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Öğe Adjustable tunneling barrier in bi-based high-Tc cross-whisker junctions(Iop Publishing Ltd, 2015) Kizilaslan, O.; Simsek, Y.; Aksan, M. A.; Koval, Y.; Yakinci, M. E.; Mueller, P.We present a study of cross-whisker junctions with electronically modified tunneling barriers. Cross-whisker junctions were successfully prepared by annealing two crossed Bi-based whisker bars at temperatures of around 840 degrees C. In addition to the artificially produced junction at the interface, intrinsic Josephson junctions in the respective bars of the cross were observed. The artificial junction exhibited reproducible and almost ideal junction characteristics. The tunneling barrier/interface properties were controlled by carrier injection in c-axis direction. Using this process, we were able to reduce the tunneling resistance from 408 Omega to 30 Omega. At the same time, the critical current did rise by a factor of 4. Subsequently, the critical current was doubled while the tunneling resistance stayed constant. We interpret this observation in terms of the counter-play between transparency of the barrier and the carrier concentration of the electrodes. In this sense, we can consider the current injection procedure as an electronic gluing process of the two cross-junction whisker bars.Öğe Crystallization of glass-ceramic Bi3Sr2Ca2Cu3Oy superconducting system(Elsevier Science Bv, 2013) Kizilaslan, O.; Aksan, M. A.In this study, the nucleation and the crystallization of the material with nominal composition of Bi3Sr2Ca2Cu3O delta have been investigated, depending on the heat treatment conditions. It was found that 4 and 8-fold branched nuclei on the samples occurred with increasing the heat treatment temperature. Finally, crystallization occurred with further increase of the temperature. The needle-like crystals (whiskers) perpendicular to the sample surface was formed above 800 degrees C, which consisted of the Bi-2212 phase. The results clearly showed that the an isotropy of surface tension is them ostim portant parameter indetermining the shape of the nucleus. (C) 2013 Elsevier B.V. All rights reserved.Öğe Effect of Co addition on the critical current density of MgB2 superconductor(Elsevier, 2014) Kirat, G.; Kizilaslan, O.; Aksan, M. A.; Yakinci, M. E.In this study, the (MgB2),Co-1-x(x), samples, where x-0.0. 0.1, 0.3, 0.6 and 0.8, were prepared by using the solid-state reaction technique. Effect of the Co-addition on the structural, magnetic, critical current density. J(c), and flux pinning properties of the superconducting MgB2 system was investigated. A magnetic transition was observed with increasing the Co-concentration in MgB2. For the pure MgB2, J(c), was obtained to be 2.78 x 10(4) A cm(-2) at 5 K and 1.7 kOe, but J(c) was suppressed by addition of Co to the system. It was found that a large amount of Co in MgB2 did not act as pinning centers which improves J(c) The magnetic behavior observed by the Co-addition originates from the magnetic properties of cobalt itself. (C) 2014 Elsevier B.V. All rights reserved.Öğe Effect of the Er-substitution on critical current density in glass-ceramic Bi2Sr2Ca2(Cu3-xErx)O10+? superconducting system(Elsevier Sci Ltd, 2016) Kirat, G.; Kizilaslan, O.; Aksan, M. A.Bi2Sr2Ca2Cu3-xErxO10+delta (x=0.0, 0.5 and 1.0) samples were manufactured using conventional glass ceramic technique and their structural, magnetic and flux pinning properties were studied. The onset temperature of diamagnetic signal decreased by the Er-substitution. The x=0.5 Er-substituted sample showed perfect diamagnetism, which is attributed to secondary phases acting as pinning centers in the crystal structure. J(c) of the samples was calculated by using Bean's equation. The highest J(c) value was obtained for x=0.5 Er-substitution. The results suggest that the Er ions in the host material act as pinning centers which lead to the increase of J(c). (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Enhancement of the critical current of intrinsic Josephson junctions by carrier injection(Iop Publishing Ltd, 2015) Kizilaslan, O.; Simsek, Y.; Aksan, M. A.; Koval, Y.; Mueller, P.We present a study of the doping effect by carrier injection of high-T-c superconducting Bi-based whiskers. The current was injected in the c-axis direction, i.e., perpendicular to the superconducting planes. Superconducting properties were investigated systematically as a function of the doping level. The doping level of one and the same sample was changed by current injection in very small steps from an underdoped state up to a slightly overdoped state. We have observed that T-c versus log (j(c)) exhibits a dome-shaped characteristic, which can be fitted by a parabola. As T-c versus carrier concentration has a parabolic form, too, it can be concluded that the critical current density j(c) increases exponentially with the doping level. The electron-trapping mechanism is interpreted in the framework of Phillips' microscopic theory. In addition, the Joule heating effect in the intrinsic Josephson junction (IJJ) was controlled by carrier injection, and the effect of the non-equilibrium quasiparticle on the I-V curves of the IJJs was also discussed.Öğe Grain alignment and its relationship with superconductivity and thermal transport of Ni-substituted Bi-2212 textured rods fabricated at two different growth rates(Springer, 2015) Ozabaci, M.; Kizilaslan, O.; Madre, M. A.; Yakinci, M. E.; Sotelo, A.The microstructure, grain alignment, superconducting and thermal transport properties of Ni-substituted Bi-2212 rods grown at two different speeds (15 and 30 mm h(-1)) through the Laser Floating Zone method, have been evaluated. Significant variations in grain size, grain alignment, electrical and thermoelectric power properties have been observed for the rods depending on the growth and substitution rates. The highest aligned structure was obtained on unsubstituted rods grown at 15 mm h(-1). Both increased substitution and growth rates degraded the grain alignment. The presence of Ni-based secondary phases showed that Ni is not totally incorporated into the crystal structure, which, in turn, caused a decrease on the average grain size of the rods. With increasing Ni concentration, peak values of thermoelectric power of the rods, which lie between 3.8 and 6.4 mu VK-1, monotonically decreased while thermal conductivity values did not show any systematic change. The activation energy of flux motion, Uo, was calculated from the field dependent resistivity-temperature curves in a range of 0-8 T. Superconducting transition temperatures, T-c(onset) and T-c(offset), and activation energy, U-o, were found to decrease with increasing Ni contents and applied magnetic field. It has been estimated from the magnetic field dependence of activation energy of the samples that plastic creep of the collective vortices is dominant in the rods.Öğe Growth Speed and Substitution Effects on Alignment and Thermal Transport Properties of Bi-2212 Textured Superconductors(Springer, 2015) Ozabaci, M.; Rasekh, Sh.; Kizilaslan, O.; Madre, M. A.; Sotelo, A.; Yakinci, M. E.Fe-substituted superconducting thin BiSrCaCuO rods with nominal compositions of Bi2Sr2Ca1Cu2-x Fe (x) O8+delta (x = 0, 0.01, 0.03, 0.05, and 0.1) were fabricated using the laser floating zone technique at two different growth speeds, 15 mm h(-1) and 30 mm h(-1). The influences of growth speed and Fe substitution on the grain alignment in the rods were evaluated by means of x-ray pole figure studies. The obtained results showed that both applied growth speed and Fe substitution play a crucial role on grain alignment, which is strongly connected with the current-carrying capacity of the rods. It was found that the rods grown at 15 mm h(-1) (G15) have stronger orientation than the rods grown at 30 mm h(-1) (G30). However, in contrast to the G15 rods, an increased substitution rate improved the orientation of the G30 rods. Another important observation is that the increase on the substitution caused a decrease on the grain size of all the rods. The decrease of critical temperature values of the rods upon substitution was ascribed to both grain size effect and formation of a nonsuperconducting Fe-rich phase detected in scanning electron microscope/energy-dispersive x-ray analyses. The thermal conductivity values of the G15 and G30 rods were found to be in the range of 0.9-1.9 and 1.1-1.18 W m(-1) K-1 at 150 K, respectively. The higher values of figure of merit (ZT), at all temperature ranges, were obtained from the highest substituted rods (x = 0.1) for both of the applied growth speeds. In addition, it was observed that the ZT of G30 rods are up to three times higher than that of G15 ones.Öğe Interlayer tunneling spectroscopy of mixed-phase BSCCO superconducting whiskers(Iop Publishing Ltd, 2016) Kizilaslan, O.; Truccato, M.; Simsek, Y.; Aksan, M. A.; Koval, Y.; Mueller, P.In this work, we present a study on the interlayer tunneling spectroscopy (ITS) of mixed-phase BiSrCaCuO (BSCCO) superconducting whiskers. The tunneling experiments were carried out on the artificial cross-whisker (twist angle of 90 degrees) junctions. A multiple superconducting energy gap in the cross-whisker junctions was observed, which is attributed to the presence of different doping levels of two Bi2Sr2CaCu2O8+delta phases (Bi-2212), rather than two different phases, in the BSCCO whiskers, namely Bi2Sr2CaCu2O8+delta and Bi2Sr2Ca2Cu3O8+delta (Bi-2212 and Bi-2223). The temperature dependence of the energy gaps was discussed in the framework of the BCS T-dependence. On the other hand, the carrier concentration of the cross-whisker junction was changed by the carrier injection process. The effects of the carrier injection on the critical current, I-c, and the ITS of intrinsic Josephson junctions were investigated in details.Öğe Magnetic relaxation behavior in the Bi2Sr2Ca2Cu3-xMoxO10+? system fabricated by glass-ceramic technique(Elsevier, 2015) Kizilaslan, O.; Kirat, G.; Aksan, M. A.In this paper, we present a study on the critical current density and magnetic relaxation behavior of the Mo-substituted BiSrCaCuO system. The Bi2Sr2Ca2Cu3-xMoxO10+y, where x=0, 0.5, 1.0 and 1.5 have been fabricated using glass-ceramic technique. It was found that the Mo-substitution to the BiSrCaCuO system lead to the formation of pinning centers and hence an increase of critical current density. The magnetic relaxation experiments were performed to understand physical phenomena behind of flux motion. Time dependence of the magnetization exhibited thermally activation flux motion in the samples. It was found that the normalized magnetic relaxation rate, S. did not vary linearly with temperature, especially at high temperatures, which is attributed to the collective creep theory. In addition, the characteristic pinning energy, U-e/K-B, in the samples was calculated using Maley's method. Correlation between the pinning energy and the magnetization showed that obtained experimental results can be fitted to the collective creep theory. (C) 2015 Elsevier B.V. All rights reservedÖğe Nanoindentation Properties of HTc Superconducting Single-Crystalline Whiskers(Springer, 2013) Kizilaslan, O.; Yilmaz, F.; Aksan, M. A.; Yakinci, M. E.In this work, superconducting and mechanical properties of BSCCO superconducting whiskers were studied in detail. Nanoindentation properties of the whiskers were analyzed by the Oliver and Pharr Method and Work of Indentation (WI) method. More accurate results using the WI method for both the hardness and the reduced modulus were obtained due to reduction of the pile-up error. The correct hardness and reduced modulus values were determined by considering the indentation size effect and the substrate (epoxy resin) effect. The modified composite hardness model and the composite reduced modulus model are the most suitable models to investigate the nanoindentation properties of the whiskers.Öğe Preparation and characterization of CaO-Al2O3-SiO2 (CAS) glass-ceramics(Elsevier, 2016) Ozabaci, M.; Aksan, M. A.; Kirat, G.; Kizilaslan, O.; Yakinci, M. E.A glass-ceramic CaO-Al2O3-SiO2 (CAS) system with CaAl2Si2O8 (Anorthite) composition was prepared at two different heat treatment temperatures following the melt-quenching in an arc-melter system. Structural and magnetic properties of the samples were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and magnetic measurements (M-H, M-T). A clear phase transformation, hexagonal to triclinic, was obtained when heat treatment temperature increased from 930 degrees C to the 1100 degrees C Two different impurity phases were obtained for the sample heat treated at 930 degrees C, but then these phases dissolved in the main matrix when temperature increased to 1100 degrees C and single phase CaAl2Si2O8 was obtained. All the samples showed diamagnetic behavior between room temperature and similar to 150 K, but then ferri- or ferromagnetic phase transformation was obtained. This transformation is found to be independent from the heating temperature and believed to be associated with the trapping of free electrons at crystal imperfections and cation ordering between Al and Si in the main matrix. (C) 2016 Elsevier B.V. All rights reserved.Öğe Thermoelectric power and thermal conductivity study of the Y3Ba5Cu8Ox system(Elsevier, 2012) Aksan, M. A.; Kizilaslan, O.; Aksan, E. N.; Yakinci, M. E.In the present study, the effect of heat treatment conditions on the structural/microstructural, electrical and transport properties, such as thermoelectric power and thermal conductivity, of the Y-358 system was investigated. The Y-123 phase instead of Y-358 were obtained with heat treatments. XRD and SEM analyses showed that the best structural formation was obtained for the sample prepared at 900 degrees C for 24 h. All the samples exhibited the T-c value around 91 K. Positive S(T) value was obtained in all the samples. S(T) data was analyzed by Two band model with linear T-term. In thermal conductivity of the samples, small peak with broad maximum was found just below T-c, unlike that of the HTc cuprates. (c) 2012 Elsevier B.V. All rights reserved.Öğe Tuning THz emission properties of Bi2Sr2CaCu2O8+? intrinsic Josephson junction stacks by charge carrier injection(Iop Publishing Ltd, 2017) Kizilaslan, O.; Rudau, F.; Wieland, R.; Hampp, J. S.; Zhou, X. J.; Ji, M.; Kiselev, O.We report on doping and undoping experiments of terahertz (THz) emitting intrinsic Josephson junction stacks, where the change in charge carrier concentration is achieved by heavy current injection. The experiments were performed on stand-alone structures fabricated from a Bi2Sr2CaCu2O8+delta single crystal near optimal doping. The stacks contained about 930 intrinsic Josephson junctions. On purpose, the doping and undoping experiments were performed over only a modest range of charge carrier concentrations, changing the critical temperature of the stack by less than 1 K. We show that both undoping and doping is feasible also for the large intrinsic Josephson junction stacks used for THz generation. Even moderate changes in doping introduce large changes in the THz emission properties of the stacks. The highest emission power was achieved after doping a pristine sample.
