Abstract
We report nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, , with K. 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 of the As(1) and As(2) sites show an opposite temperature dependence. Nearly independent behavior of the Knight shifts is observed in the normal state, and a sudden decrease in in the superconducting (SC) state suggests spin-singlet Cooper pairs. spin-lattice relaxation rates show a power-law dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector or in the single-iron Brillouin zone notation are revealed by and measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, shows a rapid decrease below without a Hebel-Slichter peak and decreases exponentially at low , consistent with an nodeless two-gap superconductor.
- Received 5 July 2017
- Revised 30 August 2017
DOI:https://doi.org/10.1103/PhysRevB.96.104512
©2017 American Physical Society