Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As 75 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

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38 Scopus citations


We report As75 nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. As75 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. As75 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 As75 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.

Original languageEnglish (US)
Article number104512
JournalPhysical Review B
Issue number10
StatePublished - Sep 25 2017
Externally publishedYes

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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