Self-organized magnetization patterns in cold atoms

Bonnie L. Schmittberger, Joel A. Greenberg, Daniel J. Gauthier

Ivor Krešić, Gordon R. M. Robb, G. Labeyrie +2 more

Laser-driven cold atoms near a plane retroreflecting mirror exhibit self-organization above a pump threshold. We analyze the properties of self-organized spin patterns in the ground state of cold rubidium atoms. Antiferromagnetic patterns in zero magnetic field give way to ferrimagnetic patterns if a small longitudinal field is applied. We demonstrate how the experimental system can be modeled as spin-1 atoms diffractively coupled by the light reflected by the mirror. The roles of both dipolar and quadrupolar magnetization components in determining the threshold and symmetry variations with a weak longitudinal magnetic field are examined. Although the magnetic structures correspond dominantly to a lattice of magnetic dipoles, the symmetry breaking to ferrimagnetic structures in a finite field is mediated by the coupling to a homogenous quadrupole (alignment), which is not possible in a spin-1/2 system. Our study provides a basis for further exploration of instabilities in driven multilevel systems with feedback.

J. Stajic

Inti Sodemann, D. A. Pesin, Allan H. MacDonald

Ivor Krešić, Gordon R. M. Robb, G. Labeyrie +3 more

Laser driven cold atoms in front of a retro-reflecting mirror exhibit self-organization above pump threshold. It was recently demonstrated how this system can be used to mimic a transverse lattice of interacting spins. The spontaneously formed lattice breaks both translational and rotational symmetries of the initial homogeneous spin state. By applying a constant longitudinal magnetic field of |Bz| ~ 100 mG a switching between antiferromagnetic square and ferri-magnetic hexagonal pattern ordering can be induced. We study the coupling between this magnetic field and modulations in the atomic magnetization.

Bonnie L. Schmittberger, Daniel J. Gauthier

The study of transverse optical pattern formation has been studied extensively in nonlinear optics, with a recent experimental interest in studying the phenomenon using cold atoms, which can undergo real-space self-organization. Here, we describe our experimental observation of pattern formation in cold atoms, which occurs using less than 1 μW of applied power. We show that the optical patterns and the self-organized atomic structures undergo continuous symmetry breaking, which is characteristic of nonequilibrium phenomena in a multimode system. To theoretically describe pattern formation in cold atoms, we present a self-consistent model that allows for tight atomic bunching in the applied optical lattice. We derive the nonlinear refractive index of a gas of multilevel atoms in an optical lattice, and we derive the threshold conditions under which pattern formation occurs. We show that by using small detunings and sub-Doppler temperatures, one achieves intensity thresholds for pattern formation that are reduced by two orders of magnitude compared to warm atoms.