WoS İndeksli Yayınlar Koleksiyonu / WoS Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7150
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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 Spin Polarized Tunneling in Large Area Mesas of Superconducting Bi 2sr2cacu2o8+δ for the Generation of the Thz Waves(Institute of Electrical and Electronics Engineers Inc., 2009) Türkoğlu, Fulya; Şimşek, Yılmaz; Köseoğlu, Hasan; Demirhan, Yasemin; Meriç Polster, Zeynep; Özyüzer, LütfiRectangular intrinsic Josephson junction mesa structures of superconducting Bi2Sr2CaCu2O8+delta (Bi2212) can be used as a source of continuous, coherent and polarized terahertz (THz) radiation. THz emitting mesas are below a certain underdoped level of Bi2212. They have small Josephson critical current in contrast to optimally doped and overdoped Bi2212. We deposited Au/Co/Au multilayer top of mesa and obtained small critical current from mesas fabricated by as-grown Bi2212 single crystals due to injection of spin polarized current. The spin injection eliminates adjustment of doping level for successful THz emission.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.Conference Object Characterization of Vo2 Films Grown by Magnetron Sputtering(Institute of Electrical and Electronics Engineers Inc., 2015) Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, Lütfi; Köklü, Mehtap[No abstract available]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.Article Citation - WoS: 33Citation - Scopus: 33Characterization of Thin Film Li0.5la0.5ti1-Xalxo3 Electrolyte for All-Solid Li-Ion Batteries(Elsevier, 2018) Ulusoy, Seda; Gülen, Sena; Aygün, Gülnur; Özyüzer, Lütfi; Özdemir, MehtapSince addition of Al in Li0.5La0.5TiO3 has enhanced ionic conductivity in bulk materials, it is important to apply this material on all solid state thin film batteries. Because some of the good ionic conductors such as Lithium Phosphorus Oxynitride (LiPON) are sensitive to oxygen and moisture and their application is limited, so amorphous Li0.5La0.5Ti1−xAlxO3 (LLTAlO) is a most promising candidate because of its stability. In this study, the crystalline LLTAlO targets were prepared changing the amount of x content by conventional solid state reactions. Using these targets, lithium lanthanum titanium oxide (LLTO) thin film electrolytes were deposited on ITO/SLG substrates by radio frequency magnetron sputtering system in Ar atmosphere. The structural and compositional properties of targets and thin films were characterized by SEM, XRD, Raman spectroscopy and XPS. It was found that all targets are crystalline while the thin films are amorphous. To understand the effect of Al doping on ionic conductivity, electrical measurements were done at room temperature by AC impedance spectroscopy forming ITO/LLTAlO/Al structure like capacitor. Highest ionic conductivity result, 0.96 × 10−6 S·cm−1, is obtained from the nominal thin film composition of Li0.5La0.5Ti1−xAlxO3 (x = 0.05) at room temperature measurements. Heat treatment is also conducted to investigate to understand its effect on ionic conductivity and the structure of the thin films. It is found that ionic conductivity enhances with annealing. Also, temperature dependent ionic conductivity measurements from 298 K to 385 K are taken in order to evaluate activation energy for Li-ion conduction.Article Citation - WoS: 56Citation - Scopus: 62Cross-like terahertz metamaterial absorber for sensing applications(Springer Verlag, 2018) Sabah, Cumali; Mulla, Batuhan; Altan, Hakan; Özyüzer, LütfiIn this work, a new multiband terahertz metamaterial absorber is designed and characterised by numerical simulation method. In addition, the utilisation of the proposed absorber as a sensor is also investigated. The dielectric and thickness sensing characteristics are analysed. The proposed multiband metamaterial absorber has the ability for utilising the terahertz region up to 2 THz. According to the results, it is found that the proposed absorber is capable of sensing unknown materials and material thickness with any of its five absorption bands. The sensitivity of the proposed sensor is 6.57 GHz / unit sensitivity for dielectric sensing and 7.66GHz/μm for thickness sensing.Article Citation - WoS: 15Citation - Scopus: 17Effect of Substrate Rotation Speed and Off-Center Deposition on the Structural, Optical, and Electrical Properties of Azo Thin Films Fabricated by Dc Magnetron Sputtering(American Institute of Physics, 2018) Türkoğlu, Fulya; Aygün, Gülnur; Köseoğlu, Hasan; Özdemir, Mehtap; Zeybek, S.; Özyüzer, Lütfi; Özdemir, Mehtap; Özyüzer, Gülnur Aygün; Özyüzer, LütfiIn this study, aluminum-doped zinc oxide (AZO) thin films were deposited by DC magnetron sputtering at room temperature. The distance between the substrate and target axis, and substrate rotation speed were varied to get high quality AZO thin films. The influences of these deposition parameters on the structural, optical, and electrical properties of the fabricated films were investigated by X-ray diffraction (XRD), Raman spectroscopy, spectrophotometry, and four-point probe techniques. The overall analysis revealed that both sample position and substrate rotation speed are effective in changing the optical, structural, and electrical properties of the AZO thin films. We further observed that stress in the films can be significantly reduced by off-center deposition and rotating the sample holder during the deposition. An average transmittance above 85% in the visible range and a resistivity of 2.02 × 10-3Ω cm were obtained for the AZO films.Article Citation - WoS: 16Citation - Scopus: 18Study of Undoped and Indium Doped Zno Thin Films Deposited by Sol Gel Method(Springer Verlag, 2018) Medjaldi, M.; Touil, O.; Boudine, B.; Zaabat, M.; Halimi, O.; Sebais, M.; Özyüzer, LütfiIn this paper, we report the effects of Indium doping concentrations (from 0 to 10wt%) on the structural, morphological, and optical properties of deposited In doped ZnO (IZO) thin films prepared by the sol–gel method through the dip coating technique. X-ray diffraction (XRD) analysis indicates that all ZnO thin films have a polycrystalline nature with a hexagonal wurtzite phase with (002) as a preferential orientation. XRD results demonstrate that the particle size of ZnO decreased with the increase in Indium concentrations. Raman scattering spectra confirmed the wurtzite phase and the presence of intrinsic defects in our samples. Energy dispersive spectroscopy (EDS) and the X-ray photoelectron spectroscopy (XPS) measurements, confirmed the presence of zinc, oxygen and indium elements which is in agreement with XPS results. The photoluminescence (PL) spectra of the films exhibit defects-related visible emission peaks, with intensities differing owing to different concentrations of zinc vacancies. UV–Vis spectrometer measurements show that all the films are highly transparent in the visible wavelength region (≥ 70%) and presented two different absorption edges at about 3.21 eV and 3.7 eV, these may be correspond to the band gap of zinc oxide and indium oxide respectively.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.