  The origin of the spin resonance observed in CeCoIn5 with Inelastic Neutron
Scattering is subject to debate. It has been shown recently that in this heavy
electron compound at low temperature an instability to a ground state with
coexisting dx2 -y2 -wave superconductivity and Spin Density Wave (SDW) order in
a magnetic field is a corollary of the consideration of a collective spin
excitation mode in a quasi-2D dx2 -y2 -wave Pauli-limited superconductor. This
provides a natural scenario for the occurence of the puzzling
high-field-low-temperature phase highlighted in CeCoIn5 . We present
perspectives on this ground state transition and propose directions for future
experiment.
