Inelastic neutron scattering study on spin resonance in CaKFe4As4

The iron-based superconductors are extensively studied since the discovery in 2008, because of much material variations and the unconventional mechanisms of their high transition temperature superconductivity. Recently, new-structure-type iron-base superconductors AeAFe₄As₄ (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were found. The AeAFe₄As₄ family exhibits T_c=32 ～ 38 K, whose T_c is similar to that of so-called 122 system, and Ae and A layers stack alternatively between Fe₂As₂ layers owing to the large difference between their ionic radii. This unique crystal structure gives us opportunity to theoretically and experimentally investigate iron based superconductivity. In the session, we report the dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe₄As₄ by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations [1]. Powder INS measurements show that the spin resonance at Q_res = 1.17(1) Å^-1, corresponding to the (π, π) nesting wave vector in tetragonal notation, evolves below T_c. The characteristic energy of the spin resonance E_res = 12.5 meV is smaller than twice the size of the superconducting gap (2Δ). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe₄As₄ is the s_± symmetry.

[1] Kazuki Iida, Motoyuki Ishikado, Yuki Nagai, Hiroyuki Yoshida, Andrew D. Christianson, Naoki Murai, Kenji Kawashima, Yoshiyuki Yoshida, Hiroshi Eisaki, and Akira Iyo, J. Phys. Soc. Jpn. 86, 093703 (2017).