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Superconductivity and normal state properties of single-crystalline Tl0.47Rb0.34Fe1.63Se2 as seen via 77Se and 87Rb NMR

We report both 77Se and 87Rb NMR studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (T_c about 32 K). Singlet superconductivity is decisively determined by a sharp drop of the Knight shift K(T) below T_c, after subtracting the superconducting diamagnetic effect. However, the Hebel-Slichter coherence peak below T_c is not observed in the spin-lattice relaxation rate 1/T_1, even with a low in-plane NMR field of 2.6 Tesla. Just above T_c, no evidence of low-energy spin fluctuation is found in the spin-lattice relaxation rate on both the 77Se and the 87Rb sites. Upon warming, however, the Knight shifts and the spin-lattice relaxation rates of both nuclei increase substantially with temperature. In particular, the Knight shift is nearly isotropic and follow a function fit of K=a+bT^2 from T_c up to 300 K. These normal state properties may be an indication of thermally activated spin fluctuations. Our observations should put a strong constraint to the theory of magnetism and superconductivity in the newly discovered iron-based superconductors.