Effect of the in-plane magnetic field on the neutron spin resonance in optimally doped FeSe0.4Te0.6 and BaFe1.9Ni0.1As2 superconductors*

We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe0.4Te0.6 (Tc = 14 K) and BaFe1.9Ni0.1As2 (Tc = 20 K). While the magnetic field up to 14.5 T does not change the energy of the resonance, it partially suppresses Tc and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore it is still unclear if the resonance is the long-sought singlet-triplet excitation directly coupled to the superconducting electron Cooper pairs.