Copper-tuned magnetic order and excitations in iron-based superconductors Fe1+yTe1-xSex*

We report neutron scattering results on the Cu-substitution effects in the iron-based superconductors, Fe$_{1+y}$Te$_{1-x}$Se$_x$. In the parent compound, it is found that Cu drives the low-temperature magnetic ground state from long-range commensurate antiferromagnetic order in Fe$_{1.06}$TeCu$_{0.04}$ to short-range incommensurate order in FeTeCu$_{0.1}$. In the former sample, the structural and magnetic ordering temperature is 40~K; in FeTeCu$_{0.1}$, the structural phase transition is not obvious and a transition to the spin-glass state is found at 22~K. Cu suppresses superconductivity in FeTe$_{0.5}$Se$_{0.5}$---$T_c$ is reduced to 7~K with a 2\% Cu doping, and no superconductivity is found in the 10\% Cu-doped sample. In the meantime, the intensity and energy of the resonance mode are suppressed in the 2\% Cu-doped sample, while there is no resonance in the non-superconducting sample. Besides, the low-temperature magnetic excitation spectra are distinct for these two samples, with the superconducting one having an ``hour-glass" shape and the other one having a ``waterfall" shape. Our results provide further insights on the interplay between magnetism and superconductivity in the iron-based superconductors.