Design trade-offs in optoelectronic parallel processing systems using smart-SLMs

Optoelectronic devices with free-space optical interconnections offer new possibilities in massively parallel processing. The trade-offs involved in system design and device selection for optoelectronic implementations are examined. System design trade-offs are approached from algorithmic and technological standpoints. From the algorithmic standpoint, new architectures based on expander graphs, that have been shown to provide low-contention fault-tolerant communication, are discussed. Optoelectronic systems which implement such random graphs can be folded to reduce the hardware cost or unfolded to increase bandwidth. They can also be partially folded by increasing the grain size or by reducing the randomness of the graph topology to reduce the complexity of the interconnection holograms. An optoelectronic and a VLSI implementation of a multistage interconnection network are compared from a technological standpoint. Physical design parameters, such as the chip size or the number of phase levels of the interconnection holograms, are related to the system design metrics such as bandwidth, volume, area and power. It is shown that the optoelectronic implementations have higher performance and are more cost-effective than VLSI implementations. These results are also used to provide general guidelines for device selection in the design of smart pixels/smart spatial light modulators based optoelectronic systems.     

Towards a kinetic theory for fermions with quantum coherence

A new density matrix and corresponding quantum kinetic equations are introduced for fermions undergoing coherent evolution either in time (coherent particle production) or in space (quantum reflection). A central element in our derivation is finding new spectral solutions for the 2-point Green's functions written in the Wigner representation, that are carrying the information of the quantum coherence. Physically observable density matrix is then defined from the bare singular 2-point function by convoluting it with the extrenous information about the state of the system. The formalism is shown to reproduce familiar results from the Dirac equation approach, like Klein problem and nonlocal reflection from a mass wall. The notion of the particle number in the presence of quantum coherence is shown to be particularly transparent in the current picture. We extend the formalism to the case of mixing fields and show how the usual flavour mixing and oscillation of neutrinos emerges again from a singular shell structure. Finally, we show how the formalism can be extended to include decohering interactions.     

Artificial pinning in thick YBCO films: Pinning potential and c-axis correlation

We have introduced artificial pinning centres in thick (>1 μm) YBCO films grown by Pulsed Laser Deposition using substrate decoration, quasi-multilayers, and target doping approaches. We have found that the frequency dependence of critical current density is consistent with a logarithmic dependence of pinning potential on current density. For most of materials used as nano-dots, artificially-induced pinning centres have a larger potential than natural ones. From angle-dependent in-field transport measurements and from Transmission Electron Microscopy we have found evidence of c-axis correlated pinning centres.     

Catalytic dehydrogenation of ethanol into acetaldehyde and isobutanol using mono- and multicomponent copper catalysts

The purpose of this work was to investigate biomass-derived ethanol dehydrogenation into acetaldehyde using several mono- and multicomponent (CuO, ZnO and Cr₂O₃)-containing catalysts supported on industrial size Al₂O₃ beads. The catalysts, prepared with either solution combustion or incipient wetness method, were characterized by using various physico-chemical methods, such as EDXA, SEM-EDXA, TEM, XRD, XPS, pyridine adsorption desorption FTIR, and ζ-potential measurements. The results revealed that the multicomponent catalysts exhibited superior activity compared to the metal oxide catalysts containing only one metal oxide. In addition, the most selective catalyst towards acetaldehyde formation, with 50% selectivity at 55% conversion of ethanol at 300 °C and WHSV 1 h^?1 was CuOCr₂O₃/Al₂O₃ prepared by using the solution combustion method, indicating that this inexpensive and rapid catalyst preparation method is promising for other applications.     

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr(1-x)Ca(x)MnO3 (x = 0.0-0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.     

Circuit lower bounds and linear codes

In 1977, Valiant proposed a graph-theoretical method for proving lower bounds on algebraic circuits with gates computing linear functions. He used this method to reduce the problem of proving lower bounds on circuits with linear gates to proving lower bounds on the rigidity of a matrix, a notion that he introduced in that paper. The largest lower bound for an explicitly given matrix is due to J. Friedman, who proved a lower bound on the rigidity of the generator matrices of error-correcting codes over finite fields. He showed that the proof can be interpreted as a bound on a certain parameter defined for all linear spaces of finite dimension. In this note, we define another parameter that can be used to prove lower bounds on circuits with linear gates. Our parameter may be larger than Friedman’s, and it seems incomparable with rigidity, hence it may be easier to prove a lower bound using this notion. Bibliography: 14 titles. Published in Zapiski Nauchnykh Seminarov POMI, Vol. 316, 2004, pp. 188–204.     

Photoinduced Colossal Magnetoresistance under Substantially Reduced Magnetic Field

The colossal magnetoresistive insulator to metal switching of almost nine orders of magnitude under the significantly reduced magnetic field is achieved by illumination for the low bandwidth manganite thin films. Similarly, by changing the measuring bias voltage through the sample the required magnetic field for insulator–metal transition can be further fine‐tuned. By applying a magnetic field of suitable strength, the samples can also be tuned to be extra sensitive to the illumination having colossal effect on the resistivity at low temperatures. This kind of utilizing of multiple external stimulants, which together change the properties of the material, could have significant impact on the new generation of phase‐change memories working under affordable conditions.     

Peculiarities of the electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration in the two-dimensional channel

The electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration are reported. We have observed anisotropy of the electron concentration and mobility measured at a low magnetic field in the [110] and $\left[ {1\bar 10} \right]$ crystallographic directions. It has been established by analysis of Shubnikov-de Haas oscillations that the conductivity through the two-dimensional electron channel InSb/AlInSb quantum well (QW) does not depend on the crystallographic direction. However, the magnetic-field dependences of the Hall coefficient and resistivity of the structures reveal a strong influence of the crystallographic directions. This has allowed one to conclude that the anisotropy of the electron transport parameters in the QW structures measured at low magnetic fields correspond to parasitic conductivity through the Al_0.09In_0.91Sb buffer layer with two pronounced anisotropic contributions: the influence of metallic In nanoclusters inhomogeneously distributed within the buffer layer and conductivity of the near-interface layer with a high anisotropic density of extended defects.     

Potassium Dodecatungsto Cobaltate Trihydrate: A New Heterogeneous Catalyst for the Synthesis of Homoallylic Alcohols and Amines

A simple and efficient procedure has been developed for one-pot synthesis of homoallylic alcohols and amines in good to excellent yields using a variety of aldehydes, amines, and allyltributylstannane in the presence of a catalytic amount of potassium dodecatungsto cobaltate trihydrate (K₅CoW₁₂O₄₀ · 3H₂O) as a heterogeneous catalyst at room temperature. The catalyst exhibited remarkable reusability.     

Effect of target density on YBCO thin films deposited from nanograined targets

Nanograined YBa₂Cu₃O_6+x${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ (YBCO) targets with grain sizes of 25–45 nm and different densities were prepared with sol–gel method. The properties of the thin films laser deposited from these targets were systematically studied. It was found that most parameters are not clearly affected by the target density, although the surface roughness decreased with target density. A small effect of decreasing J_c$J_{\text{c}}$ with increasing target density was observed, but this can be interpreted as film to film variation. On the other hand, a negative correlation between the accommodation field, B^∗$B^{\ast }$, and zero field J_c$J_{\text{c}}$ was observed and B^∗$B^{\ast }$ was found to increase with increasing number of lattice defects. Thus target density is found not to have a large effects on the superconducting or structural properties of YBCO thin films.