부산대학교 도서관

We report both 77Se and 87Rb nuclear magnetic resonance (NMR) studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (Tc≈ 32 K). Singlet superconductivity is suggested by a sharp drop of the Knight shift K(T) below Tc, after subtracting the superconducting diamagnetic effect. However, the Hebel-Slichter coherence peak below Tc 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 Tc, 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+bT2 from Tc 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. [ABSTRACT FROM AUTHOR]