Browsing by Subject "Electric field effects"
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Item Open Access Analysis of radome covered circular reflectors by complex source-dual series approach(IEEE, 1994) Oğuzer, T.; Altıntaş, A.; Nosich, A. I.Radiation from a two dimensional reflector antenna covered by a cylindrical radome is analyzed by the complex dual series approach. It is only performed for the electrically polarized incident field. The approach is based on the analytical numerical type regularization technique and not on the moment method. This method gives the exact solution with any desired accuracy and the directivity of the feed antenna can be modelled by using this method. The results can be thought as a reliable data for the validity of approximate solutions. The lossy case and the multilayer radome problems can be solved by the same method.Item Open Access Circular arrays of log-periodic antennas for broadband applications(IEEE, 2006) Ergül, Özgür; Gürel, LeventCircular arrays of log-periodic (LP) antennas are designed for broadband applications. A sophisticated electromagnetic simulation environment involving integral equations and fast solvers is developed to analyze the LP arrays both accurately and efficienuy. The resulting matrix equation obtained by the discretization of the electric field integral equation is solved iteratively via the multilevel fast multipole algorithm (MLFMA). Genetic algorithms interacting with MLFMA is employed to optimize the excitations of the array elements to increase the frequency independence and also to add the beam-steering ability to the arrays.Item Open Access Electric field effects on optical properties of semiconductor nanorods(2009-05) Gurinovich L. I.; Lutich, A. A.; Stupak, A. P.; Prislopskii, S. Va.; Artemyev, M. V.; Rusakov, E. K.; Demir, Hilmi VolkanWe studied electric field effects on optical properties of CdSe/ZnS nanorods integrated in thin films sandwiched between transparent electrodes. It was demonstrated that P-polarized component of the photoluminescence of CdSelZnS nanorods is quenched stronger by external electric field than the S-polarized component. Quantum dots are more sensitive to external electric field than the nanorods. A mechanism of external electric field influence on the luminescence spectrum of semiconductor nanorods is discussed.Item Open Access Electron initiated impact ionization in AlGaN alloys(Institute of Physics, 2002) Bulutay, C.Detailed impact ionization (II) analysis of electrons is presented for AlGaN alloys as a vital resource for solar-blind avalanche photodiode and high power transistor applications. Necessary ingredients for the II characterization are supplied from a recent experiment on the GaN end, and a Keldysh analysis for the AlN end, of the alloy AlGaN. High-field electron dynamics are simulated using an ensemble Monte Carlo framework, accounting for all valleys in the lowest two conduction bands, obtained from accurate empirical pseudopotential band structure computations. The effect of alloy scattering on II is considered and observed to be significant. For any AlxGa1-xN alloy, the electron II coefficients are found to obey the form, A exp(-K/F), for the electric field, F.Item Open Access Electronic structure of the contact between carbon nanotube and metal electrodes(American Institute of Physics, 2003) Dag, S.; Gülseren, O.; Çıracı, Salim; Yildirim, T.Our first-principles study of the contact between a semiconducting single-walled carbon nanotube ~s-SWNT! and metal electrodes shows that the electronic structure and potential depend strongly on the type of metal. The s-SWNT is weakly side-bonded to the gold surface with minute charge rearrangement and remains semiconducting. A finite potential barrier forms at the contact region. In contrast, the molybdenum surface forms strong bonds, resulting in significant charge transfer and metallicity at the contact. The radial deformation of the tube lowers the potential barrier at the contact and increases the state density at the Fermi level.Item Open Access Electronic transport through a kink in an electron waveguide(Institute of Electrical and Electronics Engineers, 1994) Yalabik, M. C.The current-voltage denendence correspondinp to electronic transport through a kink in an electronic waveguide is analyzed. No phase breaking dissipation mechanisms are considered, but the effects of the Coulomb interaction are included through a self consistent approximation. The results indicate very nonlinear transport properties, including negative differential resistance and bistability. © 1994 IEEEItem Open Access Energy-transfer rate in a double-quantum-well system due to Coulomb coupling(Elsevier, 2002) Senger, R. T.; Tanatar, BilalWe study the energy-transfer rate for electrons in a double-quantum-well structure, where the layers are coupled through screened Coulomb interactions. The energy-transfer rate between the layers (similar to the Coulomb drag effect in which the momentum-transfer rate is considered) is calculated as functions of electron densities, interlayer spacing, the temperature difference of the 2DEGs, and the electron drift velocity in the drive layer. We employ the full wave vector and frequency-dependent random-phase approximation at finite temperature to describe the effective interlayer Coulomb interaction. We find that the collective modes (plasmons) of the system play a dominant role in the energy-transfer rates.Item Open Access Guiding and bending of photons via hopping in three-dimensional photonic crystals(IEEE, Piscataway, NJ, United States, 2000) Bayındır, Mehmet; Temelkuran, B.; Özbay, EkmelFor the past decade, photonic crystals, also known as photonic bandgap (PBG) materials, have inspired great interest because of their novel scientific and engineering applications such as the inhibition of spontaneous emission, thresholdless lasers, optical circuits, antennas, waveguides, detectors, fibers, and so on. Creating defect states within the PBG are very important for such applications. Recently, we have reported the eigenmode splitting due to coupling of the localized defects and guiding of the electromagnetic (EM) waves through a periodic arrangement of such defects in three-dimensional (3D) photonic crystals. Although the modes of each cavity were tightly confined at the defect sites, overlapping between the nearest-neighbor modes is enough to provide the propagation of photons via hopping. We report on the observation of guiding and bending of EM wave through evanescent defect modes for three different PBG waveguide structures.Item Open Access Highly directional enhanced radiation from sources embedded inside two and three-dimensional photonic crystals(SPIE, 2005) Çağlayan, Hümeyra; Bulu, İrfan; Özbay, EkmelIn this work, we have experimentally and theoretically studied the emission of radiation from a monopole source embedded in a two and three dimensional photonic crystal. We have demonstrated the enhancement of radiation at the band edges and at the cavity modes including coupled cavity modes. We have shown that the emission of radiation from a source depends on the group velocities of the modes and on the electric field intensities of the modes at the source location. Moreover, we have studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. Our results show that it is possible to obtain highly directive radiation sources operating at the band edge of the photonic crystal.Item Open Access Incomplete LU preconditioning for the electric-field integral equation(2006) Malas, Tahir; Gürel, LeventLinear systems resulting from the electric-field integral equation (EFIE) become ill-conditioned, particularly for large-scale problems. Hence, effective preconditioners should be used to obtain the iterative solution with the multilevel fast multipole algorithm in a reasonable time. In this paper, we show that a threshold-based incomplete LU (ILU) preconditioner, i.e., ILUT, can be used safely for such systems, provided that column pivoting is applied for the stability of the incomplete factors. It is observed that the resulting preconditioner ILUTP reduces the solution times by an order of magnitude, compared to simple Jacobi preconditioner. Moreover, we also use the iterative solution of the near-field system as a preconditioner, and use ILUTP as the preconditioner for the near-field system. This way, the effectiveness of the ILUTP is further improved.Item Open Access Investigation of the inaccuracy of the MFIE discretized with the RWG basis functions(IEEE, 2004-06) Ergül, Özgür; Gürel, LeventThe inaccuracy of the magnetic-field integral equation (MFIE) discretized with the Rao-Wilton-Glisson (RWG) basis functions is discussed. The electric-field integral equation (EFIE) gives accurate results with the usage of RWG basis functions for the conducting surfaces with arbitrary planar triangulations. Two reasons i.e., logarithmic singularity in the field integration and solid angle expression in the MFIE, that lead to inaccuracy of the MFIE, are also discussed. It is found that the current distribution and the radar cross section (RCS) obtained by the MFIE does not perfectly match their counterparts obtained by the EFIE.Item Open Access Magnetic and electric Aharonov-Bohm effects in nanostructures(Elsevier BV, 1996) Kulik, I. O.The paper reviews and extends the magnetic Aharonov-Bohm effect (persistent current, resistance oscillation) in normal-metal rings including spin-independent and spin-dependent hopping, Zeeman splitting, magnetic textures and wheels, ring rotation and weak coupling, as well as the electric Aharonov-Bohm effect ("persistent charge") in small metallic contacts. We then discuss dynamical screening effects in a surface charge in a metal. Energy dissipation due to motion of the surface charge has a singularity at the velocity of motion equal to the phonon propagation velocity. Surface image of an external charge inside the metal is strongly distorted at the velocity of motion larger than the Fermi velocity.Item Open Access Orientational transition and complexation of DNA with anionic membranes: weak and intermediate electrostatic coupling(American Physical Society, 2019) Büyükdağlı, Şahin; Podgornik, R.We characterize the role of charge correlations in the adsorption of a short, rodlike anionic polyelectrolyte onto a similarly charged membrane. Our theory reveals two different mechanisms driving the like-charge polyelectrolyte-membrane complexation: In weakly charged membranes, repulsive polyelectrolyte-membrane interactions lead to the interfacial depletion and a parallel orientation of the polyelectrolyte with respect to the membrane; while in the intermediate membrane charge regime, the interfacial counterion excess gives rise to an attractive “salt-induced” image force. This furthermore results in an orientational transition from a parallel to a perpendicular configuration and a subsequent short-ranged like-charge adsorption of the polyelectrolyte to the substrate. A further increase of the membrane charge engenders a charge inversion, originating from surface-induced ionic correlations, that act as a separate mechanism capable of triggering the like-charge polyelectrolyte-membrane complexation over an extended distance interval from the membrane surface. The emerging picture of this complexation phenomenon identifies the interfacial “salt-induced” image forces as a powerful control mechanism in polyelectrolyte-membrane complexation.Item Open Access Photonic band gap effect and localization in two-dimensional Penrose lattice(IEEE, 2001) Bayındır, Mehmet; Çubukçu, Ertuğrul; Bulu, İrfan; Özbay, EkmelA study of photonic bandgap effect and localization in two-dimensional Penrose lattice was performed. Penrose quasicrystals consisted of dielectric rods and defect characteristics of various inequivalent sites of the crystal were investigated. It was observed that the defect characteristics of quasi periodic photonic crystals were different from the periodic case. Localization properties of the defect modes in quasicrystals depended on the position of the removed rod and different defect frequencies could be obtained by removal of rods from various positions.Item Open Access Thermomagnetic effects of nondegenerate Kane semiconductors under the conditions of mutual electron-phonon drag in high electric and arbitrary magnetic fields(2005) Babaev, M.M.; Gassym, T.M.; Tas, M.; Tomak, M.The thermopower and Nernst-Ettingshausen (NE) effect of nondegenerate Kane semiconductors are investigated by taking into account the electron and phonon heating and their arbitrary mutual drag. The electron spectrum is taken in the Kane two-band form. The electric and magnetic field dependences of the electronic and phonon parts of the thermoelectric and NE coefficients and voltages are obtained in analytical forms. It is shown that the mutual drag of electrons and phonons and degree of nonparabolicity of the electron spectrum strongly influence the thermoelectric and thermomagnetic properties of semiconductors under high electric and magnetic fields. © IOP Publishing Ltd.