Semifield planes of order p 4 and kernel GF(p 2)

In this article we determine the number of non-isomorphic semifield planes of order p⁴ and kernel GF(p²) for p prime, 3 ≤ p ≤ 11. We show that for each of these values of p, the plane is either desarguesian, p-primitive, or a generalized twisted field plane. We also show that the class of p-primitive planes is the largest. We also discuss the autotopism group of the semifields under study.     

Photoluminescence properties of SiNx/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH₂Cl₂/H₂/NH₃ plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV-VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N-H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN_x interfaces.     

GaN quantum dots by molecular beam epitaxy

The conditions to grow GaN quantum dots (QDs) by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value, the growth mode of GaN deposited on AlN can be either of the Stranski–Krastanow (SK) or of the Frank–Van der Merwe type. Accordingly, quantum wells or QDs can be grown, depending on the desired application. In the particular case of modified SK growth mode, it will be shown that both plastic and elastic strain relaxation can coexist. Growth of GaN QDs with N-polarity will also be discussed and compared to their counterpart with Ga polarity.     

Selective Electrochemical Determination of Uric Acid in the Presence of Ascorbic Acid Using a Carbon Paste Electrode Modified with β‐Cyclodextrin

Uric acid (UA) was determined in the presence of ascorbic acid (AA) by using a carbon paste electrode modified superficially by a β‐cyclodextrin film (CPE/β‐CD). The surface carbon paste electrode was prepared applying a 30 cycles potential program and using a 1 M HClO₄+0.01 M β‐CD electrolytic solution. The UA and AA solutions were used to evaluate the electrode selectivity and sensitivity by cyclic voltammetric and amperometric methods. In these experiments the detection limit for UA was (4.6±0.01)×10^?6 M and the RSD calculated from the amperometric curves was 10%. From the data obtained it was possible to quantify UA in the urine and saliva samples. Selective detection of UA was improved by formation of an inclusion complex between β‐CD and UA. The results show that the CPE/β‐CD is a good candidate due to its selectivity and sensitivity in the UA determination in complex samples like the biological fluids.     

Molecular modeling and mutagenesis reveals a tetradentate binding site for Zn2+ in GABA(A) alphabeta receptors and provides a structural basis for the modulating effect of the gamma subunit

Gamma-aminobutyric acid type A receptors (GABA(A)-R) containing alpha1beta2gamma2 subunits are weakly inhibited by Zn2+, whereas receptors containing only the alpha1beta2 subunits are strongly inhibited. We built homology models of the ion pores of alpha1beta2 and alpha1beta2gamma2 GABA(A)-R using coordinates of the nicotinic acetylcholine receptor as a template. Threading the GABA(A)-R beta2 sequence onto this template placed the 17' histidine and the 20' glutamate residues at adjacent locations in the mouth of the pore, such that a nearly ideal tetradentate site for Zn2+ was formed from two histidine and two glutamate residues between adjacent beta subunits in the alpha1beta2 GABA(A)-R. Following optimization with CHARMM, the distance between the alpha-carbons of the adjacent histidine residues was approximately 9.2 A, close to the ideal distance for a Zn2+ binding site. Loss of inhibition by Zn2+ in alpha1beta2gamma2 GABA(A)-R can be explained by the geometry of these residues in the arrangement alpha1beta2gamma2alpha1beta2, in which the nearest C-alpha-C-alpha distance between the histidine residues is 15.5 A, too far apart for an energetically optimal Zn2+ binding site. We then mutated the gamma subunit at the 17' and/or 20' positions. Zn2+ inhibition was not restored in alpha1beta2gamma2 (I282H) receptors. A novel finding is that the modeling shows the native 20' lysine in gamma2 can compete with Zn2+ for binding to the inserted 17' histidine. Sensitivity to Zn2+ was restored in the double mutant receptor, alpha1beta2gamma2 (I282H; K285E), in which the competition with lysine was removed and a more favorable Zn2+ binding site was formed.     

Novel proton-conducting polymer inclusion membranes

The preparation and characterization of new polymer inclusion membranes (PIMs) for proton transport is described. PIMs were prepared with different polymeric cellulose-based compounds and PVC as supports, tris(2-butoxyethyl)phosphate (TBEP) and 2-nitrophenyl octyl ether (NPOE) as plasticizers and dinonylnaphthalenesulfonic acid (DNSA) and dinonylnaphthalenedisulfonic acid (DNDSA) as carriers. The effects of the nature and content of the supports, plasticizers and carriers on membrane proton conductivity was studied using electrochemical impedance spectroscopy (EIS). This technique was also used to evaluate the chemical stability of a CTA–NPOE–DNDSA membrane while its selectivity was monitored with respect to sodium and calcium ions through counter-transport experiments. DSC and TGA techniques were used to determine the thermal stability of these membranes. A PIM based on CTA–DNDSA–NPOE showed the highest proton conductivity (3.5 mS/cm) with no variation of its behavior during 2 months of evaluation. FTIR characterization did not show structural changes of the membrane in this period of time. Thermal analysis indicates that it is stable up to 180 °C. An empirical functional relationship between PIM resistance and composition indicates that increasing plasticizer and carrier concentrations enhances the conductivity of the membranes, while increasing CTA content tends to decrease this property. Transport experiments showed a good selectivity of the CTA–DNDSA–NPOE membrane for protons over calcium or sodium ions.     

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10¹⁹ cm⁻³. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.     

Some AB-percolation problems in the antiferromagnetic potts model

The AB-correlated-site/random-bond percolation problem in a q-state antiferromagnetic Potts model on Bethe lattices is solved. We find the analytic expression of the AB-percolation characteristic functions in terms of the temperature, the external field and the active bond concentration p_B. The AB-threshold and the phase boundary of the system coincide at zero temperature and at most in two other points for every constant p_B > 1?σ. The properties of the Bethe lattice allow us to find the temperature dependent p_B which defines the AB-droplets, i.e. those special AB-clusters which diverge with thermal exponents along the phase boundary.     

Nature-Inspired Routing and Wavelength Assignment Algorithms for Optical Circuit-Switched Polymorphic Networks

Polymorphic optical networks simultaneously support several optical switching paradigms over a single physical network. In this way, they provide service differentiation at the optical layer by employing the most appropriate paradigm for each service. One type of such architecture is the optical circuit-switched polymorphic network (OCSPN), which combines optical circuit switching paradigms with different grades of dynamism. The performance of this network relies on the utilization of efficient routing and wavelength assignment (RWA) algorithms. In this article, we review the fundamentals of OCSPNs and present a set of efficient RWA algorithms based on ant colony optimization and genetic algorithms.     

A family of codes between some Markov and Bernoulli schemes

We construct a family of almost continuous codes between a mixing one-step Markov process with two symbols and a Bernoulli scheme.