  We use inelastic neutron scattering to study the effect of a magnetic field
on the neutron spin resonance (Er = 3.6 meV) of superconducting FeSe (Tc = 9
K). While a field aligned along the in-plane direction broadens and suppresses
the resonance, a c-axis aligned field does so much more efficiently, consistent
with the anisotropic field-induced suppression of the superfluid density from
the heat capacity measurements. These results suggest that the resonance in
FeSe is associated with the superconducting electrons arising from orbital
selective quasi-particle excitations between the hole and electron Fermi
surfaces.
