Two by Two: Cooper Pairs
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That changed in 1957, when a trio of researchers, pursuing a hunch that electrons in a superconductive state were actually attracting each other, hit on a pretty convincing hypothesis. For this they got a theory named after them (Bardeen-Cooper-Schrieffer, for John Bardeen, Leon Cooper and Robert Schrieffer) and, as a little added bonus, a Nobel Prize.
The idea behind BCS, of course, flies in the face of everything you have learned about particle physics. Opposites attract, like particles repel. That’s what makes atoms spin ’round.
Yet to every rule there’s an exception, or two or three (welcome to physics!). Chalk this one up to the extreme temperatures in which superconductivity is achieved.
Put on your parka and mittens and we’ll go take a look.
PHYSICS FACTOID: Physicists tend not to think about how cold things are – even when they’re near absolute zero – but how hot they are; cold is nothing but the absence of heat. The coldest our universe gets naturally, scientists believe, is about 3 kelvin. However, they have created temperatures within a tiny fraction of a degree above absolute zero in laboratories, including at the MagLab's Millikelvin Facility and our High B/T Facility.
The atoms of metals, in their solid state, have a crystal lattice structure. Picture a tower of interconnected cubes made with Tinker Toys, the little wooden discs being the atoms. The space between the discs represents the potential paths for our electrons. Because this metal is surrounded by liquid helium, the atoms are barely moving.
There wouldn’t be anything "super" about superconductivity if the electrons moseyed through this lattice one at a time. What actually happens, according to BCS theory, is a kind of buddy system: The electrons pair up, zipping through the lattice with greater efficiency, forming a very fluid stream. Scientists call these electrons Cooper pairs. To reprise our earlier metaphor, think how much easier it is to strike up a conversation with someone new if, rather than going solo, you have a friend providing support.
Generally speaking, this is what it looks like: When an electron enters the lattice and passes between two positively-charged atoms, those cations get tugged just slightly toward the electron, and each other. The ions do not capture the electron with their positive charms; nor do they impede it. Rather, at superconducting temperatures, they nudge it along by creating a positively-charged wave, called a phonon, behind the electron as it passes. This positively-charged phonon, in turn, draws a second electron into the picture, which piggy-backs on the first electron’s momentum. Thus they become buddy-buddy, and continue traveling that way.
You might picture it this way: Our first electron tries to start a conversation with the positively-charged ions. The ions lean in a bit to hear what he has to say, decide they’re not interested, and go back to where they were, happy to let the boorish electron (and its buddy) move on and try their luck elsewhere. The phenomenon of Cooper pairs, by its very quantum nature, is hard to visualize, but the below animation communicates the main ideas about what happens in this state.
Click the image above to see a depiction of how Cooper pairs form.
The electrons are not attracted to each other: The phonon acts as the bond. This bond is pretty stable, and certainly more efficient than single electrons zigzagging their way one by one through the crowd of cations. The electrons, you might say, watch out for each other, neither one letting his buddy stray from the straight and narrow.
There is another important aspect of this theory: These pairs don’t cooperate only with each other, but also with other Cooper pairs. Simply speaking, the pairs can overlap, or even include other pairs within a longer pairing. This creates one cohesive group of paired electrons flowing with exquisite efficiency through the superconductor – as a single wave. The transformation is such that materials in superconducting mode are considered to be in a phase of matter all their own, distinct from solids, gasses or liquids.
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