Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu
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Article Citation - WoS: 15Citation - Scopus: 15A density functional study of small Li-B and Li-B-H clusters(Pergamon-elsevier Science Ltd, 2009) Yildirim, E. K.; Guvenc, Z. B.; 1155In this work we present the results of a detailed theoretical research for the small LinBm clusters and their hydrogen storage properties by means of density functional theory. All calculations were performed by using Gaussian03 program. For the optimization of the clusters 6-311++G(d,p) basis set was employed. We have chosen global minimum of B-6 cluster as the starting point and replaced the boron atoms one at a time, in a stepwise manner, with Li atoms. For these new structures we have searched for the stable configurations, and calculated their energies, HOMO-LUMO energy gaps, Raman and infrared data, average boron, and lithium charge distribution, and vibrational frequencies. Our results show that as the number of Li atoms increases, stability of the clusters decreases and they become more reactive. In addition to that, there are significant charge transfers from boron atoms to lithium atoms on average. The hydrogen storage capabilities of the most stable isomers of LinBm and B-6 clusters have also been investigated by the same methods, and the results are compared. The Li3B3 has the highest hydrogen storage capacity among the clusters investigated in this study. Furthermore, formation of hydrogen molecules is observed in the vicinity of the clusters, some of which are attached to the Li atoms. In addition, as the number of hydrogen atoms increases, the boron atoms are separated from the other boron atoms, and they form satellite BHx (x = 3,4) clusters around the center. These are attached to the system by a bridging bond of a hydrogen or a Li atom. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.Article Citation - WoS: 0Citation - Scopus: 0A new approach to the Dirac equation(Elsevier Gmbh, 2019) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiDirac equation is based on a novel method that expresses the roots of a square-root operation in terms of matrices. Instead of this technique we propose an alternative approach. The square-root expression is represented as a hypotenuse and it is written in terms of a trigonometric summation of the remaining two edges of a right-angle triangle. Both of the positive and negative energy terms are taken into account and a new matrix equation is obtained. The plane wave solutions of the new differential equation are studied.Article Citation - WoS: 2Citation - Scopus: 4A new quadrature routine for improper and oscillatory integrals(Elsevier Science inc, 2007) Sermutlu, E.; Sermutlu, Emre; Eyyuboglu, H. T.; 17647; 7688; MatematikIn MATLAB environment, a new quadrature routine based on Gaussian quadrature rule has been developed. Its performance is evaluated for improper integrals, rapidly oscillating functions and other types of functions requiring a. large number of evaluations. This performance is compared against the other quadrature routines written for MATLAB in terms of capability, accuracy and computation time. It is found that our routine rates quite favourably. (c) 2006 Elsevier Inc. All rights reserved.Article Citation - WoS: 0Citation - Scopus: 0A new series solution associated with the quantum harmonic oscillator(Elsevier Gmbh, 2021) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiIn the non-relativistic case, the problem of quantum harmonic oscillator is solved by the aid of the Schro spacing diaeresis dinger equation. An additional term of potential energy that is related with the square of the spatial coordinate is also included in the equation. A second order linear differential equation is obtained in the stationary state. The direct solution gives the wave function in terms of the parabolic cylinder functions. This solution leads to the Hermite polynomials when the quantization of energy is considered. However, the derived expression of the wave function does not yield the correct functions for the limiting case, in which the potential energy is zero. We put forth an alternative solution for the second order linear differential equation. First of all, a new series expression is suggested as an ansatz for the power series solution of differential equations. The coefficients of the series are obtained in terms of recurrence relations. Some computational simulations are given.Article Citation - WoS: 0Citation - Scopus: 0A relativistic electron interacting with electromagnetic waves(Elsevier Gmbh, 2021) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiA relativistic electron, in an electromagnetic wave is studied. The relativistic energy-momentum relation is taken into consideration for the derivation of a wave equation for spin-1/2 particle. The electromagnetic wave is represented by the electric scalar and magnetic vector potentials. Thus the spin information of the electron is integrated into the relativistic differential equation. A relativistic quantum wave equation is derived by using the derivatives with respect to time and spatial coordinates with respect to the kinetic energy and momentum of the electron. It is shown that the obtained relativistic equation directly reduces to the Pauli equation in the non-relativistic limit.Erratum Citation - WoS: 7Citation - Scopus: 7A simple analytical EAM model for some bcc metals(Elsevier, 2010) Dursun, Ibrahim H.; Guvenc, Ziya B.; Kasap, E.; 47800An analytical embedded atom method which can treat bcc transition metal iron has been developed. In this model, a new potential was presented and a modified term has been introduced to fit the negative Cauchy pressure P-C = (C-12 - C-44)/2 for Fe element. The new model was applied to calculate thermodynamic properties of binary alloys of the bcc transition metals: Fe, V and Cr. The calculated dilute-solution enthalpies and formation enthalpies of random alloys are in good agreement with the results of first principles calculations and that of the thermodynamic calculations. (C) 2009 Elsevier B.V. All rights reserved.Editorial Citation - WoS: 1Citation - Scopus: 1A survey of the new proposal about the photon momentum(Elsevier Gmbh, 2017) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiBahadoran et al. [1] proposed that the new approach to photon momentum, introduced by Umul [2], was not correct, because they obtained a negative energy expression for the absorbed photon. In this comment, we show that this proposal is wrong, since the related authors made a mistake in their derivations. (C) 2017 Elsevier GmbH. All rights reserved.Article Citation - WoS: 3Citation - Scopus: 3A survey on Klein paradox(Elsevier Gmbh, 2019) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiThe solution of the relativistic tunneling problem with the aid of the Klein-Gordon and kinetic energy based wave equations are reviewed. The behaviors of the transmitted and reflected wave functions by the potential barrier are studied for different values of the barrier's energy. The reason of the Klein's paradox is put forth in terms of the structures of the wave equations.Article Citation - WoS: 1Citation - Scopus: 0A uniform function for the diffraction of spherical waves(Elsevier Gmbh, Urban & Fischer verlag, 2017) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiA function which occurs in the scattering problem of the spherical waves by a perfectly conducting half-plane is introduced for constructing the transition functions of the uniform theory of diffraction and uniform asymptotic theory of diffraction. The important properties of the function, related with the geometrical optics and diffracted field components, are derived mathematically. A uniform asymptotic theory of wedge diffraction is introduced based on the new function. The resultant field expressions-are investigated numerically. (C) 2016 Elsevier GmbH. All rights reserved.Article Citation - WoS: 12Citation - Scopus: 17Abstraction-based verification of codiagnosability for discrete event systems(Pergamon-elsevier Science Ltd, 2010) Schmidt, K.In this paper, we investigate the verification of codiagnosability for discrete event systems (DES). That is, it is desired to ascertain if the occurrence of system faults can be detected based on the information of multiple local sites that partially observe the overall DES. As an improvement of existing codiagnosability tests that resort to the original DES with a potentially computationally infeasible state space, we propose a method that employs an abstracted system model on a smaller state space for the codiagnosability verification. Furthermore, we show that this abstraction can be computed without explicitly evaluating the state space of the original model in the practical case where the DES is composed of multiple subsystems. (c) 2010 Elsevier Ltd. All rights reserved.Article Citation - WoS: 110Citation - Scopus: 112Active laser radar systems with stochastic electromagnetic beams in turbulent atmosphere(Optica Publishing Group, 2008) Cai, Yangjian; Korotkova, Olga; Eyyuboglu, Halil T.; Baykal, Yahya; 7688; 7812Propagation of stochastic electromagnetic beams through paraxial ABCD optical systems operating through turbulent atmosphere is investigated with the help of the ABCD matrices and the generalized Huygens-Fresnel integral. In particular, the analytic formula is derived for the cross-spectral density matrix of an electromagnetic Gaussian Schell-model (EGSM) beam. We applied our analysis for the ABCD system with a single lens located on the propagation path, representing, in a particular case, the unfolded double-pass propagation scenario of active laser radar. Through a number of numerical examples we investigated the effect of local turbulence strength and lens' parameters on spectral, coherence and polarization properties of the EGSM beam. (C) 2008 Optical Society of AmericaArticle Citation - WoS: 1Citation - Scopus: 1Airy-type relativistic matter wave(Elsevier Gmbh, 2021) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 4269; Elektronik ve Haberleşme MühendisliğiA new relativistic Airy-type matter wave is introduced as a solution of the kinetic energy based wave equation. The parametric solution of the related differential equation is obtained. The total energy and momentum of the relativistic particle are derived by using a Bohmian type of decomposition of the kinetic energy based equation. The acceleration of the particle is also evaluated. The behavior of the matter wave is investigated numerically.Article Citation - WoS: 4Citation - Scopus: 4An analytical solution for the plane wave diffraction by a resistive half-plane residing at the interface of left and right handed media(Elsevier Gmbh, 2019) Basdemir, Husnu Deniz; Başdemir, Hüsnü Deniz; 20399; Elektronik ve Haberleşme MühendisliğiThe diffraction of waves by a resistive half-plane lying on the interface of left and right handed media is investigated in this study. Because of the anomalous refraction property of left handed media, shadow boundary shifts toward to the lower surface of the half-plane. In order to obtain the shifted boundary, diffracted waves are divided to subfields. Thus, obtained diffracted waves compensates the deficiency of geometries optics fields at the transition regions. The uniform version of waves is obtained by using the uniform theory of diffraction. The wave behaviors are examined numerically.Article Citation - WoS: 12Citation - Scopus: 13An online adaptive cooperation scheme for spectrum sensing based on a second-order statistical method(Ieee-inst Electrical Electronics Engineers inc, 2012) Yarkan, Serhan; Töreyin, Behçet Uğur; Toreyin, Behcet Ugur; Qaraqe, Khalid A.; Cetin, A. Enis; 19325; Elektrik-Elektronik MühendisliğiSpectrum sensing is one of the most important features of cognitive radio (CR) systems. Although spectrum sensing can be performed by a single CR, it is shown in the literature that cooperative techniques, including multiple CRs/sensors, improve the performance and reliability of spectrum sensing. Existing cooperation techniques usually assume a static communication scenario between the unknown source and sensors along with a fixed propagation environment class. In this paper, an online adaptive cooperation scheme is proposed for spectrum sensing to maintain the level of sensing reliability and performance under changing channel and environmental conditions. Each cooperating sensor analyzes second-order statistics of the received signal, which undergoes both correlated fast and slow fading. Autocorrelation estimation data from sensors are fused together by an adaptive weighted linear combination at the fusion center. Weight update operation is performed online through the use of orthogonal projection onto convex sets. Numerical results show that the performance of the proposed scheme is maintained for dynamically changing characteristics of the channel between an unknown source and sensors, even under different physical propagation environments. In addition, it is shown that the proposed cooperative scheme, which is based on second-order detectors, yields better results compared with the same fusion mechanism that is based on conventional energy detectors.Article Citation - WoS: 183Citation - Scopus: 193Analysis of reciprocity of cos-Gaussian and cosh-Gaussian laser beams in a turbulent atmosphere(Optical Soc Amer, 2004) Eyyuboglu, HT; Baykal, Y; 7688; 7812In a turbulent atmosphere, starting with a cos-Gaussian excitation at the source plane, the average intensity profile at the receiver plane is formulated. This average intensity profile is evaluated against the variations of link lengths, turbulence levels, two frequently used free-space optics wavelengths, and beam displacement parameters. We show that a cos-Gaussian beam, following a natural diffraction, is eventually transformed into a cosh-Gaussian beam. Combining our earlier results with the current findings, we conclude that cos-Gaussian and cosh-Gaussian beams act in a reciprocal manner after propagation in turbulence. The rates (paces) of conversion in the two directions are not the same. Although the conversion of cos-Gaussian beams to cosh-Gaussian beams can happen over a wide range of turbulence levels (low to moderate to high), the conversion of cosh-Gaussian beams to cos-Gaussian beams is pronounced under relatively stronger turbulence conditions. Source and propagation parameters that affect this reciprocity have been analyzed. (C) 2004 Optical Society of America.Article Citation - WoS: 4Citation - Scopus: 5Application of the complex point source method to the Schrodinger equation(Elsevier Sci Ltd, 2010) Umul, Yusuf Z.; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiThe paraxial wave equation is a reduced form of the Helmholtz equation. Its solutions can be directly obtained from the solutions of the Helmholtz equation by using the method of complex point source. We applied the same logic to quantum mechanics, because the Schrodinger equation is parabolic in nature as the paraxial wave equation. We defined a differential equation, which is analogous to the Helmholtz equation for quantum mechanics and derived the solutions of the Schrodinger equation by taking into account the solutions of this equation with the method of complex point source. The method is applied to the problem of diffraction of matter waves by a shutter. (C) 2010 Elsevier Ltd. All rights reserved.Article Citation - WoS: 1Citation - Scopus: 1Application of the Method of Transition Boundary To the Half-Planes With Mixed Boundary Conditions(Elsevier Gmbh, 2020) Umul, Yusuf Ziya; Umul, Yusuf Ziya; 42699; Elektronik ve Haberleşme MühendisliğiThe scattering problems of waves by soft-hard and hard-soft half-planes are investigated by the method of transition boundary. The functional values at the transition boundaries are found to be identical to the ones for the soft and hard half-screens. It is shown that the factorization process should be applied by also taking into account the boundary conditions besides the principle of reciprocity. The exact diffracted field expressions are obtained for both of the half-planes.Article Citation - WoS: 53Citation - Scopus: 58Average intensity and spreading of an elegant Hermite-Gaussian beam in turbulent atmosphere(Optical Soc Amer, 2009) Yuan, Yangsheng; Cai, Yangjian; Qu, Jun; Eyyuboglu, Halil T.; Baykal, Yahya; 7688; 7812The propagation of an elegant Hermite-Gaussian beam (EHGB) in turbulent atmosphere is investigated. Analytical propagation formulae for the average intensity and effective beam size of an EHGB in turbulent atmosphere are derived based on the extended Huygens-Fresnel integral. The corresponding results of a standard Hermite-Gaussian beam (SHGB) in turbulent atmosphere are also derived for the convenience of comparison. The intensity and spreading properties of EHGBs and SHGBs in turbulent atmosphere are studied numerically and comparatively. It is found that the propagation properties of EHGBs and SHGBs are much different from their properties in free space, and the EHGB and SHGB with higher orders are less affected by the turbulence. What's more, the SHGB spreads more rapidly than the EHGB in turbulent atmosphere under the same conditions. Our results will be useful in long-distance free-space optical communications. (C) 2009 Optical Society of AmericaArticle Citation - WoS: 112Citation - Scopus: 115Average irradiance and polarization properties of a radially or azimuthally polarized beam in a turbulent atmosphere(Optical Soc Amer, 2008) Cai, Yangjian; Lin, Qiang; Eyyuboglu, Halil T.; Baykal, Yahya; 7688; 7812Analytical formulas are derived for the average irradiance and the degree of polarization of a radially or azimuthally polarized doughnut beam (PDB) propagating in a turbulent atmosphere by adopting a beam coherence-polarization matrix. It is found that the radial or azimuthal polarization structure of a radially or azimuthally PDB will be destroyed (i.e., a radially or azimuthally PDB is depolarized and becomes a partially polarized beam) and the doughnut beam spot becomes a circularly Gaussian beam spot during propagation in a turbulent atmosphere. The propagation properties are closely related to the parameters of the beam and the structure constant of the atmospheric turbulence. (C) 2008 Optical Society of America.Article Citation - WoS: 12Citation - Scopus: 12Averaging of receiver aperture for flat-topped incidence(Elsevier Sci Ltd, 2013) Kamacioglu, Canan; Baykal, Yahya; Yazgan, Erdem; 7812Using a flat-topped profile for the incident beam, the power scintillation index for weak atmospheric turbulence is formulated and analytically evaluated. Through the use of the aperture averaging factor, the averaging effect of the finite receiver aperture on the intensity fluctuations for a flat-topped incident beam is examined. The influence of the order of flatness on the averaging is investigated. At large propagation lengths, increasing the flatness parameter decreases the power scintillations and it is possible to further reduce the scintillation by increasing the receiver aperture. Increasing the structure constant increases this effect. (C) 2013 Elsevier Ltd. All rights reserved.
