Spin segregation puzzles physicists
Oct 23, 2008 7 comments
Physicists in the US are the first to segregate a Fermi gas of ultracold atoms according to their spin — with “spin-up” and “spin-down” atoms moving to opposite sides of the optical trap in which they were contained.
John Thomas and colleagues at Duke University found that about 60% of the lithium-6 atoms became segregated and that the spin-up and spin-down atoms remained apart for several seconds. However, they are puzzled as to why the segregation lasts much longer, and is more intense, than predicted by theory (Phys. Rev. Lett. 101 150401).
Spin segregation is interesting for physicists because it plays an important role in “spintronics”, where it is used to create currents of spin-polarized electrons. As lithium-6 atoms and electrons are both fermions — that is, particles with half-integer spin — the new system could therefore be used as a “quantum simulator” of spintronic devices.
Although Eric Cornell and colleagues at the University of Colorado have previously been able to segregate an ultracold atomic gas of bosons (rubidium atoms with integer spin) in terms of spin, such a system has not been useful for simulating interactions between electrons in solids because fermions and bosons behave so differently.
Hit with a radiofrequency pulse
Thomas's team used a Fermi gas of ultracold lithium-6 atoms that are trapped in the centre of a vacuum chamber by a laser beam. The team begin their experiment with all the atoms in the spin-up state. The gas is then hit with a radiofrequency pulse that puts each atom into a coherent superposition of spin-up and spin-down states. This means that until, a measurement is made on the spin of an atom, it is both spin-up and spin-down.
The team then switch on a magnetic field that varies in strength across the trap and causes the atoms to migrate to the centre. Occasionally, two atoms collide leaving one atom “spin-up” with respect to the magnetic field and the other “spin-down”. The atoms then move away from the centre with velocities that are correlated to their spins — spin-up electrons moving in one direction and spin-down electrons in the other.
The team found that this process took about 200 ms to reach a maximum spin segregation of 60% — and that this segregation endured for about 5 s. By contrast Cornell’s bosons remained segregated for a mere 200 ms — something that has been successfully described by a theory that assumes that the atoms interact strongly with each other.
However, Thomas told physicsworld.com that this “theory would predict that our spins would segregate in 7 ms and relax back to equilibrium in about 7 ms”.
Freedom to move
Thomas believes that his atoms remained segregated for seconds because they are much freer to move about the trap than were Cornell’s bosons — which couldn’t travel very far without colliding with each other. By carefully selecting the applied magnetic field, Thomas and colleagues we able to minimize the interaction strength between fermions, which means that the rarely collide. Because segregation and its subsequent decay are both driven by collisions Thomas believes that this is why these processes too much longer in his system.
As well as simulating the behaviour of electrons in solids, Thomas believes the system could be used to gain a better understanding of why spin segregation endures for such a long time could help physicists create new types of entangled spin-states.
About the author
Hamish Johnston is editor of physicsworld.com