1. Magnetic phases of spin-orbit-coupled Bose gases
Magnetic fields arise from the microscopic behavior of atoms and their electrons. In permanent magnets, neighboring atoms align and lock into place to create inseparable north and south poles. For other materials, magnetism can be induced by a field strong enough to coax atoms into alignment.
Atoms are typically arranged in the rigid structure of a solid, glued into place by their interactions and prevented from moving. But a PFC-supported team of researchers has found that magnetism can arise even when the constituent particles are free to roam around.
The researchers mapped out the magnetic properties of a cold atomic cloud, which served as testbed for a study of itinerant magnetism. They employed previously developed techniques for applying effective magnetic fields to atoms with three accessible spin states. They studied the phase transitions that the atomic cloud underwent while adjusting the parameters of two interfering lasers—namely, their frequency difference and their intensities.
They observed three distinct phases, corresponding to different settings for the laser parameters. When one laser’s frequency was shifted higher and both lasers had relatively low intensities, the atoms sat in their non-magnetic state, unperturbed by the fields. As the frequency shift was turned down and eventually flipped—so that the second laser’s frequency was higher—atoms preferred to fall into one of the two magnetic states, leading to an increase in the atoms’ motion. Atoms even grouped together by state, leading to magnetic domains similar to those that appear in ordinary magnets. When the laser intensity was ramped up, this magnetic ordering collapsed.
Interfering lasers create an effective magnetic field for a cloud of cold rubidium atoms. Image credit, Kelley/JQI
“Magnetic phases of spin-1 spin–orbit-coupled Bose gases,” D. L. Campbell, R. M. Price, A. Putra, A. Valdés-Curiel, D. Trypogeorgos, and I. B. Spielman, Nature Communications, 7, (2016)