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91.
Nabil Kahale 《Random Structures and Algorithms》1997,11(4):299-313
We study general geometric techniques for bounding the spectral gap of a reversible Markov chain. We show that the best bound obtainable using these techniques can be computed in polynomial time via semidefinite programming, and is off by at most a factor of order log2n, where n is the number of states. © 1997 John Wiley & Sons, Inc. Random Struct. Alg., 11 , 299–313 (1997) 相似文献
92.
Nabil Kallel Sami Kallel Ahmed Hagaza Mohamed Oumezzine 《Physica B: Condensed Matter》2009,404(2):285-288
Single-phase polycrystalline samples of La0.7Sr0.3Mn1-xCrxO3 with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature TC decreases with increasing x and the maximum magnetic entropy change (−ΔSM) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K. 相似文献
93.
Rongrong Zhang Ahmed Yousif Ghazal Yuzhu Liu Yan Zhang Bifeng Tang 《Optics Communications》2009,282(11):2169-6193
The photodissociation dynamics of iodocyclohexane (C6H11I) at 266 and 277 nm has been investigated by ion velocity imaging technique. The velocity distributions, angular distributions and relative quantum yields are obtained for I (2P3/2) (denoted I) and I (2P1/2) (denoted I*) fragments. The energy partitioning shows that about 70% of the available energy goes into the internal excitation of the photofragments for both dissociation channels. From the angular distributions, we found the value of the anisotropy parameter β for I* at the corresponding excitation wavelength was less than that for I. Based on the measured angular distributions and relative quantum yields, the relative fractions of each excited state to the products are determined. The curve crossing probabilities between the 3Q0 and 1Q1 states are determined 0.503 at 266 nm and 0.443 at 277 nm. 相似文献
94.
Nabil S. Youssef Eman A. El-Zahany Barsoum N. Barsoum Ahmed M. A. El-Seidy 《Transition Metal Chemistry》2009,34(8):905-914
New transition metal complexes of Co(II), Cu(II), Ni(II), and Fe(III) of the ligands 6,6′-(1E,1′E)-(4,5-dimethyl-1,2-phenylene)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one)
H2L1 and 6,6’-(1E,1′E)-cyclohexane-1,2-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(7-hydroxy-5-methoxy-2-methyl-4H-chromen-4-one)
H2L2 have been prepared and characterized using physio-chemical and spectroscopic methods. The results obtained for the complexes
indicated that the geometries of the metal centres are either square planar or octahedral. Cyclopropanation reactions of unactivated
olefins by ethyldiazoacetate (EDA) in the presence of [L1Cu]·H2O, [L2Cu]·2H2O and [L2*Co]·2H2O as catalysts were examined. The results showed that only [L2*Co]·2H2O can act as a catalyst for the cyclopropanation reaction of unactivated olefins with very high selectivity (up to 99% based
on EDA). 相似文献
95.
An integrated guidance and control system has been developed to enable underactuated marine surface vessels to operate autonomously and yield robust tracking performance in spite of significant external disturbances and modeling imprecision. A nonlinear ship model, accounting for all six degrees-of-freedom of the ship, has been used as a test bed to assess the performance of the proposed scheme. The controller combines the advantages of the variable structure systems (VSS) theory with the self-tuning fuzzy logic scheme. It does not require an accurate dynamic model of the ship or the construction of a rule-based expert system. Its asymptotic stability is ensured by knowing the upper bounds on modeling imprecision and external disturbances and by forcing the tuning parameters to satisfy the sliding conditions. The guidance system is based on the concepts of the variable radius line-of-sight (LOS) and the acceptance circle around the waypoints. The current system varies the LOS radius exponentially with the cross track error in order to achieve a fast convergence rate of the ship to its desired trajectory. The simulation results demonstrate the robust tracking characteristic of the integrated guidance and control system in spite of significant modeling uncertainties and environmental disturbances. 相似文献
96.
A large eddy simulation is used to simulate flow and heat transfer in a turbulent plane jet with two distances from the jet-exit
to impingement corresponding to twice and ten times the slot nozzle width. The resolved different unsteady vortex motions
of the jet shear layers are studied and shown to have an important influence on heat transfer at the wall. They are used to
explain existence of the second peak in Nusselt number for the case corresponding to twice the slot nozzle width. The predicted
average surface Nusselt number profiles exhibit good agreement with experiments. 相似文献
97.
Performance enhancement of the power penalty in DWDM FSO communication using DPPM and OOK modulation
Ebrahim Eld. Elsayed Bedir B. Yousif Mahmoud M. Alzalabani 《Optical and Quantum Electronics》2018,50(7):282
This paper presents the design and performance enhancement of the power penalty (PP) in a dense wavelength division multiplexing based on free space optical communication (FSOC) link using digital pulse position modulation (DPPM) and on–off keying (OOK) modulation. Such a system has a high performance, low cost, robust and power efficient, reliable, excessive flexibility, and higher data rate for access networks. The system performance is evaluated for an 8-channel wavelength-division-multiplexing for hybrid fiber FSOC system at 2.5 Gbps on widely accepted modulation schemes under various atmospheric turbulence (AT) regimes conditions. The performance of system is introduced in terms of PP, bit-error rate (BER), transmission distance and the average received optical power. The numerical results shows that the improvement of the PP using DPPM modulation of 0.2–3.0 dB for weak turbulence (WT) regimes for BER of 10?6 and above 20, 25 dB for strong turbulence (ST) regimes are reported for BER of 10?6 and 10?9, as respectively (depending on the AT level). Further, we develop of improvement the PP caused by multiple-access interference about 6.686 dB which is predicted for target BER of 10?9 in WT and 1 dB at target BER of 10?6 in ST when the 8 user are active on the system of optical network units. Additionally, the optical power budget and margin losses of a system are calculated with different link length. The proposed approach of DPPM merges superiority with higher enhancement of PP about 0.8 dB for BER equal 10?9 at FSO link length lfso?=?2000 m compared to OOK at 1 dB for WT. An improvement of 2 dB is observed using the DPPM scheme over an OOK due to capability of detect pulses under background noise conditions with increased receiver sensitivity. 相似文献
98.
Shongwe MS Al-Rahbi SH Al-Azani MA Al-Muharbi AA Al-Mjeni F Matoga D Gismelseed A Al-Omari IA Yousif A Adams H Morris MJ Mikuriya M 《Dalton transactions (Cambridge, England : 2003)》2012,41(8):2500-2514
The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)?(2)T(2) transition curve is sigmoidal with T(?)~280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), M?ssbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state. 相似文献
99.
Sonia Ben Abdelkhalek Nabil Kallel Sami Kallel Octavio Peña Mohamed Oumezzine 《Journal of magnetism and magnetic materials》2012
Critical behavior in the La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 ceramics was studied using magnetization methods. Results show that the paramagnetic–ferromagnetic transition is of second order. Based on the critical behavior analysis using the Banerjee criterion and the Kouvel–Fisher method, we find the critical exponents: β=0.395±0.010, γ=1.402±0.010, and δ=5.208±0.007, for which the magnetic interaction is satisfied within the three-dimensional Heisenberg model. Results indicate the presence of short-range interactions. The magnetic entropy change (−ΔSM) reached maximum values of 1.75, 1.45, 1.15, 0.8 and 0.43 J Kg−1 K−1 under a magnetic field variation of 5, 4, 3, 2 and 1 T, respectively. Nevertheless, these (−ΔSM) values are much low for any potential application at this moment. The nature of this phenomenon is discussed in relation to the characteristics of the magnetic phase transition and critical exponents. 相似文献
100.
Hassan Yousif Ahmed M. Almaleeh Hilal Fadhil S.A. Aljunid Ammar Elyass N.M. Saad 《Optik》2012,123(15):1352-1359
This paper investigates the performance of vector combinatorial (VC) code for spectral amplitude coding optical code-division multiple-access (SAC-OCDMA) system using a newly proposed subtract exclusive OR detection (SED) technique. VC code is constructed using Euclidean vector and algebraic ways. One of the important properties of this code is that the maximum cross correlation is one, which means that multi-user interference (MUI) and phase induced intensity noise (PIIN) are reduced. The weight for the VC code can be any positive integer number. The system performance was evaluated by comparing the theoretical studies using related equations and simulation results taken from the commercial optical systems, “VPI? TransmissionMaker 7.1”. Significant improvement in system performances has been observed when the proposed SED technique employing VC code was applied compared to other codes that utilize conventional balance detection techniques for spectral amplitude coding. 相似文献