We report both ${}^{77}\mathrm{Se}$ and ${}^{87}\mathrm{Rb}$ nuclear magnetic resonance (NMR) studies on Tl${}_{0.47}$Rb${}_{0.34}$Fe${}_{1.63}$Se${}_2$ single-crystalline superconductors ($T_c\approx$ 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 ${}^{77}\mathrm{Se}$ and the ${}^{87}\mathrm{Rb}$ 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 $K=a+{\mathit{bT}}^2$ 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.

• Received 22 February 2011

DOI:https://doi.org/10.1103/PhysRevB.83.174510