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

We report both Se-77 and Rb-87 nuclear magnetic resonance (NMR) studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (T-c approximate to 32 K). Singlet superconductivity is suggested 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 T. Just above T-c, no evidence of low-energy spin fluctuation is found in the spin-lattice relaxation rate on both the Se-77 and the Rb-87 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. Our observations should put a strong constraint to the theory of magnetism and superconductivity in the recently discovered iron selenide superconductors.