  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.
