Spin resonance of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ studied by neutron scattering

The remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase, so called spin resonance, is important to examin since it could include information of Cooper pairing. Here, we examined the spin fluctuation of hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by inelastic neutron scattering to clarify the doping dependence of spin resonance. Neutron scattering experiments were conducted using the triple-axis spectrometer PUMA at FRM ll, Germany and 2T1 at LLB, France. We have found that the behavior of the spin resonance dramatically changes around x = 0.66 [1]. Resonance peaks have been observed clearly below 2$\Delta$s in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2$\Delta$s to higher energies, peaking at values of 3$\Delta$s for x = 0.84. These results indicate a reduced impact of magnetism on Cooper pair formation in the overdoped region. [1] C.H.Lee et al., Sci. Rep. 6, 23424 (2016)