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    The Annealing Effects in the Iron-Based Superconductor FeTe0.8Se0.2 Prepared by the Self-Flux Method
    (Springer, 2014) Guler, Nilay Kantarci; Ekicibil, Ahmet; Ozcelik, Bekir; Onar, Kubra; Yakinci, M. Eyyuphan; Okazaki, H.; Takeya, H.
    FeTe0.8Se0.2 single crystals as-cast and post-annealed were prepared by the self-flux method. We have investigated the structural properties of samples by using the XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), and magnetic techniques. The SEM results clearly demonstrate that Te ions are quite well substituted for Se ions in the FeSe lattice for the samples. From the XRD and EDX spectra of the both samples, it has been concluded that the post-annealing causes no change in the tetragonal structure of FeTe0.8Se0.2. According to M-H measurements, the perfect diamagnetism has been observed only in low field at 5 and 10 K temperatures. The trend of the magnetization versus temperature curves, measured under a magnetic field of 10 Oe, also support our conclusion about diamagnetic contribution in FeTe0.8Se0.2 single crystal explored in this study. The as-cast and post-annealed samples show the onset of diamagnetism at temperatures, T-c.on(mag), 12.45 and 13.27 K, respectively. In addition, those curves indicate that the high field value and some impurities reveal ferromagnetic interactions.
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    Effect of Tungsten (W) Substitution on the Physical Properties of Bi-(2223) Superconductors
    (Springer, 2014) Turk, Nihal; Gundogmus, Hakan; Akyol, Mustafa; Yakinci, Z. Deniz; Ekicibil, Ahmet; Ozcelik, Bekir
    The superconducting Bi1.6Pb0.4Sr2Ca2Cu3-x W (x) O10+y (x=0.00, 0.05, 0.10, 0.15) bulk samples were prepared by the solid-state reaction method. The effects of W substitution on the BSCCO system have been investigated by the electrical resistivity (rho-T), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), magnetic hysteresis and critical current density measurements. It has been found that the Bi-(2212) low-T (C) phase is formed for all the substitution levels, together with the Bi-(2223) high-T (C) phase. The results obtained from the XRD data show that the Bi-(2223) phase gradually transforms into the Bi-(2212) phase with increasing W substituting for Cu. In addition, from the magnetization measurements at the temperatures below the zero resistance temperatures of the samples, we have observed that a decreasing in magnitude of |M| with the increasing measurement temperature and W concentration. Therefore, the decreasing of |M| related to superconducting volume seems to imply an existence of flux-pinning centers in our samples.
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    Structure and physical properties of iron-selenide KxFe2-ySe2
    (Elsevier Science Sa, 2015) Guler, Nilay Kantarci; Ozcelik, Bekir; Ekicibil, Ahmet; Onar, Kubra; Yakinci, M. Eyyuphan; Okazaki, H.; Takeya, H.
    We have synthesized the superconducting KxFe2-ySe2 (x = 0.8) crystal by using the one step technique. SEM images demonstrate terrace-like structures on the surface and the sample contains two phases. XRD pattern gives the (00l) peaks. The main peaks are indexed to the I4/m space group and the lattice parameter c is calculated to be 13.94076 angstrom. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperature, T-c(mag), obtained from magnetization measurement is estimated to be 31 K. The critical current value, Jc(0), deduced from the M-H loops for sample is approximately 9 x 10(4) A/cm(2) at 5 K. From magnetoresistivity measurement the T-c, T-c(onset) and T-c(offset) temperatures at the absence of the external magnetic field are found as 32.2, 33.1 K and 31.5 K, respectively. The calculated flux pinning or activation energy U is around 0.46 eV for 0 T. (C) 2015 Elsevier B.V. All rights reserved.