Dec 6 2013
Swirling, persistent vortices can be created in superfluid helium. Generally no atoms sit at the eye of these miniature hurricanes.
Figure 1. Maps of spin density for Skyrmion (left) and Meron (right) phases of magnetic atoms. Red denotes spin-up atoms and blue denotes spin-down atoms. Several phases observed in the simulation (types II and III) are not shown here. (Credit: Ryan Wilson)
Things might be different in condensates of ultracold atoms. Theorists at the Joint Quantum Institute predict that for some elements a vortex of atoms can be produced which pivots around another sample of atoms at rest in the middle. Such a quantum gimbal has been observed in condensates of two atomic species but never before in a swarm of exclusively one type of atoms in a state of lowest energy.
The JQI search for new forms of quantum reality occurs not in an actual lab. Instead it takes place in a dedicated computer which, over hundreds of hours, simulates the magnetic interactions of a million atoms or so under various conditions. When the “experiment” is over the researchers take the numerical results and plot them up as if they were maps showing the disposition of actual atoms---either their positions in space or the directions of their spins (as if the atoms were little bar magnets).
In effect the simulation probes atomic matter for new types of magnetic order. It deduces what happens when magnetic atoms, such as those of the element dysprosium, are cooled into a Bose Einstein condensate (BEC) and then subjected to laser beams that make the atoms’ interactions spin-dependent. The strength of the interaction is also proportional to the density of atoms; squeeze more and more atoms into the ensemble and a variety of patterns---so called “spin textures”---emerges in succession. The results, published in the journal Physical Review Letters (see first related publication), reveals the occurrence of coreless vortices and shows an array of five such new magnetic phases, a feat interesting enough for the JQI paper to have received an “editor’s pick” designation in the journal.
SPINOR CONDENSATES
Why use dysprosium? Because it is (along with holmium) the most magnetic of the elements. Indeed, its magnetic dipole moment is ten times stronger than that for rubidium, an element used in many Bose Einstein condensate (BEC) experiments. This makes Dy a potential workhorse for experiments attempting to find new states of magnetic matter. Dy has a complement of 66 electrons which can arrange themselves into seventeen different lowest-energy (groundstate) configurations. So when Dy is chilled low enough in temperature you get not one but seventeen different BECs, all pretty much residing on top of each other in the center of an atom trap. Another name for a BEC consisting of co-existing, multiple-spin orientations is “Spinor condensate.”
SPIN-ORBIT COUPLING
This co-existence can be made more complicated in an interesting way by exposing the atoms to a pair of laser beams. This has the effect of toggling the atoms---which lie mostly in a plane---from one internal energy level to another. The effectiveness of this toggling is proportional to the velocity of the atoms. Thus an internal state of each atom (the energy levels) is coordinated with an external state of the atom, namely its velocity. This coordination is referred to as spin-orbit coupling, an expression that dates back to the early days of quantum physics, when scientists studied the important force between an electron’s spin and the magnetic field created by the fact that the electron was orbiting within the atom. In this early work, spin-orbit coupling could be viewed as a form of self-interaction.
More recently spin-orbit coupling was demonstrated for the first time (see related articles) in a JQI experiment with ultracold rubidium atoms; here the “orbit” in spin-orbit coupling refers not to the motion of electrons within an atom but the motion of the atoms themselves.
MAGNETIC PHASES
The effect of turning on those lasers is to “dress” the atoms. After the laser light is applied the energy levels of the atoms are so prominently reconfigured that only two of the original 17 ground states are still effectively retained in the atom trap for detailed study. These two remaining spin species---called for convenience spin-up and spin-down---form an ideal way of exploring strong interactions in a gas of ultracold atoms, all participating in a single gigantic quantum state which allows the atoms to be oriented in two ways---with their spins up or down.
In conventional matter, magnetism appears in many forms. Some of the more important are ferromagnetic ordering, in which domains (small volumes within the material) aligned together by an external magnetic field remain aligned even after the field is turned off. In anti-ferromagnets, the domains alternate in an aligned-antialigned checkerboard pattern (that is, neighboring domains are oppositely aligned). In paramagnetism regions (atoms or domains) will be attracted or aligned with an external applied field but will lose the alignment when the field is turned off.
In conventional materials, the types of magnetism depend on the intrinsic nature of the material and on impurities present. In ultracold atoms, by contrast, there are no impurities and the magnetism can be engineered at will by adjusting the laser beams (the light used to “dress” the atoms) and the density of the atoms. In a JQI experiment conducted several years ago laser beams were first used to create spin-dependent forces in cold atoms. The researchers, led by Ian Spielman, noticed something else: the atoms with different spin segregated themselves in space (see related articles).
With ultracold Dy atoms, new magnetic phases are expected to appear when the strength of the inter-atomic force is turned up by adding more atoms, and this is what the present JQI work addresses. Here too an expected self- segregation of atoms takes place. In the diagrams below, the two ground-state conditions (spin up and spin down) are depicted as red and blue. And like oil and vinegar separating themselves in an unshaken bottle of salad dressing, the red and blue occupy separate regions of space.