Neutron scattering study of spin fluctuations on hole-overdoped KFe$_2$As$_2$

Spin fluctuations in Fe-based superconductors have attracted great attention since they can be a key factor of the formation of superconducting states. The inelastic neutron scattering technique is a powerful method to examine spin fluctuations, whereas measurements using a single crystal were restricted to Fe(Te,Se) or electron-doped AFe$_2$As$_2$ (A=Ba, Ca, or Sr) due to difficulty of growing a large single crystal. To overcome this problem, we have improved growth procedure and succeeded to grow single crystals of heavily hole-overdoped superconducting KFe$_2$As$_2$ ($T_c$ = 3.4 K). It was believed that no spin fluctuation can be observed in KFe$_2$As$_2$, since the nesting of the Fermi surface disappears. To confirm the hypothesis, we have studied spin fluctuations of KFe$_2$As$_2$ by neutron scattering using single crystals at JRR-3 reactor of JAERI in Tokai. As results, a well-defined low-energy incommensurate spin fluctuation has been observed at ($\pi(1\pm2\delta$),0) with $\delta$ = 0.16. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)$_2$As$_2$ at high energies. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped Fe-based superconductors, or a new type of spin fluctuation emerges by heavily hole doping.