YÖK Akademik
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Publication One Dof Thumb Mechanism and It's Dimensional Optimization(International Journal of Multidisciplinary Studies and Innovative Technologies, 2019-01-01) Yavuz, SametThis paper involves the geometrical representation, kinematic solution and dimensional optimization of a one degree of freedom parallel thumb mechanism. The aim of the designing of this mechanism grasping objects by mimicking of a human thumb. Link length optimization of the mechanism are made around an average human thumb sizes by using Firefly Algorithm which is a nature inspired optimization algorithm.Publication Improvement of breakdown characteristics in AlGaN/GaN/AlxGa1 xN HEMT based on a grading AlxGa1 xN buffer layer(2010-01-01) Kelekçi, ÖzgürTo improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/AlxGa1 xN double hetero structure (DH-HEMTs) were designed and fabricated by replacing the semi-insulating GaN buffer with content graded AlxGa1 xN(x¼x1!x2, x1>x2), in turn linearly lowering the Al content x from x1¼90% to x2¼5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive AlxGa1 xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi-insulating GaN buffer, the fabricated DH-HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.Publication Classical Yang-Baxter equation from supergravity(2018-01-01) Bakhmatov, I.; Kelekçi, Özgür; Colgáin, E. Ó; Sheikh-Jabbari, M. M.We promote the open-closed string map, originally formulated by Seiberg & Witten, to a solution generating prescription in generalized supergravity. The approach hinges on a knowledge of an antisymmetric bivector Θ, built from antisymmetric products of Killing vectors, which is specified by the equations of motion. In the cases we study, the equations of motion reproduce the classical Yang-Baxter equation (CYBE) and Θ is the most general r-matrix solution. Our work generalizes Yang-Baxter deformations to non-coset spaces and unlocksgravityas ameanstoclassify r-matrix solutions to the CYBE.Publication Large superconformal near-horizons from M-theory(2016-01-01) Lozano, Y.; Kelekçi, Özgür; Montero, J.; Colgáin, E.Colgâin; Park, M.We report on a classification of supersymmetric solutions to 11D supergravity with SOð2;2Þ × SOð3Þ isometry, which are AdS=CFT dual to 2D CFTs with N ¼ð0;4Þ supersymmetry. We recover the Maldacena, Strominger, Witten near-horizon with small superconformal symmetry and identify a class of AdS3 ×S2 ×S2 ×CY2 geometries with emergent large superconformal symmetry. This exhausts known compact geometries. Compactification of M-theory on CY2 results in a vacuum of 7D supergravity with large superconformal symmetry, providing a candidate near-horizon for an extremal black hole and a potential new setting to address microstates.Publication Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1 xN/AlN/GaN high electron mobility transistors with InGaN back barriers(2011-01-01) Lisesivdin, S.B.; Kelekçi, Özgür; Ozcelik, S.; Ozbay, E.The effects of the In-mole fraction (x) of an InxGa1 xN back barrier layer and the thicknesses of different layers in pseudomorphic AlyGa1 yN/AlN/GaN/InxGa1 xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non linear Schrodinger–Poisson equations. Strain relaxation limits were also calculated for the investigated ¨ AlyGa1 yN barrier layer and InxGa1 xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.Publication Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AlN/GaN heterostructures(2010-01-01) Arslan, Engin; Kelekçi, Özgür; Şafak, Yasemin; Altındal, Şemsettin; Özbay, EkmelThe temperature dependent capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated by considering the series resistance (Rs) effect in the temperature range of 80–390 K. The experimental results show that the values of C and G/x are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point ( 2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/x vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/x values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. Rs values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C–V curves and the increase in Rs with temperature can be explained by the lack of free charge carriers, especially at low temperatures.Publication Anomalous Nernst Effects of [CoSiB/Pt] Multilayer Films(2013-01-01) Lee, H. N.; Kelekçi, Özgür; Kim, T. W.; Noh, H.We report a measurement for the anomalous Nernst effects induced by a temperature gradient in [CoSiB/Pt] multilayer films with perpendicular magnetic anisotropy. The Nernst voltage shows a characteristic hysteresis which reflects the magnetization of the film as in the case of the anomalous Hall effects. With a local heating geometry, we also measure the dependence of the anomalous Nernst voltage on the distance d from the heating element. It is roughly proportional to 1/d1.3, which can be conjectured from the expected temperature gradient along the sample from the heat equation.Publication An alternative IIB embedding of F(4) gauged supergravity(2013-01-01) Jeong, Jaehoon; Kelekçi, Özgür; Colgâin, Eoin ÔThrough the construction of a complete non-linear Kaluza-Klein reduction ansatz from type IIB supergravity to Romans’ F(4) gauged supergravity, we identify a recently discovered supersymmetric AdS6 solution as the IIB uplift of the supersymmetric vacuum of Romans’ theory. We present new IIB uplifts of a number of known solutions of Romans’ theory and comment on supersymmetry in higher-dimensions where it is expected.Publication Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H–SiC(2011-01-01) Caban, Piotr; Kelekçi, Özgür; Strupinski, Wlodek; Szmidt, Jan; Wojcik, Marek; Gaca, Jaroslaw; Caliskan, Deniz; Ozbay, EkmelThe influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4H–SiC by low pressure metalorganic vapor phase epitaxy was studied. It was shown that growth pressure has an impact on the surface roughness of epilayers and their crystal quality. GaN coalescence thicknesses were determined for the investigated growth pressures. The GaN layers were characterized by AFM and HRXRD measurements. HEMT structures were also fabricated and characterized. Among the growth pressures studied, 50, 125 and 200 mbar, 200 mbar was found to be most suitable for GaN/SiC epitaxy.Publication Supersymmetry and non-Abelian T-duality in type II supergravity(2015-01-01) Lozano, Yolanda; Kelekçi, Özgür; Macpherson, Niall T; Colgáin, Eoin ÓWe study the effect of T-duality on supersymmetry in the context of type II supergravity. For both U(1) Abelian and SU(2) non-Abelian T-duality, we demonstrate that the supersymmetry variations after T-duality are related to the variations before T-duality through the Kosmann spinorial Lie derivative, which vanishes when the Killing spinors are independent of the T-duality directions. As a byproduct of our analysis, we present closed expressions for SU(2) T-duality in a class of spacetimes with diagonal Bianchi IX symmetry and comment on specific examples of T-dual geometries, including a novel AdS3 geometry with large = (4, 4) superconformal symmetry.