Cooper Pairs Unpaired

In a low-temperature superconductor electrons don’t travel singly but in weakly tethered pairs, Cooper pairs. In a new experiment at the Forschungszentrum Karlsruhe in Germany, physicists have been able to send the two partners from Cooper pairs down separate wires spaced more closely than the effective size of the Cooper pairs themselves (see figure).

The Cooper pairs (which have the property that if one electron’s spin is up, then the spin of its partner must be down) start out in a piece of superconducting aluminum and proceed to a frontier where they can travel down either of two normally-conducting and magnetized iron wires. (In general, when Cooper pairs move from a superconducting into a normally-conducting material they can maintain their pair status for a bit into the new material---a distance referred to as the normal-metal coherence length---before breaking up.)

By magnetizing the wires so as to filter out pairings of any electrons that don’t have the characteristic Cooper opposite-spin-orientation, and by varying the distance between wires, and by measuring the resistance across the iron wires, the experimenters can learn specific things about the Cooper pairing mechanism (such as how large the pair is under various circumstances).

This work is part of the larger study of spintronics---the exploitation of electron spin for performing high-control electronics---and entangled states---the quantum behavior in which two spatially separated objects have a correlated behavior. (Beckmann et al., Physical Review Letters, 5 November 2004; contact Detlef Beckmann, detlef.beckmann@int.fzkde, 49-7247-82-6413).