77Se NMR study of the spin dynamics and the pairing symmetry on K0.7Fe2-xSe2 single crystals

We report the $^{77}$Se NMR studies on high-quality ternary iron chalcogenide K$_{0.8}$Fe$_{2-x}$Se$_2$ single crystals with a $T_C\approx 29.5K$, to search for bulk evidence of the pairing symmetry and the electron correlations. Both the Knight shift and the spin-lattice relaxation rate are isotropic with field orientation and show a prominent pseudogap-like behavior with temperature. However, the Korringa ratio $S$, defined as $S=(4\pi k_B/\hbar) (\gamma_n/\gamma_e)^2 T_1TK^2_S$, is found to be temperature independent, which suggests a canonical Fermi liquid. Such a Fermi liquid behavior with isotropic spin dynamics suggests that the SDW fluctuations are not seen, in contrast with obvious spin fluctuations in iron pnictide and the binary iron chalcogenide $\alpha$-FeSe with a high $T_c$. In fact, our value of $S\approx 1.8$ suggests that the weak ferromagnetic fluctuations may present in the system instead. Below $T_C$, the Knight shift drops sharply with temperature, which gives the bulk evidence for a singlet pairing. However, the spin-lattice relaxation rate does not show a Hebel-Slichter coherence peak.