Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As75 NMR

J. Cui, Q.-P. Ding, W. R. Meier, A. E. Böhmer, T. Kong, V. Borisov, Y. Lee, S. L. Bud'ko, R. Valentí, P. C. Canfield, Y. Furukawa
2017 Physical review B  
We report ^75As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor CaKFe_4As_4 with T_ c = 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 ν_ Q of the As(1) and As(2) sites show an opposite temperature (T) dependence. Nearly T independent behavior of the Knight shifts
more » ... are observed in the normal state, and a sudden decrease in K in the superconducting (SC) state clearly evidences spin-singlet Cooper pairs. ^75As spin-lattice relaxation rates 1/T_1 show a power law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wavevector q = (π, 0) or (0, π) in the single-iron Brillouin zone notation are revealed by 1/T_1T 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/T_1 shows a rapid decrease below T_ c without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s^± nodeless two-gap superconductor.
doi:10.1103/physrevb.96.104512 fatcat:m2wtmmdpznev5mfgs4nms4amje