Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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.Article Citation - WoS: 2Citation - Scopus: 3Use of Combination of Accelerator-Based Ion-Beam Analysis Techniques To the Investigation of the Corrosion Behavior of Cocrmo Alloy(Elsevier Ltd., 2014) Noli, F.; Misaelides, P.; Lagoyannis, A.; Pichon, L.; Öztürk, OrhanNuclear Reaction Analysis - NRA in combination with d-RBS (Ed: 1.35 MeV) was applied in order to investigate the corrosion behavior of CoCrMo alloy. The corrosion resistance of the alloy was compared to that of modified CoCrMo samples by several techniques as plasma nitriding and oxidizing at moderate temperature (∼400 °C). Electrochemical techniques in simulated body fluid 0.9% NaCl (37 °C) were applied in order to accelerate the corrosion process. The nitrogen depth distribution before and after the corrosion was determined using the 14N(d,α)12C and the 14N(d,p)15N nuclear reactions whereas the oxygen by the 16O(d,p)17O. The surface morphology and microstructure was investigated using microscopy techniques. It was found that surface treatments produce thick nitrided layers (5-6 μm) consisting of a supersaturated nitrogen solution (nitrogen concentration is ∼30 at.%) in the matrix (expanded phase γN) and a thin oxygen solution (0.3 μm). The samples subjected to plasma nitridation and oxidation exhibited the lowest deterioration and better resistance to corrosion compared to the single nitrided or single oxidized and the untreated material. This could be attributed to the modified surface region with the high nitrogen content and the presence of oxygen.Article Citation - WoS: 22Citation - Scopus: 24Preparation of the Ferrocene-Substituted 1,3-Distal P-Tert Based Qcm Sensors Array and Utilization of Its Gas-Sensing Affinities(Elsevier Ltd., 2014) Sayın, Serkan; Özbek, Cebrail; Okur, Salih; Yılmaz, MustafaThe article describes the synthesis of the new ferrocene-substituted calix[4]arene derivative 4 as sensitive layer, and suggests that the immobilization of it is on a quartz crystal microbalance gold electrode via the drop-casting method in order to produce the calix[4]arene-based QCM sensor. In addition, sensor response of the calix[4]arene-based QCM sensor was investigated towards carbon dioxide (CO2) and carbon monoxide (CO). The sensing and selectivity studies show that the ferrocene-substituted calix[4]arene-based QCM sensor is an effective gas sensor toward both carbon dioxide and carbon monoxide.Article Citation - WoS: 12Citation - Scopus: 13Emission of the Thz Waves From Large Area Mesas of Superconducting Bi 2sr2cacu2o8+? by the Injection of Spin Polarized Current(Elsevier Ltd., 2013) Türkoğlu, Fulya; Özyüzer, Lütfi; Köseoğlu, Hasan; Demirhan, Yasemin; Preu, S.; Malzer, S.; Şimşek, Yılmaz; Wang, Huabing; Muller, P.Rectangular Au/Co/Au/Bi2Sr2CaCu2O 8+δ (Bi2212) mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals in order to obtain small critical current from as-grown mesas by the injection of spin polarized current and so eliminate the adjustment of doping level for successful THz emission. We have performed c-axis resistance versus temperature (R-T), current-voltage (I-V) characteristics and bolometer measurements. It is the first time that THz emission has been observed from as-grown mesas due to injection of spin polarized current. © 2013 Elsevier Ltd. All rights reserved.Article Citation - WoS: 31Citation - Scopus: 40Structural and Magnetic Characterization of Plasma Ion Nitrided Layer on 316l Stainless Steel Alloy(Elsevier Ltd., 2009) Öztürk, Orhan; Okur, Salih; Riviere, Jean PaulIn this study, an FeCrNi alloy (316L stainless steel disc) was nitrided in a low-pressure R.F. plasma at 430 °C for 72 min under a gas mixture of 60% N2-40% H2. Structural, compositional and magnetic properties of the plasma nitrided layer was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and magnetic force microscopy (MFM). The magnetic behaviour of the nitrided layer was also investigated with a vibrating sample magnetometer (VSM). Combined X-ray diffraction, cross-sectional SEM, AFM and MFM, as well as VSM analyses provide strong evidence for the formation of the γN phase, [γN-(Fe, Cr, Ni)], with mainly ferromagnetic characteristics. The uniform nature of the γN layer is clearly demonstrated by the XRD, cross-sectional SEM and AFM analyses. Based on the AFM and SEM data, the thickness of the γN layer is found to be ∼6 μm. According to the MFM and VSM analyses, ferromagnetism in the γN layer is revealed by the observation of stripe domain structures and the hysteresis loops. The cross-sectional MFM results demonstrate the ferromagnetic γN phase distributed across the plasma nitrided layer. The MFM images show variation in the size and form of the magnetic domains from one grain to another.Article Citation - WoS: 18Citation - Scopus: 22Microstructural and Mechanical Characterization of Nitrogen Ion Implanted Layer on 316l Stainless Steel(Elsevier Ltd., 2009) Öztürk, OrhanNitrogen ion implantation can be used to improve surface mechanical properties (hardness, wear, friction) of stainless steels by modifying the near-surface layers of these materials. In this study, a medical grade FeCrNi alloy (316L stainless steel plate) was implanted with 85 keV nitrogen ions to a high fluence of 1 × 1018N2+ / cm2 at a substrate temperature <200 °C in an industrial implantation facility. The N implanted layer microstructures, thicknesses and strengths were studied by a combination of X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), atomic force microscopy (AFM) and nanohardness measurements. AFM was also used for the surface roughness analysis of the implanted as well as polished materials. The CEMS analysis indicate that the N implanted layer is ∼200 nm thick and is composed of ε-(Fe,Cr,Ni)2+xN-like nitride phase with mainly paramagnetic characteristics. The nanohardness measurements clearly indicate an enhanced hardness behaviour for the N implanted layer. It is found that the implanted layer hardness is increased by a factor of 1.5 in comparison to that of the substrate material. The increased hardness resulting from nitrogen implantation is attributed to the formation of ε nitride phase.Article Citation - WoS: 12Citation - Scopus: 12Ge Nanocrystals Embedded in Sio2 in Mos Based Radiation Sensors(Elsevier Ltd., 2010) Aktağ, Aliekber; Yılmaz, Ercan; Mogaddam, Nader A.P.; Aygün, Gülnur; Cantaş, Ayten; Turan, RaşitIn this work, the effects of gamma radiation on the Raman spectra of Ge nanocrystals embedded in SiO2 have been investigated. SiO2 films containing nanoparticles of Ge were grown using the r.f.-magnetron sputtering technique. Formation of Ge nanocrystals was observed after high temperature annealing in an inert atmosphere and confirmed by Raman measurements. The intensity of the Raman signal originating from Ge nanocrystals was found to decrease with increasing gamma radiation. The study also includes the gamma radiation effects on MOS structure with Ge nanocrystals embedded in SiO2. The gamma radiation effects from 500 up to 4000 Gray were investigated. Capacitance-voltage measurements were performed and analyzed. Oxide traps and interface trap charges were calculated. Results show that MOS structure with Ge nanocrystals embedded in SiO2 is a good candidate to be used in radiation sensors, especially at high radiation doses. © 2010 Elsevier B.V. All rights reserved.Article Citation - WoS: 150Citation - Scopus: 146Direct Observation of Tetrahertz Electromagnetic Waves Emitted From Intrinsic Josephson Junctions in Single Crystalline Bi2sr2cacu2o8+?(Elsevier Ltd., 2008) Kadowaki, K.; Yamaguchi, H.; Kawamata, K.; Yamamoto, T.; Minami, H.; Kakeya, I.; Welp, U.; Özyüzer, Lütfi; Koshelev, A. E.; Kurter, Cihan; Gray, Kenneth E.; Kwok, W. K.We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν = c/nλ = c/2nw and ν = 2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40 K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437-438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5 μW from single mesa with w = 60 μm at 648 GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960 GHz in the fundamental mode in the w = 40 μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5 THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437-438 (2006) 111, M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.Article Citation - WoS: 10Citation - Scopus: 12The Superconducting Transition Width and Illumination Wavelength Dependence of the Response of Mgo Substrate Ybco Transition Edge Bolometers(Elsevier Ltd., 2007) Öktem, B.; Bozbey, Ali; Avcı, İlbeyi; Tepe, Mustafa; Abukay, Doğan; Fardmanesh, M.Dependence of the phase and magnitude of the response of MgO substrate YBa2Cu3O7-δ (YBCO) transition edge bolometers to the near infrared radiation on the superconducting transition width is presented in this work. The bolometers were made of YBCO thin films of 200 nm thickness that were grown on single crystal MgO (1 0 0) substrates by DC inverted cylindrical magnetron sputtering. We have measured the responses of both large and small area devices with respect to the bias temperature and radiation modulation frequency. We have observed that the superconducting transition width has major effects on the response of the bolometers such as; on a dip of the phase of the response versus modulation frequency curve around 1 Hz, the rate of decrease of the magnitude of the response, and dependence of the phase of the response on temperature at mid-range modulation frequency. We have investigated a correlation between the superconducting transition width and the YBCO film surface morphology of the devices. In addition, the illumination wavelength dependence of the optical response of both wide and narrow transition width devices has been investigated. Here we present the analysis and the possible mechanisms that can affect the response of the bolometers at the superconducting transition region.Article Citation - WoS: 1Effect of Magnetic Field on Quasiparticle Branches of Intrinsic Josephson Junctions With Ferromagnetic Layer(Elsevier Ltd., 2007) Özyüzer, Lütfi; Özdemir, Mustafa; Kurter, Cihan; Hinks, David G.; Gray, Kenneth E.The interlayer tunneling spectroscopy has been performed on micron-sized mesa arrays of HgBr2 intercalated superconducting Bi2212 single crystals. A ferromagnetic multilayer (Au/Co/Au) is deposited on top of the mesas. The spin-polarized current is driven along the c-axis of the mesas through a ferromagnetic Co layer and the hysteretic quasiparticle branches are observed at 4.2 K. Magnetic field evolution of hysteretic quasiparticle branches is obtained to examine the effect of injected spin-polarized current on intrinsic Josephson junction characteristics. It is observed that there is a gradual distribution in quasiparticle branches with the application of magnetic field and increasing field reduces the switching current progressively.