Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As-75 NMR

We report 75As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. 75As NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. 75As spin-lattice relaxation rates 1/T1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q=(π,0) or (0,π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the 75As quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s± nodeless two-gap superconductor