Magnetic phase diagram of light-mediated spin structuring in cold atoms

Self-organization in driven dissipative systems is an active field of research with implications in physics, chemistry, and biology. When applying a red-detuned retroreflected laser beam to a large cloud of cold atoms, we observe the spontaneous formation of 2D structures in the transverse plane corresponding to high contrast spatial modulations of both light field and atomic spins. By applying a weak magnetic field, we explore the rich resulting phase space and identify specific phases associated with both dipolar and quadrupolar terms of the atomic magnetic moment. In particular, we demonstrate spontaneous structures in optically induced ground state coherences representing magnetic quadrupoles. Our results illustrate the wealth of behavior exhibited by laser-driven atomic media with complex level structure under optical feedback.