially improved in the number of layers and wiring density thereof.

Superconductivity in sodium-and lithium-containing alkali-metal fullerides

Abstract: THE discovery1 of superconductivity, with a transition temperature Tc of 18 K, in potassium-doped C60 was followed by the synthesis of other superconducting M3C60 phases: Rb3C60 (Tc = 28K and 30 K; refs 2, 3), CsRb2C60 (Tc = 31K; ref. 4) and Cs2RbC60 (Tc = 33 K; ref. 4). A monotonic relationship is observed5 between Tc and lattice constant a0 for these face-centred cubic M3C0 compounds, all of which have a0 values larger than that of pure C60. Here we study this relationship over a wider range using mixed alkali compounds incorporating sodium (Na2MC60, where M is K, Rb or Cs) and lithium (Li2CsC60). The Na2MC60 compounds have face-centred cubic structures and are superconducting, with Tc = 12 K (M = Cs), 2.5 K (M = Rb) and 2.5 K (M = K); these Tcs are lower than those predicted on the basis of high-pressure studies6–8. This departure from the previous relationship between Tc and a0 for lattice parameters smaller than that of pristine C60 may provide new insights into what controls superconductivity in these materials.

A Study on the “Runaway Greenhouse Effect” with a One-Dimensional Radiative–Convective Equilibrium Model

Abstract: A simple one-dimensional radiative–convective equilibrium model is used to investigate the relationship between the surface temperature and the outgoing infrared radiation at the top of the atmosphere. The model atmosphere has a gray infrared absorption coefficient and is composed of a radiative equilibrium stratosphere and a moist adiabat troposphere. An upper limit of the outgoing infrared radiation is found to exist. The existence of the upper limit is characterized by the radiation limits that appear when the optical depth of the entire atmosphere becomes sufficiently deep and the temperature structure around the levels where the optical depth is about unity approaches a fixed profile. This appearance of an upper limit differs from that found by Komabayashi and Ingersoll, which is obtained from the constraint of the stratospheric radiation balance. As one of those radiation limits, the outgoing infrared radiation has an asymptotic limit as the surface temperature increases. This is caused by ...

Method for fabricating a semiconductor device having a multi-layered interconnection structure

Abstract: A method of fabricating a multi-layered interconnection structure which comprises the steps of: forming a first wiring layer on a silicon oxide film having a compressive stress; forming a thick (2 to 3.5 μm) fluorine-containing silicon oxide film at a temperature not higher than 200 ° C.; etching back the fluorine-containing silicon oxide film to flatten the surface of the film; forming a silicon oxide film having a compressive stress; forming a through-hole in position; and forming a second wiring layer. Since the fluorine-containing silicon oxide film is used as part of an insulating film, a resistance to cracking, flatness and reliability are significantly improved.

Resource reallocation for flow-enforced user traffic

Abstract: In a packet (ATM) network, a source node at the entry of the network is responsive to a connection request from a user terminal for invoking a CAC (call admission control) algorithm to accept or reject the request depending on the amount of resource requested by the user, and allocates a portion of a free bandwidth resource exclusively to the user for the duration of the call according to established contract parameter values. Each node of the network responds to a reallocation request from the user for transmitting a copy of the request to a downstream node to elicit an acceptance message therefrom, and reserving a portion of a pool bandwidth resource and invoking the CAC algorithm to additionally reserve a portion of the free bandwidth resource. The node proceeds to allocate the reserved pool bandwidth to the user in response to the acceptance message indicating that the same amount of the reserved pool bandwidth is available in the downstream node. The allocation of the pool bandwidth is temporary. When a portion of the free bandwidth resource is reserved using the CAC algorithm, this portion is exclusively allocated to the user until the end of the call and the temporarily allocated pool bandwidth is returned to the pool resource for other users.

Method of forming silicon oxide film containing fluorine

Abstract: It is an object of the present invention to provide an excellent silicon oxide film formed at a temperature of 200° C. or less, a method of forming the silicon oxide film, and a selective growing method. According to the present invention, by using a vapor containing alkoxyfluorosilane as a main component, a silicon oxide film containing fluorine is formed at a temperature of 200° C. or less in a reaction chamber having a predetermined temperature and a predetermined pressure. In addition, an organic film such as a photoresist film is used as a mask to selectively form the silicon oxide film. Although the silicon oxide film containing fluorine and formed on the basis of the present invention is formed at a very low temperature of 30° C., this silicon oxide film has a water content smaller than that of a silicon oxide film formed at a temperature of 250° C. in a conventional method. In addition, the film properties of the silicon oxide film according to the present invention are better than those of the silicon oxide film formed in the conventional method. Furthermore, a two-layered aluminum wiring structure can be easily formed by the selective growing method.

The unc-18 gene encodes a novel protein affecting the kinetics of acetylcholine metabolism in the nematode Caenorhabditis elegans.

Abstract: Genes affecting acetylcholine (ACh) levels without influencing choline acetyltransferase activity have been identified in Caenorhabditis elegans. We have examined one such gene, unc-18. We isolated a transposon-insertion allele for unc-18 and used it to clone a genomic region containing the unc-18 locus. The unc-18 location within this region was determined by rescuing the unc-18 mutant phenotype in a germline transformation experiment and identifying transcripts affected by four independent unc-18 mutations. A single-sized poly(A)+ RNA was synthesized from the gene. Expression of the transcript appears to be stage specific: The transcript is found in abundance at the early larval stage but in decreased amounts at the fourth larval and the adult stages. These results show that the unc-18 gene plays a role in development as well as in the kinetics of ACh metabolism.

Variable modulation communication method and system

Abstract: In a variable modulation communication method, the transmission quality of a transmission path is monitored. If the transmission quality is good, communication is performed while the transmission time is shortened by using a multilevel modulation scheme. If the transmission quality is bad, communication is performed while the transmission time is prolonged by using a modulation scheme having a smaller number of levels than the multilevel modulation scheme. A variable modulation communication apparatus is also disclosed.

Piezoelectric ceramic transformer being driven with thickness extensional vibration

Abstract: A piezoelectric ceramic transformer comprises a low impedance portion 11 including a plurality of piezoelectric ceramic layers 111 to 115 having different thicknesses, a high impedance portion 12 and internal electrodes 16. The low and high impedance portions 11 and 12 are subjected to a uniform polarization treatment in a thickness direction and with the same vector of polarization. The low impedance portion 11 includes one or more piezoelectric ceramic layers having a thickness different from those of the other layers therein. No delamination of electrodes or breaking of ceramics occur upon polarization treatment. Therefore, a compact piezoelectric ceramic transformer for power source which has a high mechanical quality coefficient Qm and a high electric power transmission efficiency is provided.

Method and arrangement for estimating data sequences transmsitted using Viterbi algorithm

Abstract: To reduce the number of operations required for estimating a transmitted data sequence using a Viterbi algorithm, a method and apparatus are provided for (a) storing a plurality of sampled values of an incoming signal in a shift register in a predetermined interval; (b) receiving a plurality of the sampled values from the shift register, and estimating channel responses, at a current time point, of a plurality of first signal sequences which are derived from the plurality of sampled values and each of the length of which is reduced by deleting at least one sampled data at the oldest time point; (c) receiving the channel responses estimated at (b) and checking to see if each of the estimated channel responses is determinate, producing a first signal sequence if an estimated channel response of a first signal sequence is found determinate, and producing an estimated channel response as a first signal sequence at a time point preceding the current time point in the event that the estimated channel response of the first signal sequence is found indeterminate; (d) storing the first signal sequences produced at (c), and producing an estimated channel response which has been stored at a time point preceding the current time point; and (e) determining a plurality of branch metrics using the estimated channel response obtained at (c) and an original first signal sequence before the original first signal sequence is reduced in length.

A 30-ns 64-Mb DRAM with built-in self-test and self-repair function

Abstract: A 64 Mw*1 b/16 Mw*4 b DRAM with 30-ns access time which uses a double-metal layer and 0.4- mu m CMOS technology is reported. The external power supply is 3 V, while memory cell arrays operate at 2.2 V. Key circuits for the 64-Mb DRAM are (1) a latched-sense, shared-sense circuit with open bit-line read-out and folded bit-line rewrite operations (LOF) to reduce inter-bit-line coupling noise, (2) alternatively activated and separately end-located word drivers and X decoders to reduce word-line selection delay, and (3) built-in self test and repair circuits using spare memory cells to reduce test costs and increase chip reliability. >

Global scheduling independent of control dependencies based on condition vectors

Abstract: The authors present a global scheduling method based on condition vectors. The proposed method exploits global parallelism. The technique can schedule operations independent of control dependencies. It transforms the control structure of the given behavior drastically, while preserving semantics to minimize the number of states in final schedule. The method can parallelize multiple nests of conditional branches and optimize across the boundaries of basic blocks. It can also optimize all possible execution paths. An algorithm is proposed which generates a single finite state machine controller from parallel individual control sequences derived in the global parallelization process. Experimental results prove that the global parallelization is very effective. >

Vacancy in Si: Successful description within the local-density approximation.

Abstract: We have performed large-scale total-energy electronic-structure calculations within the local-density approximation for the negatively charged vacancy in Si. The obtained Jahn-Teller distortion, the electronic structures, and the hyperfine coupling tensors are in good agreement with the experimental data available, indicating the validity of the one-electron theory, contrary to the prevailing picture based on model calculations.

Pitch pattern generation apparatus

Abstract: A pitch pattern defining intonation for a text-to-speech system is generated in accordance with a part of speech (e.g., noun, verb, adjective, adverb, etc.) of each word which can be determined more accurately than the syntactic structure of a sentence. The pitch pattern is generated in response to the combinations of parts of speech of adjacent words in a sentence based on the fact that any combination in parts of speech of two words at both sides of each word boundary reflects the strength of connection in meaning of the adjacent words.

Digital mobile communications system and method for providing intensity/coverage reference maps using base stations and mobile stations

Abstract: A digital mobile communications system which maintains synchronization among sending data of base stations, measures the time delay of radio waves from mobile stations under control at the control base station and instructs the value of time delay to each mobile station to adjust the timing of sending data of mobile stations to establish synchronization of communications between the base station and the mobile stations, wherein the system comprises means for measuring at mobile stations relative time delays of radio waves from at least one base station other than the control base station with respect to the radio waves from the control base station and periodically reporting the relative time delays to the control base station, and means for calculating at the base station the distances between the mobile station and the base stations and further determining the current geographic positions of the mobile station.

Random access data communication system with slot assignment capability for contending users

Abstract: In a random access communication system comprising a central station and user stations, each user station transmits a packet to the central station on a randomly selected timeslot of a time-division multiple access channel and defines a plurality of "minislots" within the selected timeslot and transmits a burst on one of the minislots. The central station monitors the defined monislots to detect the burst transmission, and assigns as many timeslots as required if more than one burst transmission is detected within a timeslot interval. A slot assignment signal is sent from the central station to the user stations containing a negative acknowledgment of the transmitted packet. The user station from which the packet was transmitted is responsive to the negative acknowledgment to select one of the assigned timeslots and retransmit to the central station a copy of the packet on the selected assigned timeslot.

Thin film capacitor

Abstract: There is provided a thin film capacitor including (a) a semiconductor substrate, (b) an interlayer insulating film formed on the semiconductor substrate, (c) a contact formed throughout the interlayer insulating film such that the contact has an upper surface upwardly projecting, (d) a lower electrode formed on the interlayer insulating film such that the lower electrode covers the upper surface of the contact therewith, (e) a capacitor insulating film covering the lower electrode and the interlayer insulating film therewith, and (f) an upper electrode formed on the capacitor insulating film The thin film capacitor prevents peeling between the contact and the lower electrode even in an annealing step

The transition and autocorrelation structure of tes processes

Abstract: TES (Transform-Expand-Sample) is a versatile class of stochastic sequences which can capture arbitrary marginals and a wide variety of sample path behavior and autocorrelation functions. In TES, the initial variate is uniform on [0,1) and the next variate is obtained recursively by taking the fractional part (i.e., modulo-1 reduction) of a linear autoregressive scheme. We show how this class gives rise to uniform Markovian sequences in a general and natural way, by observing that marginal uniformity is closed under modulo-1 addition of an independent variate with arbitrary distribution. We derive the transition function of TES sequences and the autocovariance function of transformed TES sequences using Fourier and Laplace Transform methods. The autocovariance formulas are amenable to fast and accurate calculation and provide the theoretical basis for a computer-based methodology of heuristic TES modeling of empirical data. A companion paper contains various examples which show the efficacy of the TES appr...

Dynamic channel assignment cordless telecommunication network

Abstract: In a microcellular cordless telephone network, radio speech channels are divided into several channel groups such that no intermodulation products occur among the channels of each group. In each microcell is located a base unit to which one of the channel groups is dynamically assigned. To achieve the channel assignment, each base unit generates priority values of the channel groups as representations of usage of the channel groups, consistently updates them through a statistical process and assigns one of the channel groups having a maximum priority value to the base unit.

Autonomous reuse partitioning in cellular systems

Abstract: A simple, decentralized procedure for adaptive channel allocation, autonomous reuse partitioning (ARP), is proposed. Radio channels are viewed in the same order at any base station, and the first channel that satisfies a carrier-to-interference ratio (CIR) threshold is allocated. ARP easily achieves reuse partitioning, in which channels higher in the order are reused at shorter distance by mobile stations from which stronger signal levels are received. An efficient frequency reuse pattern helps ARP to improve traffic handling capacity while avoiding interference. Performance was evaluated by means of computer simulation in which a system accommodating portable units with 70 channels is assumed. Compared with random channel allocation, ARP doubles traffic handling capacity and decreases the probability of interference by 1/4. Actual transmitter power control is shown to improve capacity by 9%. ARP improves traffic handling capacity at the cost of CIR margin in each channel. This can create problems for mobile stations, such as car-mounted units, which suffer rapid fluctuation in signal levels. >

Modeling and analysis of a variable bit rate video multiplexer

Abstract: The authors develop a new analytic model for a variable bit rate video source, which can model a variety of video applications by a suitable choice of model parameters. Using the model they studied the performance of a video multiplexer that multiplexes several full motion video sources. The video sources were assumed to use a layered coding technique. A service strategy is proposed that guarantees delivery of packets marked essential, whereas nonessential packets may be dropped in the event of congestion. The study showed that with layered coding, good tail probabilities can be obtained for a given number of sources at the expense of some loss for nonessential packets. The performance of the video multiplexer was investigated when the video sources were correlated, and when independent video sources with different thresholds for essential packet rate were multiplexed together. >