Superconducting and normal-state properties of single-crystalline Tl_0.47Rb_0.34Fe_1.63Se₂ as seen via ⁷⁷Se and ⁸⁷Rb NMR

We report both ⁷⁷Se and ⁸⁷Rb nuclear magnetic resonance (NMR) studies on Tl_0.47Rb_0.34Fe_1.63Se₂ single-crystalline superconductors (T_c ≈ 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/T₁ , 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 ⁷⁷Se and 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+bT² 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.