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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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    Enhanced physical properties of single crystal Fe0.99Te0.63Se0.37 prepared by self-flux synthesis method
    (Elsevier Science Sa, 2016) Onar, K.; Oezcelik, B.; Guler, N. K.; Okazaki, H.; Takeya, H.; Takano, Y.; Yakinci, M. E.
    In this study, we have systemically studied the physical, electrical and magnetic properties of Fe0.99Te0.63Se0.37 single crystalline samples prepared by self-flux method. We found that the self-flux method is a suitable synthesis technique for this alloys if setting of experimental parameters made carefully. The M-H curve affirms that samples are typical type-II superconductor. Strong sign of bulk superconductivity, even after high field measurements, were seen. Calculated J(c)(mag) values, at zero field, were found to be 7.7 x 10(5) Acm(-2) and 2.6 x 10(4) Acm(-2) for 5 K and 10 K respectively. The upper critical field H-c2(0) has been determined with the magnetic field parallel to the sample surface and yielding a maximum value of 65 T. At the zero field coherence length, xi, value was calculated to be 2.24 nm for 10% T-c(offset) which is significantly larger (approximately 6 fold) than the unit cell, a, and indicating the absence of weak link behavior in the sample. Calculated mu H-0(c2)(0)/k(B)T(c) rate indicated comparably higher value (3.66 T/K) than the Pauli limit (1.84 T/K) and obtained results were suggested unconventional nature of superconductivity in our samples. (C) 2016 Elsevier B.V. All rights reserved.
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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.