  We develop a theory of cavity quantum electrodynamics for a 2D electron gas
in the presence of Rashba spin-orbit coupling and perpendicular static magnetic
field, coupled to spatially nonuniform multimode quantum cavity photon fields.
We demonstrate that the lowest polaritonic frequency of the full Hamiltonian
can vanish for realistic parameters, achieving the Dicke superradiant quantum
phase transition. This singular behaviour originates from soft spin-flip
transitions possessing a non-vanishing dipole moment at non-zero wave vectors
and can be viewed as a magnetostatic instability.
