Microscopic coexistence of antiferromagnetic order and superconductivity   in Ba$_{0.77}$K$_{0.23}$Fe$_{2}$As$_{2}$
  We report ^{75}As nuclear magnetic resonance studies on an underdoped single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below T_N = 46 K, the NMR peaks for H//c split and those for H //a shift to higher frequencies, which indicates that an internal magnetic field along the c axis develops below T_N. The spin-lattice relaxation rate 1/T_1 measured at the shifted peak with H//a, which experiences the internal field, shows a distinct decrease below Tc({\mu}_0H = 12 T) = 16 K, following a T^3 relation at low temperatures. Our results show unambiguously that antiferromagnetic order and superconductivity coexist microscopically. The unusual superconducting state with the coexisting magnetism is highlighted. 
Z. Li, R. Zhou, D. L. Sun, J. Yang, C. T. Lin, and Guo-qing Zheng
cond-mat.supr-con