Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 4Citation - Scopus: 5Enhanced Optoelectronic Properties of Magnetron Sputtered Ito/Ag Multilayers by Electro-Annealing(AVS, 2022) Uyanık, Zemzem; Türkoğlu, Fulya; Köseoğlu, Hasan; Ekmekçioğlu, Merve; Ata, Bengü; Demirhan, Yasemin; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiIndium tin oxide/silver/indium tin oxide (ITO/Ag/ITO) multilayers have attracted much attention to fulfill the growing need for high-performance transparent conducting oxide electrodes. To make these transparent multilayers work better, electro-annealing, which is a method of self-heating by electric current, can be effective. Moreover, the effect of current on ITO/Ag/ITO multilayers should be investigated to make sure that electronic devices will be reliable over their lifetime. In this study, ITO/Ag/ITO multilayer electrodes with varying Ag thicknesses were grown by DC magnetron sputtering at room temperature. Structural, optical, and electrical properties of these multilayers were investigated before and after electro-annealing. Measurement results revealed that improved optical transmittance and sheet resistance can be obtained by the optimization of Ag thickness for the as-grown ITO/Ag/ITO layers. The highest figure of merit (FoM) value of 17.37 × 10−3 Ω−1 with optical transmittance of 85.15% in the visible region and sheet resistance of 11.54 Ω/□ was obtained for the Ag thickness of 16.5 nm for as-grown samples. The electro-annealing of as-grown ITO/Ag/ITO multilayers led to improved optical behavior of the multilayer structure over a wide spectral range, especially in the near-infrared range. Electro-annealing also provided an improvement in the crystallinity and sheet resistance of the electrodes. The improvement of the electrical and optical properties of the structure enabled a FoM of 23.07 × 10−3 Ω−1 with the optical transmittance of 86.80% in the visible region and sheet resistance of 10.52 Ω/□. The findings of this work provide proper knowledge of the properties of ITO/Ag/ITO multilayers under electrical current and suggest that the overall performance of the multilayers can be improved by the electro-annealing process.Article Echoes From the Event Horizon of a Superfluid Vortex(IOP Publishing, 2022) Güven, Kaan; Demirkaya, BetülA vortex formed in the superfluid state of a Bose-Einstein condensate may exhibit superradiance a la blackhole for radially propagating acoustic fluctuations. The analogy is usually based on the so-called draining bathtub model of the vortex, in which an event horizon and ergosphere emerges when the radial velocity of the superfluid exceeds the propagation speed of sound in the condensate. The acoustic fluctuations mimic a massless scalar field in the curved Lorentzian space-time of the vortex and are governed by the Klein-Gordon wave equation. One common main approximation is the constant background density of the superfluid even in the presence of the vortex. This sets a constant relativistic sound speed. However, the vortex state solution of the Gross-Pitaevskii equation clearly shows that both the density and the speed of sound vary radially near the vortex core, where the event horizon and thus the superradiance will take place. What changes would this complex interdependence bring to the formulation and to the outcomes of the superradiance based on constant density approximation? Here, we recount this question posed under the guidance of Prof. Tekin Dereli and present recent results. We show that the self-consistent density modifies the amplification dynamics near the event horizon significantly, thereby altering the temporal and spectral fingerprint of the superradiance of the vortex.Article Citation - Scopus: 1Oxidizer Gases Effects on the Diameter-Controlled Synthesis of Carbon Nanotubes(MIM Research Group, 2021) İnce Yardımcı, Atike; Öğütlü, Ahmet Sabri; Öğütlü, DenizIn this study, the influence of the oxidizers on the synthesis of carbon nanotubes by C2H4 decomposition over Fe catalyst has been investigated. CO2, O2, and H2O have been used as oxidizers, and to control catalyst particle formation and their sizes in the pretreatment stage. The same oxidizers have also been used in the growth stage to maintain the catalyst particle size, remove amorphous carbon formation to keep catalyst particle active. The results of scanning electron microscopy indicated that the average diameters of nanotubes decreased from 13.4±1.2 nm to 6.2±0.5 nm and extremely dense nanotubes were obtained when we added a small amount of CO2. Adding O2 extremely decreased the areal carbon nanotube density while widens the diameter distribution. H2O addition resulted in larger average diameters and made the growth strongly pretreatment dependent. Within the parameters tried for catalyst pretreatment and CNT growth processes, CO2 seemed the best choice for a weak oxidizing assistant. The strong dependency of the average diameter on pretreatment conditions indicated that pretreatment is a very important step in deciding the final diameters and their distribution.Article Citation - WoS: 11Citation - Scopus: 13Effect of Cnt Incorporation on Pan/Ppy Nanofibers Synthesized by Electrospinning Method(TÜBİTAK, 2020) İnce Yardımcı, Atike; Tanoğlu, Metin; Yılmaz, Selahattin; Selamet, YusufIn this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H2SO4/HNO3/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.Article Citation - WoS: 12Citation - Scopus: 13Electronic and Magnetic Properties of Graphene Quantum Dots With Two Charged Vacancies(Elsevier, 2020) Kul, Erdoğan Bulut; Polat, Mustafa; Güçlü, Alev DevrimElectronic and magnetic properties of a system of two charged vacancies in hexagonal shaped graphene quantum dots are investigated using a mean-field Hubbard model as a function of the Coulomb potential strength ? of the charge impurities and the distance R between them. For ?=0, the magnetic properties of the vacancies are dictated by Lieb's rules where the opposite (same) sublattice vacancies are coupled antiferromagnetically (ferromagnetically) and exhibit Fermi oscillations. Here, we demonstrate the emergence of a non-magnetic regime within the subcritical region: as the Coulomb potential strength is increased to ??0.1, before reaching the frustrated atomic collapse regime, the magnetization is strongly suppressed and the ground state total spin projection is given by Sz=0 both for opposite and same sublattice vacancy configurations. When long-range electron–electron interactions are included within extended mean-field Hubbard model, the critical value for the frustrated collapse increases from ?cf?0.28 to ?cf?0.36 for R<27Å. © 2020 Elsevier LtdConference Object Citation - WoS: 3Citation - Scopus: 3Photometric Multi-Site Campaign on Massive B Stars in the Open Cluster Chi Persei (ngc 884)(IOP Publishing, 2008) Saesen, S.; Pigulski, A.; Carrier, F.; De Ridder, J.; Aerts, C.; Handler, G.; Drummond, R.; Kalomeni, BelindaIn 2005 a photometric observation campaign started on the open cluster chi Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialkow (Poland), which show seven confirmed beta Cephei stars, four candidate B-type pulsators and other interesting variable stars.Article Citation - WoS: 6Citation - Scopus: 6Experimental and Density Functional Theory Study on Humidity Sensing Properties of Copper Phthalocyanine (cupc)(IOP Publishing, 2019) Farzaneh, Amir; Esrafili, Mehdi D.; Okur, SalihThe quartz crystal microbalance (QCM) technique was applied to investigate humidity sensing properties of a copper phthalocyanine (CuPc) thin film prepared by drop cast method. The humidity adsorption and desorption kinetics of (CuPc) thin film was evaluated. The QCM and electrical measurements results showed that humidity sensing properties of CuPc is very sensitive to humidity changes and reversible adsorption/desorption performance which is an indicative of a good humidity sensor even at room temperature. Reproducible experimental results indicated that CuPc thin films have an abundant potential for humidity sensing applications at ambient temperature. According to the first-principle density functional theory calculations, the promising humidity sensing properties of CuPc can be attributed to the considerable charge transfer from the water molecule into Cu atom.Article Citation - WoS: 5Citation - Scopus: 5Defect Induced Anderson Localization and Magnetization in Graphene Quantum Dots(Elsevier, 2018) Altıntaş, Abdulmenaf; Güçlü, Alev DevrimWe theoretically investigate the effects of atomic defect related short-range disorders and electron-electron interactions on Anderson type localization and the magnetic properties of hexagonal armchair graphene quantum dots using an extended mean-field Hubbard model and wave packet dynamics for the calculation of localization lengths. We observe that randomly distributed defects with concentrations between 1 and 5% of the total number of atoms leads to localization alongside magnetic puddle-like structures. Although the localization lengths are not affected by interactions, staggered magnetism and localization are found to be enhanced if the defects are distributed unevenly between the sublattices of the honeycomb lattice.Article Citation - WoS: 22Citation - Scopus: 24Are There Really Conformal Frames? Uniqueness of Affine Inflation(World Scientific Publishing Co. Pte Ltd, 2018) Azri, HemzaHere, we concisely review the nonminimal coupling dynamics of a single scalar field in the context of purely affine gravity and extend the study to multifield dynamics. The coupling is performed via an affine connection and its associated curvature without referring to any metric tensor. The latter arises a posteriori and it may gain an emergent character like the scale of gravity. What is remarkable in affine gravity is the transition from nonminimal to minimal couplings which is realized by only field redefinition of the scalar fields. Consequently, the inflationary models gain a unique description in this context where the observed parameters, like the scalar tilt and the tensor-to-scalar ratio, are invariant under field reparametrization. Overall, gravity in its affine approach is expected to reveal interesting and rich phenomenology in cosmology and astroparticle physics.Article Citation - WoS: 6Citation - Scopus: 7Thermally and Optically Tunable Sub-Terahertz Superconducting Fishnet Metamaterial(Elsevier Ltd., 2018) Sabah, Cumali; Mulla, Batuhan; Altan, Hakan; Özyüzer, LütfiIn this paper, a novel fishnet metamaterial structure is designed and analyzed under different material combinations and under different active controlling techniques. The results indicate that, the proposed fishnet metamaterial has a single resonance with double negativity at 0.39 THz when quartz substrate and aluminum is utilized in the design. Moreover, when the metallic parts are replaced with YBCO, the proposed design also exhibits double negativity with a stronger resonance and can be used as a switch between the double negative and single negative modes if the temperature is altered. In addition to these, when substrate (quartz) is replaced with MgO, the resonance shifts from 0.39 THz to 0.26 THz and shows double negativity. Moreover, switching properties under illumination can also be obtained when the silicon is utilized in the design (MgO-YBCO combination). According to these results, it is found that, in the case that the conductivity of silicon exceeds a certain value, the character of the resonance changes from double negative to the single negative mode.