NMR Study of Superconductivity and Spin Fluctuations in Intercalated Iron Selenides A$_y$Fe$_{2-x}$Se$_2$

The role of spin fluctuations in superconductivity is an essential topic in both cuprate and Fe-based superconductors. NMR works in several Fe-based superconductors proposed that the low-energy antiferromagnetic spin fluctuations (AFSF) is a possible pairing glue for superconductivity. However, studies on other systems such as KFe$_2$As$_2$ and Li$_{1-x}$FeAs does not support a strong correlation between low-energy spin fluctuations and superconductivity. In the newly discovered A$_y$Fe$_{2-x}$Se$_2$ superconductors with $T_c\sim$ 32 K, our NMR study identifies unambiguously a paramagnetic superconducting phase, which is phase separated from the block antiferromagnetic state. The low-energy AFSF is not seen at all, even though the T$_c$ is high. The A$_y$Fe$_{2-x}$Se$_2$ are singlet superconductors evidenced from the NMR Knight shift $K$; However, the absence of the coherence peak in the spin-lattice relaxation rate $1/T_1$ suggests an unconventional behavior of superconductivity. In fact, we found that both the $K$ and the $1/T_1T$ increase dramatically with temperature and follow a $a+bT^2$ form from Tc up to 300 K. Such behavior is strong evidence for spin fluctuations with a high-energy, local nature in 3D systems, and inconsistent with a band-gap effect. Furthermore, $K$ and $1/T_1T$ saturate above 400 K, indicating an energy scale of 35 meV, which is distinct from the low-energy spin fluctuations. The above temperature enhanced spin fluctuations seem to be universal in Fe-based superconductors. \\[4pt] References: W. Yu et al., Phys. Rev. Lett. 106, 197001 (2011); Long Ma et al., Phys. Rev. B 83, 174510 (2011); L. Ma et al., arXiv:1103.4960.