Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Conference Object Investigation of In-Gap Field Enhancement at Terahertz Frequencies for a Metasurface Enhanced Sensor(Institute of Electrical and Electronics Engineers, 2021) Tugay, Halime; Altan, Hakan; Demirhan, Yasemin; Özyüzer, Lütfi; Sabah, CumaliThe arrangements of subwavelength inclusions in a metasurface can serve as an effective absorber for the terahertz region. When such an absorber is combined with a unique material, the absorption can induce effects that can lead to a change in the materials electrical properties. Vanadium dioxide shows a passive and reversible change from monoclinic insulator phase to metallic tetragonal rutile structure by using external stimuli such as temperature (340K), photo excitation, electric field, mechanical strain or magnetic field [1,2]. Upon absorption of the THz radiation, the high electric fields that are generated inside the gaps of the metasurface can serve as trigger points, as was shown previously using kV strength THz E-fields.Other Citation - WoS: 1Türkiye’de Lisansüstü Eğitim Üzerine Gözlem ve Öneriler: Inter-university Modeli(2021) Erdem, RecaiTürkiye'deki henüz gelişimini tamamlamamış üniversitelerde de bilimsel ve akademik yeterlilikleri üst düzeyde öğretim üyeleri bulunabilmektedir. Fakat söz konusu üniversitelerdeki hem toplam öğretim üyesi sayısının hem de uluslararası düzeydeki öğretim üyesi oranının düşüklüğü nedeniyle bu üniversitelerdeki lisansüstü eğitiminin yeterli düzeyde olup olmadığı konusu zaman zaman tartışılabilmektedir. Öte yandan, uluslararası düzeydeki öğretim üyesi sayımızın ihtiyacımızın çok gerisinde olması nedeniyle henüz gelişimini tamamlamamış üniversitelerdeki kaliteli öğretim üyesi potansiyelini etkin bir şekilde kullanmamız gerekmektedir. Bu yazıda, üniversitelerimizdeki, özellikle de gelişme sürecindeki üniversitelerimizdeki öğretim üyesi potansiyelini lisansüstü eğitimin kalitesini artırmada etkin bir şekilde kullanmayı amaçlayan inter-university modeli tartışılmaya açılmaktadır. Bu bağlamda, bu makalede önce, ABD, Kanada, Hindistan gibi ülkelerde uygulanmakta olan çeşitli inter-university modellerine değinilmiş ve daha sonra ise Türkiye'nin ihtiyaçları ve şartlarına uygun bir inter-university modelinin ana hatları önerilmiştir.Conference Object Native and Light Induced Defect States in Wide Band Gap Hydrogenated Amorphous Silicon-Carbon (a-Si1 : H) Alloy Thin Films(Springer, 1997) Güneş, MehmetIn this study, wide band gap a-Si1-x C-x:H alloy thin films prepared with and without hydrogen diluation of (SiH4, CH4) were characterized using optical absorption, dark conductivity, steady-state photoconductivity, sub-bandgap absorption obtained with both photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP), and electron spin resonance (ESR) techniques. Experimental results of steady-state photoconductivity and sub-bandgap absorption for different generation rates were analyzed using a detailed numerical model based on Simmons-Taylor statistics. The densities, energy location and nature of the native and light induced defect states in diluted and undiluted a-Si1-xCx:H alloy thin films were derived from the best fits to the experimental data. The extracted parameters for defect states were compared with those of a-Si:H films both in the annealed and light degraded states.Conference Object Can Cpt Be Violated Through Extended Time Reversal?(World Scientific Publishing, 2001) Erdem, Recai; Ufuktepe, ÜnalWe consider the implications of the extension of time reversal through Wigner types and group extensions. We clarify its physical content and apply the results in a toy model. Finally we point out the possibility of violation of CPT in this framework.Conference Object Probing the Density of States of High Temperature Superconductors With Point Contact Tunneling Spectroscopy(Springer Verlag, 2005) Özyüzer, Lütfi; Zasadzinski, John F.; Miyakawa, Nobuaki; Gray, Kenneth E.Tunneling spectroscopy measurements are performed on single crystals of single CuO2 layer Tl2Ba2CuO6+delta, double CuO2 layer Bi2Sr2CaCu2O8+delta (Bi2212) and polycrystal quadruple CuO2 layer CuBa2Ca3Cu4O12+delta using the point contact tunneling technique. I-V and dI/dV-V characteristics are obtained at 4.2 K. In spite of different number of layers and T-c values, all three high-T-c superconductors exhibit similar spectral features including dip and hump features reminiscent of strong-coupling effects in conventional superconductors. The doping dependence of Bi2212 is studied and several effects of the hole concentration on spectral features are found. A novel effect is that the energy gap increases in the underdoped region even as T-c decreases. Combining the doping dependence of the energy gap and the dip energy provides additional information in order to understand the mechanism of high-T-c superconductivity. Point contact tunneling studies of the doping dependence of the energy gap in Bi2212 also helped to understand local variations of the gap magnitude observed by scanning tunneling microscopy, indicating that this type of spectroscopy is an integral part of the tunneling technique.Conference Object Reactive Ion Beam Etching of Superconducting Bi2212 by Ta/Pr and Pr'/ta/pr Masks for the Generation of Thz Waves(Institute of Electrical and Electronics Engineers Inc., 2009) Köseoğlu, Hasan; Türkoğlu, Fulya; Demirhan, Yasemin; Meriç Polster, Zeynep; Özyüzer, LütfiGeneration of powerful THz radiation from intrinsic Josephson Junctions (Ills) of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) may require mesas with large lateral dimension. However, there are difficulties in fabrication of perfect rectangular mesas. Mesa lateral angles should be close to 90 degrees to obtain IJJs with same planar dimensions for synchronization of IJJs. Since thick photoresist (PR) layer shades the lateral dimension of mesa during ion beam etching, we patterned Ta/PR and PR'/Ta/PR masks on Bi2212 and used selective ion etching to overcome this problem. The reactive ion beam etchings have done with ion beam of Ar, N(2) and O(2) and we have obtained mesas about 1 mu m with lateral angle of approximately 50 to 75 degrees which is better than the mesas fabricated with single layer mask.Article Citation - WoS: 13Noise in Hydrogenated Amorphous Silicon(Institute of Electrical and Electronics Engineers, 2002) Johanson, Robert E.; Güneş, Mehmet; Kasap, Safa O.Published work on conductance fluctuations in hydrogenated amorphous silicon is surveyed. There are many reports of 1/f noise. some describing unusual features Such as non-Gaussian statistics. The relative insensitivity to doping and temperature is highlighted. In addition to the 1/f noise. random-telegraph-like noise is often reported. The successes and failures of generation-recombination models for 1 f noise and current filament models for the telegraph noise are summarised.Conference Object Terahertz Transmission Through Patterened Vanadium Oxide Thin Films on Dielectric Substrates(SCITEPRESS, 2017) Akkaya, M.; Demirhan, Yasemin; Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, Lütfi; Sabah, Cumali; Altan, HakanPatterned and unpatterned films of vanadium oxide grown on dielectric substrates such as fused silica and sapphire were grown and analysed by varying the temperature using terahertz time domain spectroscopy. After investigating the critical transition temperature near 340K, a well-known cross-shaped pattern was studied to observe any resonances upon transmission. Due to the poor conductivity of the films the frequency selective nature of the structure was not observed, however an etalon effect could be seen in the sapphire substrate as opposed to the fused silica substrate above the critical temperature. Dependence of the refractive index difference between substrates upon transmission of the THz pulse is likely in explaining this observed difference.Book Part Future Applications of Artificially-Synthesized Organic Molecules Containing Transition-Metal Atoms(Elsevier, 2018) Mayda, Selma; Kandemir, Zafer; Bulut, NejatArtificially-synthesized organic molecules which contain transition-metal atoms offer new possibilities for applications in the electronics, pharmaceutical, and chemical industries. Hence, developing an understanding of the electronic properties of this kind of organic molecules is important. With this purpose, here we study the electronic properties of metalloproteins, metalloenzymes, and Ru-based dye molecules as examples for this kind of organic molecules. In particular, we perform combined Hartree-Fock (HF) and quantum Monte Carlo (HF+QMC) calculations, as well as combined density functional theory (DFT) and QMC (DFT+QMC) calculations to study the electronic properties of these molecules. Our results show that new electronic states named as impurity bound states (IBS) form in metalloproteins, metalloenzymes, and Ru-based dye molecules. We show that the electron occupancy of IBS is critically important in determining the low-energy electronic properties of these molecules. In this respect, the IBS may play a central role in developing new applications based on artificially-synthesized organic molecules containing transition-metal atoms. © 2018 Elsevier Inc. All rights reserved.Conference Object Stress-Energy Connection: Degravitating the Vacuum Energy(World Scientific Publishing Co. Pte Ltd, 2013) Demir, Durmuş AliThis talk summarizes recent studies on the gravitational properties of vacuum energy in a non-Riemannian geometry formed by the stress-energy tensor of vacuum, matter and radiation. Postulating that the gravitational effects of matter and radiation can be formulated by an appropriate modification of the spacetime connection, we obtain varied geometro-dynamical equations which properly comprise the usual gravitational field equations with, however, Planck-suppressed, non-local, higher-dimensional additional terms. The prime novelty brought about by the formalism is that, the vacuum energy does act not as the cosmological constant but as the source of the gravitational constant. The formalism thus deafens the cosmological constant problem by channeling vacuum energy to gravitational constant. Nevertheless, quantum gravitational effects, if any, restore the problem via the graviton and graviton-matter loops, and the mechanism proposed here falls short of taming such contributions to cosmological constant.