Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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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: 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: 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.Article Investigation of the Tunneling Spectra in Hgbr2-Intercalated Bi-2212 Single Crystals Below and Above Tc(Elsevier Ltd., 2007) Kurter, Cihan; Mazur, Daniel; Özyüzer, Lütfi; Hinks, David G.; Gray, Kenneth E.Interlayer tunneling spectroscopy measurements were performed on mesa arrays of Bi-2212 single crystals, intercalated with HgBr2. Tunneling conductances were obtained over a wide temperature range to examine the spectral features, especially the behavior of the quasiparticle peaks corresponding to superconducting energy gaps (SGs). Experimental spectra showed that gap-like features are still present even for the temperatures far above the transition temperature, Tc. This evidence is consistent with the idea that the SG evolves into a pseudogap above Tc for HgBr2-intercalated Bi-2212 single crystals.Article Citation - WoS: 13Citation - Scopus: 12Mgb2 Superconducting Thin Films Sequentially Fabricated Using Dc Magnetron Sputtering and Thermionic Vacuum Arc Method(Elsevier Ltd., 2007) Okur, Salih; Kalkancı, M.; Pat, Suat; Ekem, Naci; Akan, Tamer; Balbağ, Zafer; Musa, G.; Tanoğlu, MetinIn this work, we discuss fabrication and characterization of MgB2 thin films obtained by sequential deposition and annealing of sandwich like Mg/B/Mg thin films on glass substrates. Mg and B films were prepared using DC magnetron sputtering and thermionic vacuum arc techniques, respectively. The MgB2 thin films showed superconducting critical transition at 33 K after annealing at 650 °C.Article Citation - WoS: 10Citation - Scopus: 9Structural and Low-Field Magnetic Characterization of Superconducting Mgb2 Wires(Elsevier Ltd., 2004) Kılıç, Ahmet; Okur, Salih; Güçlü, Nusret; Kölemen, Uğur; Uzun, Orhan; Özyüzer, Lütfi; Gençer, AhmetSuperconducting MgB2 composite wires were prepared by packing blend of MgB2 inside of Cu tubes using powder in tube (PIT) method. The produced samples of the wires were then characterised by using SEM, XRD and AC susceptibility measurements, The measured fundamental susceptibility is compared with Bean model. We have obtained an empirical functions for the penetration field Hp = Hα(1-t)β, where t is the reduced temperature. In addition, ac losses were calculated at the same fixed temperatures to compare theoretical solutions. There is a qualitative agreement between the experimental results and theory.Article Citation - WoS: 22Citation - Scopus: 23Implications of Tunneling Studies on High-Tc Cuprates: Superconducting Gap and Pseudogap(Elsevier Ltd., 2001) Miyakawa, Nobuaki; Zasadzinski, John F.; Oonuki, S.; Asano, M.; Henmi, D.; Kaneko, Tsutomu; Özyüzer, Lütfi; Gray, Kenneth E.Tunneling spectra have been measured on high-Tc cuprates including single crystals Bi2Sr2-xLaxCuO6+δ (Bi2201) and Bi2Sr2CaCu2O8+δ (Bi2212) using superconductor-insulator-normal metal point contact or superconductor-insulator-superconductor break junction methods. The doping dependence of the energy gap parameter is similar in both Bi2212 and Bi2201, increasing monotonically to very large values in the underdoped regime even as Tc decreases. This doping dependence of superconducting gap is similar to that of pseudogap temperature, T*, indicating this is consistent with the scenario whereby the low-energy pseudogap is due to some type of precursor of superconductivity. The high-energy feature observed as the hump structure may be another kind of pseudogap whose energy scale is much larger than superconducting gap, and it may be magnetic in origin.Article Citation - WoS: 14Citation - Scopus: 14High Energy Secondary Peak Structure in Tunneling Spectra (hump) as Possible Magnetic Pseudogap(Elsevier Ltd., 2000) Zasadzinski, John F.; Özyüzer, Lütfi; Miyakawa, Nobuaki; Hinks, David G.; Gray, Kenneth E.It is demonstrated that tunneling spectra in various high Tc cuprates display generic features. The principal conductance peaks in superconductor-insulator-normal metal (SIN) junctions indicate the superconducting gap in the density of states (DOS), Higher energy features include a dip and hump structure with a strength that is asymmetric in bias voltage. The dip and hump features follow the doping trends of the superconducting gap, Δ, with a rough scaling as ∼2Δ and ∼3Δ respectively. Tunneling spectra in underdoped Bi2Sr2CaCu2O8+δ display a more pronounced hump feature suggestive of a second gap in the DOS. It is observed that the hump feature in the tunneling density of states is consistent with other experimental observations of the so-called high energy pseudogap which may have magnetic origins