N.C.S.R. "Demokritos"

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Oxypnictides

The discovery of new superconductors continues to challenge our knowledge of the phenomenon of superconductivity. Twenty years after the discovery of high-T_c layered copper oxides, the discovery of superconductivity in electron doped quaternary rare-earth (RE) transition metal pnictides (oxypnictides) with the general formula REFeAsO1-xFx, with significant critical temperature, regenerates interest in superconductivity. These compounds form layered structures, where As-Fe-As layers are sandwiched between ReO layers. The crystal structure has a tetragonal unit cell with P 4/nmm space group. The charge, spin and orbital state of the iron and the basic mechanism responsible for superconductivity are not well understood. Besides the studies related to the basic superconducting mechanism it is also interesting to study the vortex matter properties. The intermediate critical temperature, in comparison with high-Tc cuprates and conventional low-Tc superconductors makes the vortex matter properties of the oxypnictides an interesting topic for both practical and fundamental reasons. Issues like the role of the thermal fluctuations, the melting transition of the vortex matter, the order-disorder transition, and the kind of the pinning centers comprise a set of open scientific questions needing detailed study. Several studies concerning the vortex matter properties of oxypnictides have recently shown that the thermal fluctuations play an important role. In addition, the high critical fields (low coherence lengths) impose significant limitations on the possible pinning centers. We have studied using Moessbauer spectroscopy the oxypnictide superconductor REFeAsO0.82F0.18 with RE=Nd for the first time. The Moessbauer spectra at 295 K predominantly consist of a singlet (94% of the spectral area) with an isomer shift IS=0.43(1) mm/s with respect to alpha-iron at 295 K, and a minor doublet with IS=0.29(1) mm/s and DEQ=1.756(3) mm/s which is attributed to an FeAs2 impurity. The spectrum remains unchanged down to 4.2 K. The isomer shift of the singlet with zero quadrupole splitting indicates that the iron in the Nd oxypnictide is probably in a low spin Fe2+ (S=0) state. Using magnetic measurements we have also studied the magnetic properties of ╬ŁdFeAsO0.82F0.18 superconductor. Low-field magnetic measurements revealed that an external magnetic induction of B=0.01 T destroys the superconducting path between grains (weak-link behavior). Nd ions behave as isolated paramagnetic centers. This is evident since for magnetic inductions larger than 0.5 T a paramagnetic moment arises from the Nd that overcomes the superconducting diamagnetic signal. Hysteresis loops at several temperatures revealed that the critical current falls rapidly with temperature. The temperature variation of the reversible magnetization shows a behavior that is reminiscent of high-Tc cuprates, indicating the important role of thermal fluctuations.

1) Magnetic measurements of superconducting NdFeAsO0.82F0.18 oxypnictide, M. Pissas, D. Stamopoulos, Zhi-An Ren, Xiao-Li Shen, Jie Yang, Zhong and Xian Zhao, Supercond. Sci. Technol. 22 (2009) 055008.

2) A Moessbauer study of the superconducting NdFeAsO0.82F0.18 oxypnictide compound, M Pissas, Y Sanakis, V Psycharis, A Simopoulos, E Devlin, Zhi-An Ren, Xiao-Li Shen, Guang-Can Che and Zhong-Xian Zhao, Supercond. Sci. Technol. 21 (2008) 115015.

Department of Materials Science, N.C.S.R. "Demokritos", 153 10 Aghia Paraskevi, Attiki, phone: +30 210 6503381, fax: +30 210 6519430
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